Written by s • Thuja Green Giant Guide
Introduction
There are many evergreen trees to choose from for the garden, but one of the most useful is the Arborvitae, or Thuja. These soft-leaved, upright trees grow in a wide range of climates and soil conditions. They make excellent specimens and are among the very best trees available for making hedges and screens, a vital and basic part of almost any garden. So basic are these trees to many gardens that it is hard to imagine a world without them.
Although there are only a few wild types of Arborvitae, they are very widely grown in gardens around the world and well-known to many gardeners, who rely on them for the basic structure of their garden. As an evergreen screen they work 365 days a year, so they always give good value. There are hundreds of different forms, varying enormously in size and shape and including some of the best fast-growing trees for hedges, such as Thuja Green Giant, a remarkable tree that is always top of the list for screening and hedging plants.
The name Arborvitae comes from the Latin phrase ‘arbor vitae’, which means ‘tree of life’. This name was commonly given to several evergreen trees that did not ‘die’ during winter but remained constantly alive and green. It is that permanence that makes them so appealing in the garden, since they give a stable background to the seasonal changes of the other trees and flowers in the garden. An alternative account of this name is that the early settlers found Native Americans making tea from the twigs and leaves, which was a valuable source of Vitamin C during the winter months. West Coast Native Americans call it ‘long-life maker’ and eastern Native Americans may have used a similar name, leading to settlers calling it the ‘tree of life’.
The name Thuja is the one preferred by botanists and professional gardeners, since it is the Latin name given by them by the great botanist Carl Linnaeus (or Carl von Linné). He was a Swedish botanist who lived in the 18th century and devised a system of organizing and naming plants to help international botanists discuss them accurately. Thuja was the name he gave to one of the plants in this group, and it is still their name today. The name is used both as a common name and as a scientific name. When meant scientifically it is written in italics.
What Do Thuja Look Like?
Thuja plants are part of the big group of plants called conifers. These are the trees that have cones, not flowers, and normally have narrow green needles, not thin, broad leaves and flowers like most other plants. Almost all are evergreen and they are often found in colder regions of the world. Although commonly called ‘evergreens’, there are also evergreen flowering trees, so that name can be confusing.
There are just six species of Thuja and all of them are trees of varying sizes, usually upright and conical in shape when mature. The bark is brown and stringy, often shedding in strips.
They have needles that are not like those of, for example, pines, sticking out from the stem. They are flattened and pressed against the stems, overlapping each other and making fan-like sprays of green foliage. Younger foliage protrudes a little from the stems and has a spine on the end of the leaf. We can assume that this is to give the young seedlings some protection from grazing animals until they become tall enough for height to give them protection. Older trees rarely if ever show this feature, making them soft and pleasant to the touch.
Since these are conifers they do not have flowers, but produce cones. These form at the end of the stems so they are often not seen much on trimmed plants. There are separate male and female cones. Both types are small, made up of a tiny cluster of 8-12 scales and are single, not in groups. These cones look like tiny buds. The male cones produce pollen which is blown by the wind to the female cones. The female cones develop into small, round seed cones which are about ¼ inch across. These are usually green and leathery, but in the Oriental Arborvitae (T. orientalis) they are hard and woody, with a pronounced spine on each scale[1]. After the cones open and the seeds are released the cones of all kinds of Thuja become brown.
Thuja in the Garden and Landscape
Thuja or Arborvitae plays a large role in the garden, with the taller kinds making wonderful hedges that clip into dense screens or they can be left unclipped to make more informal barriers. Planted alone or in small groups they are excellent accent specimens around the house, in lawns or in shrub beds. They can be used to frame a doorway or entrance, outline a driveway or stand as dramatic solitary specimens. The many unusual forms, with colored or exotically-formed foliage and in many sizes, can be used as interesting specimens too, in beds, rock gardens or gravel-covered areas. Thuja can also be grown in containers of various sizes and used on terraces and decks as vertical or rounded accents that need very little attention to always look good.
As Screens:
Thuja trees make great screening plants. They are naturally dense, upright and lush green all-year-round. They need little or no clipping to maintain an attractive form and can be planted as a screen and left to develop naturally. They soon reach 20 or 30 feet in height, creating a solid barrier that filters wind, noise and pollution and gives complete privacy.
Particularly important for this function is Thuja Green Giant, which has a rapid growth-rate, perfect foliage all year round and a dense form. Since it will grow three feet a year when young, it rapidly fills in and gains height, making an excellent screen very quickly. It can grow unclipped up to 60 feet, so it makes the perfect tall screen too, although with clipping it can be kept at any height.
For smaller gardens, especially in colder areas, Thuja Emerald Green is an excellent shorter screen, quickly growing to around 12 feet tall.
As Hedges:
Thuja trees also make great hedges because they take well to clipping and shearing, so they can be turned into formal hedges easily. They quickly fill-in and become solid and dense, making the perfect backdrop to all kinds of garden designs and styles. If clipped from an early stage they can be as short as two feet, or as tall as 30 feet or even more. By choosing the right variety, a perfect hedge for any climate can be easily grown to almost any size and form. For larger hedges Thuja Green Giant is the outstanding and premier choice, while for smaller hedges in cooler areas Thuja Emerald Green is perfect.
As Accent Specimens:
Thuja do not of course have to be grown in rows, and as single specimens or groups they make beautiful accent plants in the foundation planting around a house or in shrub beds. With a wide variety of forms available, from upright to rounded, and in green or golden foliage, there is lots of variety to choose from. Evergreens give stability to the garden and a permanence that other plants lack. Since they can be trimmed, controlling size is easy, so they can be used to frame a door, fill a corner, grow beneath windows and occupy lots of places in the garden where they will be right at home.
As Container Plants:
As well as growing in the garden, Thuja make perfect low-maintenance container plants. A matched pair in large pots makes a welcoming entrance feature, or they can be placed around a terrace or patio. Although all types can be grown in pots, the dwarf varieties are usually the ones chosen, since they will live for many years without out-growing the container and they will need little or no clipping.
The History of Thuja
The name Thuja was used several hundred years before the birth of Christ by the Greek Theophrastus, a disciple of Aristotle. He is often called ‘the Father of Botany’ and wrote a book called Historia Plantarum (Enquiry into Plants), in which he used Thuja to name a tree from Morocco a little like a Cypress.
In the 1530’s plants and seeds of the White Cedar, T. occidentalis, also called the American Arborvitae, were introduced into France from the colony of Quebec[2]. Seeing a similarity, botanists of the time called it a Thuja, spelling it ‘Thuya’, which is anyway how it should be pronounced. In his earlier works Linnaeus used that spelling, but by the time he brought out his major work Species Plantarum (1753), the official start-date for all plant names, he spelt it ‘Thuja’, so that is how it is named and spelt today.
The Oriental Arborvitae, T. orientalis, was introduced into Holland from China and Japan and it was being grown in Holland early in the 18th century. Later French missionaries in China sent seeds to Paris, and from there it was introduced by Philip Miller to England around 1740. It was named by Linnaeus a few years later. It was re-introduced from Japan by the plant collector Robert Fortune in 18612.
Western Red Cedar, T. plicata, was discovered at the end of the 18th century in Nootka Sound, on the west coast of Canada, by a Spanish expedition of discovery led by Alessandro Malaspina and José de Bustamante y Guerra. It was introduced into Britain and Scotland from Oregon in the early 1850’s.
The Japanese Arborvitae, T. standishii, was discovered in cultivation in China and Japan by Robert Fortune and introduced into Britain in 1860. It may have been brought to America from Britain, but seeds must certainly have come directly from Japan to the West Coast, probably with Japanese immigrants.
The Sichuan Arborvitae, T. sutchuenensis, was first discovered in 1892 in China[3].
The Korean Arborvitae, T. koraiensis, was discovered and described in 1919 by the Japanese botanist Takenoshin Nakai.
The Species and Varieties of Thuja
Although there are just a handful of species of Thuja, their discovery is a mini-history of botanical expeditions and plant discovery around the world. Most have been developed in cultivation to produce hundreds of special forms, officially called ‘cultivars’, although here we will often use the word ‘variety’ for them as well.
Thuja occidentalis, White Cedar or American Arborvitae
This was the first species of Thuja known in Europe, and one of the very first North American plants grown there. A seedling was brought back to France by the explorer Jacque Cartier in 1536 and grown in the gardens of the King of France. Strangely, this plant grows much better in the colder parts of North America than it does in most of Europe, where it is not highly regarded.
White Cedar grows naturally from the Canadian Province of Manitoba east around the Great Lakes and into Québec, Prince Edward Island, New Brunswick, and Nova Scotia. It occurs in the states of Vermont, New Hampshire and Maine, as well as in isolated pockets in, Connecticut, Kentucky, Maryland, Massachusetts, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, and West Virginia.
It is found growing especially in swampy areas and wetlands, where other more-vigorous species of trees find it hard to complete, as well in rocky areas and cliffs for the same reason. However the largest trees grow on better-drained land.
Although White Cedar can grow very tall, most trees seen in cultivation are around 30 feet in height, rarely to 60 feet, with a trunk diameter close to the ground of 24 to 36 inches. The tallest tree currently known is growing on South Manitou Island, Michigan and is 112 feet tall with a trunk diameter at ground level of 69 inches. The tips of the branches are flattened and the underside is pale green. The cones are about 1/3 of an inch long, oblong in shape, made up of eight or ten scales.
White Cedar is hardy to zone 2, so it will survive winters of minus 50 degrees, and it does well all the way into zone 7, making it the hedging plant of choice for colder states. Although cheap wild plants are sometimes sold for hedging, it is difficult to produce a very dense hedge with such plants and varieties such as ‘Emerald Green’ are a much better choice.
Garden Varieties of White Cedar
There are literally hundreds of varieties of White Cedar which have been produced by gardeners and plant breeders in North America and Europe. The main variations are in: foliage color, often golden yellow; foliage shape, from thread-like to fan-like or crested; form, with rounded forms being very common, as well as narrow, upright forms; and overall size, with many dwarf forms suitable for rock gardens.
Some well-known varieties of special value are:
‘Emerald Green’:This is a form that is denser, smaller and narrower than wild plants, making it an ideal choice for a specimen that does not need clipping, or for a dense hedge. The foliage is a rich, bright-green, without the yellowish color that is often seen in wild plants. It was originally a seedling selected in Denmark around 1950 and at first it was called ‘Smaragd’ but this name was abandoned after it was introduced into North America and replaced with ‘Emerald Green’ because of the foliage color.
This plant is an ideal choice for hedging in the coldest areas, since its dense growth makes it possible to do much less clipping to develop a strong hedge. The rich dark color is kept all winter and there is normally no winter burn, unless the hedge is exposed to salt-spray. It also makes an excellent specimen plant, framing a doorway or as an accent in shrub beds.
‘Hetz’s Midget’: of the many round or globe forms of White Cedar, this is probably the best, forming a perfect green mound 3 feet tall and wide. It was developed in the first half of the 20th century by Frank C. Hetz, who owned a nursery in Pennsylvania. It grows well in sun and partial-shade, is pest free, just as hardy as its bigger brothers and fits perfectly into plantings around the house, or as an easy globe for a planter or container.
‘Danica’: this globe form was introduced in 1969 from a seedling, again grown in Denmark. It is very small and slow-growing, eventually forming a perfect globe a little less than 2 feet tall and across. The foliage is carried in vertical sprays, rather than horizontally, as it normal for White Cedar.
‘Golden Globe’: This is another popular globe form, rather larger, reaching about 4 feet tall and across, eventually up to 8 feet after 70 years. The foliage, especially when new in the spring, is a bright golden-yellow color. It was introduced in 1946 and is a golden mutation of an old American variety called ‘Woodwardii’, whose exact origin is not known.
‘Holmstrup’: is another Danish selection similar in many ways to Emerald Green, but denser, more pyramidal and smaller, so it is ideal for a specimen, but
a little too dense to make a flat-fronted hedge – the plants will not grow together well.
Thuja plicata, Western Redcedar
When we move to the west coast we find White Cedar replaced by the Western Redcedar, Thuja plicata, also known as Western Arborvitae, the second species of Thuja native to North American.
This tree grows naturally in Oregon, Washington State and over the border in British Columbia, In nature it is a very large tree, toping 200 feet, with trunks up to 13 feet in diameter, but in cultivation it is much smaller, usually growing a little over 50 feet tall, or rarely to 70 feet, and with a spread of 15 to 25 feet. When grown as a specimen it keeps the branches low to the ground for very many years, but when crowded together, unless clipped, the lower branches are lost, giving a tall trunk covered with fibrous reddish-brown bark that has a strong and pleasant aroma. It lives for many centuries; the oldest tree known being almost 1,500 years old.
The tree has typical Thuja foliage but there are whitish bands on the underside of the leaves, distinguishing it from White Cedar. When crushed the foliage releases an attractive aroma. The cones are about ½ inch long when mature, with about 14 scales.
The wood is widely used for outdoor construction and furniture, since the wood of mature trees contains a natural preservative, thujaplicin, which acts as a fungicide and prevents decay for up to 100 years. For this reason the wood, which is quite soft and easily worked, is favored for boat-building, roof shingles and garden construction, since it will not rot. Untreated it weathers to an attractive soft-grey color with pronounced grain. About 5% of people are strongly allergic to the dust produced by cutting and working Western Redcedar.
Western Redcedar grows best in cooler, but not cold areas, so it is not as hardy as White Cedar, but thrives in moister areas in zones 5 to 7. It is not drought resistant and prefers damp soil. It is often found in the wild growing in wet areas. It is unusual in being quite shade-tolerant, especially when young, meaning that trees can be successfully planted in shady areas and allowed to grow up into the sun. Western Redcedar is a fast-growing tree, growing faster than spruce in many cases. As a forestry tree it can be ready to harvest in as little as 50 years, although stands are best left for 100 years.
In the garden it can be grown both as a specimen, as a hedge or as a more natural screen. It does best in damper parts of the West Coast, where spectacular specimens can be seen, or in cooler parts of the east. In hotter areas the hybrid ‘Green Giant’ is a much better choice.
There are some garden varieties (cultivars) of Western Redcedar that have been produced and may sometimes be available:
‘Excelsa’: this fast-growing tree forms an upright column with short horizontal branches and glossy, dark-green foliage. Developed in Germany it is suitable as a hedging plant.
‘Fastigiata’: this narrow upright form has dense foliage and makes an attractive specimen for limited space, as well as a hedge that needs much less clipping. The variety ‘Hogan’ is similar.
‘Hillieri’: a slow-growing bush with crowded branches and leaves in irregular whorls. Reaching just 8 to 10 feet, this form was developed in England by the famous Hillier’s Nursery around 1900.
There are a number of dwarf forms with golden foliage, including ‘Rogersii’, ‘Stoneham Gold’ and ‘Zebrina’. The last, also known as ‘Zabrian’, has, as its name suggests, alternating bands of yellowish and more greenish foliage, giving an overall yellow-green appearance to the bush.
Thuja orientalis, the Oriental Arborvitae
The remaining species of Thuja all occur in the Far East, beginning with the Oriental Arborvitae. This plant is now more correctly known as Platycladus orientalis, a genus containing just this plant, distinguishing it from Thuja on the basis of differences in the female cone and also in the seeds lacking the ‘wings’ seen in other Thuja species[4]. Although this name was first proposed in 1949, gardeners are slow to adopt name changes and it is still called Thuja in many gardening works and catalogues.
The Oriental Arborvitae occurs naturally from the North-east of Iran, through China, up into eastern Russia and the Korean peninsula. The original natural distribution has been blurred by extensive planting of this tree in the past over a wide area. This includes planting along the ancient Silk Route through Central Asia. It has also been so extensively planted in Japan that it has become part of the natural forests. It has been suggested that it may only be genuinely wild in Iran2.
It forms a large shrub or small tree 30 to 40 feet tall in cultivation, although over 60 feet in the wild. The trunk is 2 feet or sometimes more in diameter. Trees can live for 1,000 years and the tallest tree, found in Tajikistan, is 115 feet tall. In gardens it is often seen as a dense, rounded and fat-conical tree with many stems, looking more shrub-like. It is one of the best choices for xeriscaping or planting in any dry area.
It is distinctive from other Arborvitae in having the branches and their fans of leaves held in a vertical position, rather than the common more horizontal position of other Thuja. They are the same color of green on both sides with no white bands. The roundish, egg-shaped cones are up to1 inch long and blue-green in color, with a grayish waxy coating making them look purplish. The scales of the cones are distinctively thick and woody, with a hooked, horn-like protrusion at the top. The seeds are wingless. It is for these differences with other Arborvitae that they have been re-named Platycladus.
The Oriental Arborvitae also differs in its soil requirements, preferring well-drained and even sandy conditions, thriving in drier stony soils, even alkaline ones. This makes it a great garden choice as an evergreen shrub or tree for drier regions or quick-draining soils in wetter ones. It also prefers warmer temperatures, doing best in zones 6 to 11, making it the natural choice in more southerly and western states. It is drought-tolerant when established and although not salt tolerant it will even grow on crushed –coral and shell soils of coastal areas. It is also very resistant to almost all pests and diseases.
Over 40 cultivars exist, but most are rare in gardens. Some common garden varieties include:
‘Aurea’: grows 12 to 18 feet tall with yellowish foliage.
‘Aurea Nana’: this dwarf form has golden-yellow foliage and reaches 5 feet tall in about 10 years. It has an upright, rounded form, showing clearly the vertical branches of the species. It is less winter-hardy and does better in shade than the original species and may also be less drought-resistant.
‘Elegantissima’: this tight, upright broad-conical form in a yellowish-green color can reach 15 to 20 feet tall. It was developed and released in 1958 by a British nursery. The foliage turns more greenish in summer and bronzy in winter.
‘Filiformis Erecta’: (also known as forma flagelliformis) has drooping sprays of foliage which are cord-like and in clusters. It is often seen in China and Japan in gardens as a novelty. It grows just 5 to 6 feet tall.
‘Sunkist’: is a tiny green shrub-form growing just 24 inches tall and wide.
Thuja standishii, the Japanese Arborvitae
On the Japanese Islands of Honshu and Shikoku, higher in the mountains, natural stands of the Japanese Arborvitae can be found. As well, this tree is widely grown as a crop in Japan, where it is valued for its waterproof and softly-scented wood. This wood is used for sake barrels and cups and to build Shinto temples.
It forms a tree about 60 feet tall, sometimes reaching 100 feet, with a slender trunk no more than three feet across. This tree closely resembles the Western Redcedar, although there are recognizable differences between the cones. It forms a much more open tree than the other Thuja trees.
There are no particular cultivated forms of this tree and the tree is in fact very little grown outside Japan, although it is hardy to zone 6, but it is one of the parents of the important hybrid, Thuja Green Giant.
Thuja sutchuenensis, the Sichuan Arborvitae
While many Thujas are common in the wild and also widely cultivated, often in many forms, this plant is an endangered species that is not in normal cultivation at all. It was discovered by the French missionary and plant collector, Paul Guillaume Farges in 1892 in Sichuan, Western China. It was harvested locally for its highly-valued scented wood. For the next 100 years it was never seen and it was assumed to have been wiped out by over-cutting. It became the only conifer officially declared extinct. However in 1999 it was rediscovered near the spot Farges had originally found it, on some very inaccessible slopes of the mountains[5].
The area has been designated a Special Protection Area to preserve these few remaining trees. Despite its scarcity and protected status, prayer beads claiming to be made from the scented wood of this tree can be found for sale on the internet.
This species forms a tree or large shrub between 30 and 65 feet tall, with a slim 12 inch trunk. It has white markings on the underside of the leaf similar to Thuja plicata, but the leaves and cones are smaller than other Thujas, which distinguishes this species from them.
Thuja koraiensis, the Korean Arborvitae
This is a second rare and endangered species very rare in cultivation. It occurs in mountainous regions of Korea, chiefly today in the South, since it is threatened by North Korean government policies of forest clearing of slopes for agriculture. Only a very small number of mature specimens are believed to be still growing in the North[6]. It forms a small tree, 10 to 30 feet tall, with sprays of foliage. This tree makes an interesting garden plant, since the underside of the leaves are almost pure white, with broad bright-white bands, giving the tree an attractive and unusual appearance. Winter hardy to zones 5 or 6, this tree could become much more common in gardens.
| Species | Branches | Branchlets | Upper leaves of shoots | Cones | ||||||
| On the leader | On the branchlets | |||||||||
| vertical plane | horizontal plane | Both sides green | White bands beneath | With oil glands | Without oil glands | Furrowed | Not furrowed | Thick and woody | Small and leathery | |
| T. orientalis | X | X | X | X | X | |||||
| T. sutchuenensis | X | X | X | X | X | |||||
| T. occidentalis | X | X | X | X | X | |||||
| T. plicata | X | X | X | X | X | |||||
| T. standishii | X | X | X | X | X | |||||
| T. koraiensis | X | X | X | X | X | |||||
Table 1 – Identification features of Thuja species[7]
Hybrid Thuja
When different but related trees grow in different parts of the world, of course there is no chance of them ‘meeting up’ and producing seeds. However when we cultivate together trees that are brought from different continents, breeding together is something that can and does happen, either naturally or with the help of plant breeders.
One of the significant consequences of the meeting of two species is that when they cross together the strengths of each mask the weaknesses of the other, producing trees that are stronger, hardier, faster-growing and more vigorous than either of the parent trees. Plant breeders call this ‘hybrid vigor’ and it is an important and valuable consequence of plant breeding.
There exists just one Thuja hybrid, but this extraordinary plant changed completely the garden use and importance of these plants. Before its development, Thuja was important in cold areas and also of interest for some of the unusual and attractive forms that had been developed. But it hardy merited the enormous attention from most gardeners that now centers on this plant, making it one of the biggest selling garden plants in the nursery industry and the center-piece of many garden plans. This plant is called Thuja ‘Green Giant’.
Thuja ‘Green Giant’
The story of this jolly green giant is long and complex[8]. It begins in a nursery in Denmark in 1937. This was D.T. Poulsens Planteskole og Frøhandel, founded by Dorus Theus Poulsen, a gardener and botanist born in 1850 in Frijsenborg, Denmark[9]. In 1878 Poulsen started a nursery with his two sons, which soon became the most famous and prestigious nursery in Denmark. The family began to breed roses, their hybrid Polyantha roses became famous and they were the predecessors of today’s Floribunda roses. The family soon had several nurseries across Denmark. D.T. Poulsen himself died in 1925, but his sons continued his work, breeding and hybridizing many different plants. In 1937 they spotted an interesting plant, which they thought was a hybrid Thuja. It is not clear if this had occurred naturally or by deliberate crossing of the parent trees.
As happened right across Europe at this time, World War II stopped all activity and prevented the exchange and movement of new plants both in and out of all European countries. So it was not until 1967, thirty years later, that some young plants came to the National Arboretum in Washington D.C. from the Poulsen nursery in Kvistgaard, Denmark. Actually several different plants were brought over, and they became confused, so that when one of them had grown to 30 feet tall by the mid-90s and was attracting excited interest from experts, it was no longer clear exactly which of the original plants it was.
Three scientists undertook to solve this puzzle; Susan Martin, from the National Arboretum; Kim Trip, from the New York Botanic Garden; and Robert Marquard, from the Holden Arboretum in Ohio. After extensive searching through records and carrying out DNA analysis, they not only identified which original Poulsen plant this was, but demonstrated that is was indeed a hybrid, between Thuja standishii and Thuja plicata, the Japanese Arborvitae and the Western Redcedar8.
This cross-pacific meeting, taking place in a field in Denmark, gave the gardening world a new treasure, a hardy evergreen with blemish-free rich-green foliage, extremely rapid growth, a graceful and dense upright form and the ability to grow in a wide range of soils and climatic conditions. This was ‘hybrid vigor’ spelled out large.
A search for a suitable name was settled when a nurseryman from Tennessee, Don Shadow, suggested ‘Green Giant’[10]. One nursery in particular took an interest in that single specimen standing in the grounds of the National Arboretum. Wayland Gardens had begun in the 1920s when two European nurserymen got together in Ohio and started a successful nursery business. Following the deaths of its founders the nursery was bought in 1975 by Park Seed Company, who moved the whole business to South Carolina. There it flourished and became one of America’s biggest nurseries, introducing many new plants to gardeners.
As the last century came to a close, Wayland Gardens propagated hundreds of plants of Thuja Green Giant from that original plant, and began to promote and publicize it. By 2004 it became the biggest selling item in their catalogue. The timing was perfect; with many mature gardens in the south-east needing old, diseased hedges removed and replaced, which meant that literally millions of plants had to be grown so that there could be new, perfect hedges right across the country[11].
Choosing the Right Thuja
Hardiness Zone
Because of their natural distribution over large parts of the world, different Thuja grow in lots of different climates, so no matter where you live, there is a suitable type for your climate and growing conditions. Depending on your growing zone, different species and types will do best:
| USDA Hardiness Zone | Suitable Thuja | |||
| White Cedar | Thuja Hybrid | Western Redcedar | Oriental Arborvitae | |
| Zone 2 | Thuja Emerald Green | |||
| Zone 3 | Thuja Emerald Green | |||
| Zone 4 | Thuja Emerald Green | |||
| Zone 5 | Thuja Emerald Green | Thuja Green Giant | Western Redcedar | |
| Zone 6 | Thuja Emerald Green | Thuja Green Giant | Western Redcedar | Oriental Arborvitae |
| Zone 7 | Thuja Emerald Green | Thuja Green Giant | Western Redcedar | Oriental Arborvitae |
| Zone 8 | Oriental Arborvitae | |||
| Zone 9 | Oriental Arborvitae | |||
| Zone 10 | Oriental Arborvitae | |||
Rainfall and Drought-resistance
White Cedar and Western Redcedar prefer areas with significant rainfall and a more humid atmosphere. This is especially true for Western Redcedar, which does not do well in drier areas. Thuja Green Giant is more drought-resistant than either of these species, making it the ideal choice, even for drier parts of zones 5, 6 and 7.
In warmer, drier areas the Oriental Arborvitae is most suitable, growing well throughout the South-west and South-east. This is one of the best shrubs for dry locations, or where no watering is possible.
Light and Shade
Since most Thuja live below larger forest trees, most are able to tolerate some shade and will grow well with full sun for only 50% of the day. The more sunlight, the denser the growth will be, which is a factor to consider when installing hedges and screens. Of course, regular clipping can help compensate for more open growth by stimulating denser branching.
As a general rule Thuja trees will do best in full sun, but all except the Oriental Arborvitae will grow well in partial shade. So when choosing a variety, if you live in a zone where there are choices, of the most commonly grown plants, Thuja Emerald Green and other White Cedars have the best shade tolerance, followed by Thuja Green Giant and then Western Redcedar. Oriental Arborvitae should be grown in full sun.
Soil Types
Thuja are tolerant of most types of soil, but do best in ones that hold a good amount of water. Especially when young and growing vigorously, they benefit from a good, regular supply of water, but not from being in soil that is constantly flooded. As already mentioned, Oriental Arborvitae is very drought-resistant and prefers dry, sandy or even stony soils.
Thuja will grow well in acidic, neutral and slightly alkaline soil, so adjusting the pH is not normally necessary. A good amount of organic material added to the soil when planting and as mulch each year for the early years, will ensure good dense growth in almost any soil.
Choosing a Planting Location
Spacing
When planting your Thuja in rows or groups it is important to space them correctly. If they are too close they will not develop well, but if they are too far apart they may take years to meet, or even always stay slightly separated. The ideal spacing will allow the plants to grow, yet mean that in a few years they will form a single mass of plants.
For a screen, there are two methods of spacing and planting used:
- Single row: this is the most obvious method and for a shorter hedge plants should be placed 3 to 6 feet apart, depending on their final size and your needs. If your screen or hedge is against a building, plant 6 feet from the wall, to protect the foundations. If you are planting against a fence, plant 3 feet from the fence so that the lower part of the plants remains bushy and the hedge remains on your side of the property line. Use the wider spacing given here for vigorous plants like Thuja Green Giant, and the smaller spacing for others like Thuja Emerald Green.
- For a shorter hedge, less than 8 feet tall, plant 3 feet apart.
- For a larger hedge or screen, plant 4 to 6 feet apart.
- Double row: This method will give a quicker screen and use slightly fewer plants for the density of screen created. Make a double row, allowing 3-4 feet between the rows. Space the plants 6 to 10 feet. Stagger the plants so each one is in the space of the other row. This method will give a good dense screen in a short time.
To calculate how many plants you need for a screen, first measure the distance. Then decide on the spacing. Divide the distance by the spacing and round-up to the next number if the answer is a fraction. For a double row multiply this number by 2. Remember that the first plant will be placed half the spacing distance from the end of the row, and the last plant will be placed in the same way. Place these plants first and then balance out the remaining plants in-between at even distances.
Remember that whatever spacing you use it is important to keep everything at the same distance and place the trees very evenly spaced. Use a measuring tape to get everything laid-out before planting. The extra work will be worth it when you see the perfect screen you have created.
Note: If you want your screen right to the ground it is important to start trimming early in the life of your screen. Although Thuja are better than many other plants at retaining their foliage closer to the ground, if you let the plants grow untrimmed until they are the height you want they will be thin lower down and may not give you the screening you are trying to develop.
For a Specimen take the width given for your tree, add it to the final widths of the nearest other trees or shrubs and divide by two. Plant the trees at that distance apart. That way your garden will not become overgrown and crowded, forcing you to constantly prune and clip your plants.
Planting in groups is a great way to work with Thuja trees when used as specimens. Always plant your shrubs using odd numbers, 3, 5, 7, 9, 11, etc. This will give your planting a more natural look. Look at the width you particular tree will reach and space your plants at about 75% of that. In a few years they will have grown together into an attractive group.
Planting Thuja
It is worth taking a little effort to give Thuja trees a good start in life. Once the planting positions have been decided, dig over the area, going down to the full depth of a spade and turning the soil over while removing weeds and their roots. Don’t worry about taking out stones unless they are bigger than your clenched fist. Add some organic material to the soil to encourage root development. This could be garden compost, well-rotted manure or rotted leaves. If you don’t have these materials, then some peat-moss can be used instead and that is readily available everywhere. Mix a big bucket of this organic material into the soil of the planting area of each tree.
When planting hedges it is often easier to dig a trench along the line, rather than individual holes. This makes it easier to space the plants correctly and adjust them to get a perfectly even and straight row.
The night before planting, give the potted trees a heavy watering. Dig a hole two or three times the width of the pot, but only to the same depth as the pot. Once you have dug your hole, add soil if necessary and press with your foot in the bottom of the hole to make sure the soil is firm underneath the plant. Thuja should be planted at the same depth in the ground as they were in the pot – do not bury the lower foliage, or leave part of the root-ball exposed above ground.
Place the plant in the hole and put back about three-quarters of the soil. Using the feet, firm the soil down around the roots, so that there are no air-pockets in the soil and so that the tree is held firmly and can’t shift around. Fill the hole to the top with water and wait for it all to drain away. When all the water has gone, replace the rest of the soil and make everything level and neat. No more water needs to be added, unless the surrounding soil is very dry. Mulch the roots with a thick layer of organic mulch. This should be 2-3 inches thick and completely cover the root zone and a little further out. Keep the mulch off the foliage and trunk of the tree. The mulch will reduce weeds, conserve water and keep the soil cooler during the hottest weather.
You do not need to use a stake on your tree, in fact that is a bad idea, rather like giving a child a walking stick – your plant will be stronger and tougher without being held up artificially.
During the first growing season water your plants deeply at least once a week, and twice a week during hot, dry weather. Leaving a hose running slowly near the base of the plant is better than spraying water onto the soil – you disturb the soil less and the water will have a chance to penetrate deeply around the roots. For hedges and screens installing a trickle hose along the base of the hedge makes watering much easier. This can be left in place as the hedge grows so that it can be used during dry spells in future years.
Growing Thuja varieties in Planters and Containers
Thuja, especially the smaller varieties like Thuja Hetz’s Midget and Thuja Golden Globe, make good, low-maintenance, permanent specimen plants for pots and planter boxes. Choose a pot about twice the diameter of the pot your tree arrives in and you will not need to re-pot for several years at least. Make sure the pot has drainage holes – drill one if necessary in the base. Do not stand the container in a saucer, but let it drain freely. Remember that terracotta and clay pots need more watering than plastic pots. Clay pots should be soaked in water for a few hours before using them.
Use potting compost designed for outdoor planters, not garden soil. Cover the drain hole with a couple of stones and fill the container with some potting compost to place the plant so that the top of the root-ball is one inch below the top of the pot. Fill around the root-ball until the top of it is reached and cover it with just a small amount of the compost. Make sure there is about one inch of space left free at the top to hold the water when you water your plant. Firm down around the plant only a little, do not press down hard. Water the tree thoroughly after planting, until water flows out of the drain hole.
Water whenever you see the top layer of the compost has become dry. Use a liquid fertilizer for evergreens regularly during the growing season.
Caring for Thuja Trees
One of the great things about Thuja trees is that they don’t need much care. Thuja Green Giant in particular is one of the most low-maintenance plants available. However a little seasonal care will give the best possible results and make sure that your trees grow vigorously, look lush and green and remain healthy.
Fertilizer
Each spring add a layer of mulch over the roots of your plant. This should be 3 inches deep and should extend out beyond the line of the foliage, but not touch the trunk of your tree. Use something organic like garden compost or rotted leaves rather than bark or stones, which will not add any nutrients to the soil. Old mulch from previous years can be removed if it is woody and hard, but otherwise it can just be covered with the new mulch.
When your plants are young, some fertilizer is helpful to encourage them to grow vigorously and become well-established. Use a fertilizer designed for evergreen trees. This will give a good supply of the nitrogen these trees need to keep them healthy and deep green.
When they are young a liquid fertilizer is best, but for mature plants a granular fertilizer is more suitable and it should be applied in spring. This should be sprinkled over the root zone and can be place over mulch too. Evergreen fertilizer has a lot of nitrogen in it, so look for something with a high first number, like 20-10-10. A light sprinkle of fertilizer over the whole root-zone is all that is needed and you should avoid heavy fertilizing. This zone extends about two feet further out from the plant than the spread of the foliage. Keep fertilizer away from the trunk. If you want to grow your Thuja trees organically, products like soya-bean meal, cotton-seed meal or alfalfa pellets are suitable, although usually rich mulch will be all that is needed.
Trimming and Shearing Thuja Trees
All the different kinds of Thuja can be grown without any clipping and they will become attractive plants and make dense screens. A little clipping in late summer of young plants will help to make them denser later in life.
However if you want to trim your screen or hedge regularly, for looks or to keep it smaller, then this should begin during the first year. Do not wait until the trees reach the desired height and then start clipping. Worse, do not wait until they become too large and then try to cut them back hard. Thuja will not re-sprout from bare branches, so it cannot be lopped and trimmed as many deciduous trees can be. When clipping or trimming, you should always leave some foliage in the clipped area, as bare branches will never sprout new leaves.
Begin clipping lightly as soon as the plants are established. Always trim so that the upper part is a little narrower than at ground level. In other words, the sides should slope inwards slightly. This only need be by a few degrees, but if there is no inward slope the lower branches will become thin over time and then become bare and may eventually die. Because the upper growth will always be more vigorous, this means trimming more from the upper part of the plants than the lower part. Many new gardeners make the mistake of taking the same amount off all over the plant, resulting in hedges that bulge out towards the top, causing the lower parts to thin and die over time..
The best times for trimming Thuja hedges and screens are late spring, early summer and early to mid-fall. Avoid trimming during hot and dry weather, especially with White Cedar hedges. In mild areas hedges can also be trimmed in winter, but be careful not to trim Thuja hedges in late fall or winter if you live in cold areas with periods of severe freezing weather in winter.
Use gas or electric hedge shears, or hand clippers, for your hedge. Do not use coarse cutting machinery like chain saws (yes, some people do!). Cut branches so they grow horizontally and do not try to ‘tuck in’ shoots, or leave long shoots growing upwards on the outside, or your hedge will fall apart easily, especially under snow.
Pests and Diseases
Thuja trees are tough, hardy trees that do not have serious problems, but there are a few things to watch for. All Thuja are eaten by deer. If low-maintenance is your goal, thenThuja Green Giant is especially pest and disease resistant, making it the ideal choice for almost any area.
Potential problems for White Cedar[12]
Spider Mites: these minute relatives of spiders are sometimes found, especially in hot, dry conditions. They are too small to be seen easily, but cause a yellowing or ‘bronzing’ of the foliage. Sometimes fine webbing can also be seen, indicating a heavy infestation. Spraying the plants regularly with water during dry weather usually prevents them from becoming a problem, or insecticidal soap will normally kill them.
Cedar Leafminers This tiny moth has caterpillars that live inside the leaves, turning them yellow or brown. It only lives on the young tips, so it is rarely seen in clipped plants, but sometimes it is seen in unclipped ones. If rare serious infections occur, systemic insecticides give good control.
Shoot blight: this fungus is only seen on weak plants, causing the stems to brown and die. Plants that are well-watered and fertilized will normally not suffer from this problem.
Winter Dieback: the foliage can turn brown in spring and die if your plants lack water during the winter. Always soak the ground under your cedar trees in late fall if you live in a very cold area. Similar damage can be caused by salt used for winter de-icing.
Potential Problems for Western Redcedar and other Thuja[13]
Cypress bark beetle: this tiny beetle burrows beneath the bark, causing twigs and branches to die. It is usually only seen on weak and dry trees, so keep trees well-watered and fertilized to prevent this pest.
Scale Insects: these insects live under a hard shell and look like bumps on the bark or leaves. Small amounts are not a problem and larger infestations can be treated with systemic insecticides.
Bagworms: these insects, which eat the leaves, make bags of web and twigs, which hang from the branches like Christmas decorations. Large quantities can cause significant leaf-loss. The organic sprays Spinosad and BT give good control if applied in May.
Canker fungus: This disease can cause branches and even whole trees to die. There is no control, but trees that are well cared-for rarely suffer from this problem.
NOTE: Thuja Green Giant is almost always free of pests and diseases, so it is a vastly superior choice from zone 5 and on into warmer areas.
| Pests and Diseases of Thuja | ||
| Symptoms | Cause | Treatment |
| Yellow and bronze foliage | Spider mites | Water jet or insecticidal soap |
| Yellow and brown shoot tips | Cedar leafminer | Clipping or systemic insecticide spray |
| Dead foliage in spring | Winter dryness or salt spray | Deep watering before winter |
| Brown bumps on stems and leaves | Scale Insects | Systemic insecticide if severe |
| Small bags of web and twigs hanging on branches | Bagworms | Spinosad or BT sprays |
[1] Manual of Cultivated Conifers, Krüssman, G., Timber Press 1972/1985 translation (by Epp, M.E.)
[2] Trees and Shrubs Hardy in the British Isles, Bean, W.J. (1914-1975) on-line edition
[3] American Conifer Society, http://conifersociety.org/conifers/conifer/thuja/sutchuenensis/
[4] Mabberley’s Plant-Book, Mabberley, D.J., 3rd Ed. Cambridge University Press.
[5] Thuja sutchuenensis: A rediscovered species of the Cupressaceae. (2002). Xiang, Q.; Fajon, A.; Li, Z.; Fu, L.; Liu, Z. Botanical Journal of the Linnean Society 139 (3): 305–310.
[6] Thuja koraiensis Kim, Y.-S., Chang, C.-S., Lee, H. & Gardner, M. 2011. The IUCN Red List of Threatened Species.
[7] Modified from: Manual of Cultivated Conifers, Krüssman, G., Timber Press 1972/1985 ( translation: Epp, M.E.)
[8] Thuja ‘Green Giant’, the US National Arboretum. http://www.usna.usda.gov/Newintro/grgiant.pdf
[9] D.T. Poulsen, Dansk Biografisk Leksikon. http://www.denstoredanske.dk/Dansk_Biografisk_Leksikon/Landbrug,_skovbrug_og_gartneri/Gartner/D.T._Poulsen
[10]Green Giant Arborvitae, by Gerald Klingaman. University of Arkansas, Division of Agriculture .http://www.uaex.edu/yard-garden/resource-library/plant-week/green-giant-arborvitae-1-27-06.aspx
[11] Critics of the plant-patent system are quick to point out that Thuja Green Giant was released by the National Arboretum patent-free. This meant that any nursery could propagate and grow it, making it possible to keep up with the extraordinary demand for this plant.
[12]Pests and Diseases of the Eastern White Cedar. Pest Diagnostic Clinic, University of Guelph, Ontario, Canada http://www.guelphlabservices.com/files/PDC/006Eastern%20White%20Cedar.pdf
[13] Arborvitae Problems, Horticulture Center, Cornell University http://ccenassau.org/resources/arborvitae-problems
