Thuja, more commonly known as arborvitae, are popular evergreen trees in the Cupressaceae family. These dwarf to large-sized conifers are used extensively in landscapes plantings in our area, as well as in the Midwest and West.
Two of the five species of Thuja are native to the United States. Thuja occidentalis is commonly known as American arborvitae (or Eastern white cedar, though it is not a true cedar). A relatively small tree that grows 30 to 60 feet in height but at a rapid rate of growth, American arborvitae is native to the Northeast and west to the Great Lakes Region. Thuja plicata, called Western red cedar (though also not a true cedar), is native to the Pacific Northwest. It is a much larger tree, growing to heights of well over 200 feet.
We receive a lot of questions about arborvitae, because they are popular, often well-suited to the Northeastern United States and readily available to the home gardener. Most cultivars are fast-growing and can form a green screen for a property quite quickly. The natural growth habit is to retain branches all the way to the ground which enhances their value for hedging or screening use. With proper siting and care, arborvitae are also traditionally insect and disease resistant.
Increasingly wet and warming weather, however, is having an effect on the suitability of these plants in our area and their reputation as easy-to-grow. Extra care should be taken to choose the right plant for the position intended and to allow air flow around arborvitae when planting. Arborvitae need consistent moisture and excellent draining soil. Without proper site preparation they can succumb to disease and drainage problems that reduce their vigor and attractiveness irreparably. Areas where water pools after heavy rain are not suitable for these plants. They are attractive to deer and considered likely to be severely damaged in a deer heavy location.
Choosing the best species and cultivar is also important for long life and ease of care, as some arborvitae can discolor in winter and some lose foliage, yielding a thin and ratty appearance with age.
In our area, conifers can be planted in early spring (March to May) and early fall (September to October). Try to plant your conifers on an overcast day when the tree will lose less water through transpiration (the evaporation of water from plants).
Arborvitae should be planted in fertile, moist, well-drained soils. They are considered difficult to establish and preparing the site for good drainage and to receive consistent moisture cannot be overstated. You may need to work a large area of soil to improve its draining properties and consider installation of reliable irrigation to prepare the site. If you are planting in a container, it must have adequate drainage.
Arborvitae perform best in full sun, although light shade is acceptable. In heavy shade, plants become loose, open, and lose their attractively dense constitution. Thuja plicata is a Pacific Northwest native and more shade tolerant than T. occidentalis or its relative Platycladus orientalis (sometimes sold as Thuja orientalis). But T. plicata also does best in areas with cool, wet summers.
Many arborvitae species and cultivars grow quickly into large trees. They need to be spaced to accommodate that growth and to allow each tree the air movement and nutrition it needs. If the young trees are planted too closely, they will run into difficulty as they grow. They will struggle to compete for water and nutrition from the soil and will be prone to fungal diseases. Thuja 'Green Giant', for instance, is a hybrid cultivar (T. plicata x T. standishii) that can grow up to 4 feet in a year. If a hedge or screen planting is desired, they need to be planted at least ⅔ of their minimum, mature width apart. That implies a minimum of eight feet between trees. Plant arborvitae a full tree width from a fence or building.
Dig a hole that is 2 to 3 times the width of the root ball and the same depth as the height of the ball (a wide and shallow hole). Arborvitae need great soil texture and drainage so do not choose a position with poorly draining, compacted soil or it will end in poor tree health. If you have particularly dense soil and loosen just the soil in the planting area, it will create a basin that will collect water and weaken the tree. A much larger area must be improved.
When planting a container grown plant, rest the container on its side and carefully slide the conifer out of the pot. If the plant is root bound, loosen the roots with your hand, a hand fork or a knife so they are not spiraling in a circle around the root ball.
Balled and burlapped conifers should be placed in the hole intact. Position the conifer firmly in the hole and make any adjustments while the burlap is still on the root ball. Cut the burlap, string and wire away from the trunk with a sharp knife or pruning shears and wire-cutters and gently pull the material away from the root ball. Try to slide the burlap out from underneath the root ball, gently tilting the tree to the side if necessary. Always remove the burlap unless the root ball is falling apart. If it is falling apart, remove as much of the burlap as possible without disturbing the root ball. Untreated burlap will eventually decompose, but it hinders the initial root growth that is important for the successful establishment of the tree. Fill in the hole and water.
Make sure that the trunk flare (the point where the roots and the trunk meet) is slightly higher than ground level. This positioning will compensate for the tree settling once it has been planted.
Newly planted conifers will need regular watering for 3 to 6 months until the roots get established. For the first few weeks, check every 2 to 3 days to make sure the soil is not dry (stick your finger 2 to 3 inches into the soil) and water deeply if needed. You will need to give it supplemental watering during dry periods in the summer and fall for several years until it becomes more stable.
Once established, arborvitae continue to need about an inch of water a week right up until the time the ground freezes. They are not tolerant of dryness or soil that remains wet, so an attentive watering regimen is important. Do not water with above ground methods which can easily spread fungal spores as water splashes on low limbs. Watering at ground level or with underground irrigation is preferable.
Conifers are not heavy feeders and most soils do not require any fertilizer. If you question the health of your soil, perform a soil test and fertilize according to the needs indicated. You can have your soil accurately tested through a lab by contacting your county's cooperative extension office. (To find your county's cooperative extension office, begin with this map by state and then navigate from the state page to your county.) Garden centers sell bio-stimulants for conifers that enhance root growth and are beneficial to newly planted conifers.
Conifers prefer cool soil temperatures. Mulching is an important step in the planting process and the maintenance of conifers. Mulch should be 1 to 3 inches deep and should not touch the base of the conifer (when mulch comes in contact with the trunk of the conifer it encourages decay and harbors disease). Composted bark and shredded leaf mulch are two good mulch options.
As a general rule, you do not need to stake conifers. They only need staking if you plant them in a windy location. Stake large conifers for one year until the roots get established. Weeping and pendulous conifers also need staking until they can support themselves.
While many conifers will not require any pruning, dead, damaged or diseased branches should be removed as soon as they are noticed, regardless of the season. Arborvitae can create some new growth from lateral buds on young branches and, if necessary, can be moderately pruned before new growth appears, in late winter or early spring. Cut branches back to a side branch (lateral) and do not cut back more than ⅓ of the total length of the branch. This type of pruning will produce a fuller and more compact habit.
Most of the Cupressaceae family, including arborvitae, respond well to regular trimming of young growth and can be trimmed as formal hedges, provided that this more structured pruning is initiated from the start. They will not form new buds and growth from older wood.
Once arborvitae is mature in growth, pruning is best avoided unless essential. If, for instance, you remove a considerable amount from the tops of your trees to reduce an overgrown border, the top growth will not be replaced and the tops will remain relatively bald. Remember, arborvitae must be trained by pruning beginning when they are young, establishing a size limit annually. If yours have grown too tall and you need to create a lower stand of attractive trees, replace them. The new, young trees can be pruned into a hedge at the height you wish.
Most arborvitae will not grow well in the hot summer conditions above USDA zones 7 or 8, or in winters colder than zone 4. Check that the Thuja variety that you select is appropriate for your area. Thuja occidentalis and its cultivars are much better adapted to the conditions of our area than Thuja plicata, which is native to areas with cool, moist summer weather. Other Thuja species may be difficult to locate for your garden.
Avoid container grown conifers that have thick mats of roots on the soil surface or roots circling around the main stem. Do not buy a plant that has had its root flare buried below the soil surface. When selecting balled and burlapped conifers, look for moist root balls that do not have large, torn roots sticking out of the ball. As a general rule, it is better to purchase smaller plants unless: (1) you are planting in an area with heavy foot traffic or (2) you are planting a dwarf conifer and do not want to wait years until it gets to the desired size.
Some popular cultivars for east coast gardens are listed below, but there are many more. New York Botanical Garden is home to more than 30 varieties of Thuja, including examples from the species T. standishii (Japanese arborvitae), T sutchuenensis (Sichuan arborvitae) and T. koraiensis (Korean arborvitae) and many of the selections listed here. Come for a visit to see the mature appearance of the plants you are considering. Garden locations of arborvitae can be found through the NYBG Plant Tracker.
Keep your tree consistently moist during the summer and fall and then give it a soaking drink in the late fall before the soil freezes. Cover the ground around the base of the tree with up to a three inch layer of organic mulch but keep the mulch six inches from the trunk of the tree to reduce the likelihood of disease.
Arborvitae are prone to winter desiccation. If yours are planted in an exposed location, consider creating a screen that protects the trees from the prevailing winter winds and sunscald from the south and southwest. A burlap screen on stakes allows air circulation and is preferable to wrapping an entire tree, unless you live in an area of heavy snow and ice accumulation. A full encircling barrier can be used if you also need to deter animal pests.
Tall columnar forms of arborvitae are easily damaged by heavy snow and ice loads and wrapping the entire tree in burlap during the late fall is one solution in these circumstances. The objective is to prevent branch breakage, so rope, landscape wrappings or cloth can also be used to bind the upper branches together in a column. If deer damage and punishing winds are also considerations, then wrapping the tree in burlap may be the best solution. Remove the covering in spring.
These wonderful plants are so useful and so readily available that the importance of proper siting and planting is not always considered. If your arborvitae is showing some discoloration it is more likely to be a natural seasonal condition or a care and siting issue than a pest or disease. Plants that are stressed by their environment become weaker and more likely to succumb to disease. Here are some of the common issues a homeowner can encounter.
Bagworms: Thyridopteryx ephemeraeformis caterpillars live in conical bundles covered in arborvitae needles that hang from the tree like strange cones. They strip branches of foliage and can kill a tree if present in large numbers.
Deer Damage: arborvitae are very attractive to deer which create areas of foliage reduction from ground level to about 4 feet up. Consider deer fencing, deer deterrent water sprayers and winter wraps or barriers.
Royal Horticultural Society Encyclopedia of Conifers by 
A Natural History of Conifers by 
Designing with Conifers by 
Pruning of Trees, Shrubs and Conifers by 
