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Arborvitae (Thuja): Home

Thuja occidentalis 'Smaragd' in the Benenson Ornamental Conifer Collection at NYBG; photo by Ivo Vermeulen
Thuja occidentalis 'Smaragd' in the Benenson Ornamental Conifer Collection at NYBG; photo by Ivo Vermeulen
 

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.

Planting

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 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.

Scaled needles, as seen on this Thuja 'Green Giant', are typical of arborvitae

Fans of scaled needles, as seen on this Thuja 'Green Giant', are typical of mature arborvitae

Watering

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.

Feeding

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.

Mulching

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.

Staking

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.

The brilliant foliage of Thuja plicata 'Goldy'; photo by marlon Co
The brilliant foliage of Thuja plicata 'Goldy'; photo by Marlon Co

Pruning

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.

Selecting arborvitae

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.

  • Thuja occidentalis 'DeGroot's Spire' - an interesting, narrow cultivar with twisted foliage and wavy margins. Grows 6 or more inches a year, reaching 5 to 10 feet tall. Bronze in winter. Hardy in USDA zones 3 to 7.
  • Thuja occidentalis 'Hetz Wintergreen' - a robust, narrow cultivar with superior strength under snow loads. This tree grow 12 to 18 inches per year to a size of 12 to 15 feet, and half as wide, at maturity. Retains good color in winter. Hardy in USDA zones 4 to 8.
  • Thuja occidentalis 'Lutea' (George Peabody arborvitae) - a conical cultivar with foliage color changing from yellow when new, through gold tones to green. Grows more than 12 inches per year to a size of 12 feet and taller. Hardy in USDA zones 4 to 8.
  • Thuja occidentalis 'Mr. Bowling Ball'™ (syn. T. o. 'Linesville') - a slow-growing, nearly perfectly round, dwarf cultivar. Requires no pruning and grows to an average 3 feet tall at maturity. Hardy in USDA zones 5 to 7.
  • Thuja occidentalis 'Nigra'  (black arborvitae) -  a dense cultivar with dark green foliage and good winter color retention. Conical, growing 6 to 8 inches per year to reach 6 feet or taller. Hardy in USDA zones 3 to 7.
  • Thuja occidentalis 'Rheingold' -  begins as a golden ball with coppery winter color when immature and grows to an upright, six foot tree at maturity, adding 6 to 12 inches per year. It can be sheared to maintain immature characteristics. Hardy in USDA zones 3 to 7.
  • Thuja occidentalis 'Smaragd' ('Emerald Green') - a popular, narrow cultivar, that maintains its emerald green color very well in the winter. This tree grows up to 12 inches a year becoming a 6 to 10 foot plant at maturity. Hardy in USDA zones 4 to 8.
  • Thuja occidentalis 'Techny' - a compact cultivar with excellent, dark green, winter color retention, growing up to a foot a year to reach 10 feet at maturity. Hardy in USDA zones 3 to 8.
  • Thuja occidentalis 'Wareana' (Ware's arborvitae) - a dense, conical, slow-growing, dwarf cultivar with superior hardiness. Grows only 1 to 6 inches a year but will eventually reach up to 10 feet. Retains winter color quite well. Hardy to USDA zone 4, and if well mulched, zone 3.
Thuja occidentalis 'Durant' beginning to bronze in early winter; photo courtesy of Flickr cc/ F.D. Richards
Thuja occidentalis 'Durant' beginning to bronze in early winter; photo courtesy of Flickr cc/ F.D. Richards

Winter precautions

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. 

Why is my arborvitae turning brown?

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.

Browning in the months after planting

  • Transplant shock: all trees take some time to establish themselves and need attentive patience while they do, but arborvitae are considered particularly difficult. If your arborvitae branches are brown and wilting in the season it was planted, now is the time to review the planting and watering instructions above. Did you plant your arborvitae at a time of year that is too great a challenge to the plant?  Was the burlap left on or under the root ball when planting? Determine whether your arborvitae is in the right location, with correctly prepared, freely draining soil and receiving the water it needs. A declining, newly-planted tree or shrub is likely to be suffering from some condition of soil or environment rather than from a disease or pest.
  • Fertilizer burn of transitional plants: while fertilizer can be applied to established plants, newly planted ones may suffer from a sudden influx of fertilizer applied to the tree or a nearby lawn. This often manifests as needle burn at the tips. You can have this problem even when you have not fertilized, if your plants are near the border of your property and your neighbor has applied a high-nitrogen lawn treatment.

Browning in the fall

  • Annual needle replacement: even an evergreen plant replaces its foliage. When you see brown needles on the inner part of arborvitae in fall, it may not be a problem: it is normal to see brown leaves in older growth areas, as that foliage is just aging and will eventually be shed, sometimes dramatically. The brown needles may remain on the tree for months before coming off in branchlets

Browning in winter and early spring

  • Winter bronzing: the bronzing of foliage over the entire tree is a natural, though frequently unwelcome, occurrence in arborvitae and more prominent in some cultivars than others. The discoloration takes place during the coldest weather and is not accompanied by a significant change in leaf texture. The needles regain their natural color as spring arrives and the plant generates greater chlorophyll in its needles.
  • Winter kill is an understandable problem following a period of intense cold or heavy snow, but periods of warmth and sun in winter can also lead to plant deterioration. Warm weather or intense sun in winter can cause needles to release moisture that cannot be replaced from frozen root systems. Lack of snow cover can deprive conifer roots of an insulating layer that protects them. If severe enough, the resulting desiccation goes beyond bronzing and needles look yellow or brown and dried out.  An exposed side of the plant may be wholly affected but frequently there is no pattern to the injuries. As injured needles begin to fall, the branch may also die or new needles may grow at the tip creating a "poodle tail". The plant will not return to its original condition, even with care. The best you can do is prevent winter kill as much as possible.
  • Road salt damage: salt laid down on roads or paths in winter can be plowed, shoveled or simply drift into your arborvitae's root system. Heavy salt in soil is toxic to most plants. While arborvitae are moderately tolerant, repeated salt exposure will take its toll and will become noticeable in late winter to early spring.
  • Leafminers: small, tan moths (Argyresthia thuiella) arrive in summer and lay eggs. Damage caused by tiny feeding larvae becomes noticeable in late winter when brown leaf tips crumble and fall.

Browning in late spring, summer and early fall

  • Drought stress: during a hot, dry summer, plants often fail to receive sufficient water. Keep soil constantly moist. As a preventive measure, you can supply artificial irrigation and bark mulch.  Once vegetation has been damaged by drought stress, it will not become green again. In severe cases, root damage can occur, resulting in dying plants. How do you check to see if your plant is dead or still alive? Slice off a bit of bark with a knife, so that you can "look under the hood". If you see green, the plant is probably still alive (on a partially dead plant, it may take you a few tries before you find green somewhere). If all that you find is brown, the plant is most likely dead, and you should make plans to remove it.
  • Herbicide drift:  weed killers and other chemicals applied in the area may be accidentally broadcast to the roots of the tree (especially lawn weed and feeds) or arrive airborne to the foliage.
  • Spruce spider mites: Oligonychus ununguis are most active in May and June and then again as weather cools in September. They suck the coloration out of the needles, leaving areas of yellowed foliage. Look closely for speckled needles and tiny webs.
  • Fungal diseases: infections initiate in damp, spring weather, particularly on trees that are stressed by aggressive pruning, poor siting, drought or winter injury. Most of these are twig blight diseases that make tips of branchlets discolor and die. Look for tiny, black , pimple-like fungal fruiting structures on needles and branchlets, and cankers on branches in spring and summer to help identify the fungal disease present. To prevent spread, water only at ground level; prune back to beyond the affected plant material and dispose of it, as well as fallen needles and branches, well away from gardening areas.
  • Scale insects: several types of soft scale insects feed on arborvitae. Look for small, waxy white or glossy brown bumps on the needles. They  cause stunting and discoloration.

Browning anytime

  • Oversaturated soil: plants may be drowning from water soaked soil and poor drainage, due to improper siting or preparation. Roots cannot grow if they are constrained by dense soil which may also may create a "bathtub" around the root ball. In wet weather, water fills the air spaces in the soil and does not drain, choking off oxygen and damaging the roots. Needles and entire branches die-back. Mulching excessively creates a similar, overly wet, situation.
  • Dog damage: should be considered if you have pets in the area and needles are browning under a 2 foot high mark. Repeated urination can darken needles and will even affect the soil and overall plant health if the problem is persistent.

Other problems to look out for

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.

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