The genus Thuja is part of the family of Cupressaceae (cypress); its cousins include sequoia, junipers and Japanese cedars. In North America, several native Thujas have provided the genetic basis for dozens of landscaping arborvitae, white cedar and red cedars, including ground-hugging, tree-height and shrubby plants. Many varieties are hardy to USDA Zone 3 and require little special care, making "arbs" a common landscape evergreen in North America.
Flat, scale-like leaves characterize Thujas, no matter what size. They grow in horizontal sprays, adding leaves each season to make long, finger-like shoots. Plants grow from a single trunk or clusters of stems. Leaves exude resin from a gland on their backs. Thuja wood is very hard and aromatic.
Thuja occidentalis and plicata species are native to North America. T. occidentalis, also called eastern or American arborvitae or white cedar, is most often used for landscaping. T. plicata, also called giant arborvitae or Western red cedar, is a large tree used for lumber and roofing shakes. Choose the right shape and size plant and place it in the landscape based on its mature size; some arborvitae are fast growers.
According to North Dakota State University (NDSU), arborvitaes prefer sun to light shade in a moist, well-drained soil; too much shade or too little water causes leaves to turn brown. They will also die back with too much water or if their roots sit in soggy soil. Never plant Thujas under larger trees; they will not be able to compete successfully. Water them deeply once a week if they do not receive an inch of rain.
Fertilize and Mulch
Native arborvitae varieties are undemanding in North American soils. Since soil types vary across the continent, have a soil analysis completed at your state agricultural extension to find out if your plants need fertilizer. Most evergreens will thrive on a nutritious mulch of compost, humus and pine needles. The arborvitae loves sunshine, but its shallow roots need shade and protection from rodents. Up to 3 to 4 inches of compost soil conditioner-mulch provides good cover, some nitrogen and keeps soil on the acid side.
Prune and Protect
Fort Valley State University Extension recommends trimming young arborvitae for shape their first year in late winter before leaves start to bud. Hand trim, never shear the soft leaves and avoid pruning back to areas where leaves have dropped--they may not grow back. Cut back overgrown Arborvitae in early spring or re-plant shorter varieties. Wrap columnar and tall, multi-stemmed plants with burlap in winter to prevent desiccation of tender leaves on the north and west sides of the plant and to shield them from foraging deer who favor the soft-needled evergreens. Tie multi-stemmed branches together with twine to prevent spreading by heavy snow.
Bagworms and sawfly larvae are the only pests that seek out arborvitae, but they seldom pose a major problem to healthy plants. Prune out affected branches and destroy the wood or use a biological spray containing Bacillus thuringiensis (Bt). Chemical controls are systemic; acephate, malathion, nicotine sulfate, pyrethrum and permethrin pesticides should be marked for bagworm or sawfly control. Follow package directions closely when using any chemicals.