COLUMBIA, Mo. – University of Missouri Extension horticulture specialists are getting reports of browning arborvitae across the state.
If your arborvitae is turning brown, it might be a sign of drought stress that began last fall, says Debi Kelly, University of Missouri Extension field horticulturist.
Arborvitae is a popular landscape plant for borders and screens because it grows quickly—up to 2 feet per year—provides privacy and is generally low-maintenance and long-lived. These evergreens grow in dense, column-shaped cones that can reach up to 60 feet in height.
Because arborvitae depend heavily on water, consistent deep watering is essential, especially when they are newly planted and during extended dry periods. Applying a layer of mulch can help retain soil moisture.
Water thoroughly through September, and continue watering until the ground freezes in your area. During winter, water during thaw periods when possible. Loose soil and mulch help water penetrate more effectively to the root zone.
Kelly also notes that other issues may be at play. If arborvitae needles brown and drop prematurely during hot summers—even with adequate watering—resulting in thin areas, spider mites may be the cause, especially when damage appears on the south or west side of the tree.
You also might notice that the older and inner needles of the tree begin to yellow and fall to the ground. This is part of the natural life cycle of arborvitae, says Kelly, and is nothing to get alarmed about.
For best growth, plant arborvitae in full sun and moist, well-drained soil. Plants need at least six hours of sunlight each day to maintain their vibrant green color. Arborvitae also benefit from protection from strong winds, which can cause windburn. If browning appears on only one side of the plant, wind exposure may be the cause.
Avoid fertilizing arborvitae during late summer through winter, as excess nitrogen can increase stress on the plant, Kelly says. Instead, fertilize at appropriate intervals during the growing season. Apply fertilizer in early spring, using a high-nitrogen formulation such as 20-15-15 or 10-5-5.
If browning is visible, prune damaged branches. Late spring, when new growth appears, is the ideal time to prune. “Pruning arborvitae helps maintain the health, size, form and vigor of trees and shrubs in the landscape,” says MU Extension horticulturist David Trinklein. “Pruning can slow the spread of decay or disease by removing infected tissue and allowing the plant to seal, or compartmentalize, damage.” For more information, see Chapter 8 of the “Missouri Master Gardener Core Manual” at https://extension.missouri.edu/publications/mg8.
About arborvitae
According to the Arbor Foundation, early French explorers and Native Americans used arborvitae as a natural medicine. The name arborvitae is a Latin translation of the French phrase meaning “tree of life.” The plant contains vitamin C and was used as a treatment for scurvy. Arborvitae wood was used in the construction of early canoes.
For more information about arborvitae, contact your local MU Extension horticulture specialist.
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