Q: Two summers ago I planted a young arborvitae (about 2 feet tall) in front of my house (east side). Last summer, it was looking good. This summer, 85% of the foliage is a rust color. Can you give me any idea what might be happening? — Ken in Nashwauk
A: Arborvitae, one of the most widely used evergreens in Minnesota, is a great choice for cornerstone or privacy plantings. The damage you’re describing could result from deicing salt damage or twig blight, but winter damage is the most likely cause. This may be true even for well-established trees.
Winter sun, wind and cold temperatures can bleach and dry out evergreen foliage, damage bark and injure or kill branches and roots. When the roots are in frozen soil they are unable to replace lost water, resulting in drying and browning of the needles. Sunny days during the winter warm plant tissue, which becomes active. Then when the sun disappears, temperatures can drop, injuring or killing the foliage. When temperatures fall below 28 degrees, bright sun can destroy the green chlorophyll. It is not able to repair itself at these temperatures and may lose its color (bleaching).
Dry conditions going into the winter can also make plant tissues more susceptible to cold damage, especially on evergreens. Watering heavily only in late fall is not sufficient to avoid injury. Trees need to be well hydrated through summer and fall, stopping just before freeze-up. Snow insulates plants from wind and sub-zero temperatures so low-snow years may also see more damage.
For better protection, consider plant placement on the north and northeast sides of buildings or try screening your plants with evergreen boughs or barriers such as burlap. You may surround the tree with a barrier but leave the top open for light and air circulation. This UM link offers more detail on protecting your plants in winter: extension.umn.edu/planting-and-growing-guides/protecting-trees-and-shrubs-winter.
Note that anti-desiccant and anti-transpirant sprays are not considered effective for protecting evergreen foliage. In the ‘Diagnose a Problem’ section in the link above, you may also look more closely at disease and insects of arborvitae under “What’s wrong with my plant?”
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to firstname.lastname@example.org.