U of M Extension Agroforestry Educator
---- — Drought conditions over the last two-plus years have left trees and other perennial plants visibly stressed this fall. Tree stress symptoms include abundant seed production, leaf scorch, early fall colors, leaf drop, limb die-back and yellowing or browning of leaves and needles.
Fortunately, several measures can help enhance tree and shrub health.
Trees and shrubs — especially conifers (such as pine, spruce and cedar) and those planted in the last three years — should be watered generously until the soil freezes. Mulching newly planted trees also helps reduce winter root damage.
Young maples and thin-barked trees may benefit from sunscald protection to prevent the bark from cracking this winter and spring. This usually involves plastic tubes or tree wraps, which are removed in spring. These practices can also help reduce winter animal damage.
Deciduous trees and shrubs can incur shoot die-back and bud death during the winter. Flower buds are more susceptible to injury than vegetative buds. A good example of this is forsythia, where plant stems and leaf buds are hardy, but flower buds are very susceptible to cold-temperature injury.
Die-back: Little can be done to protect trees and shrubs from winter die-back. Plants that are marginally hardy should be planted in sheltered locations (microclimates). Plants in a vigorous growing condition late in the fall are most likely to suffer winter die-back, so avoid late-summer pruning, fertilizing, and overwatering. Fertilize in the spring on sandy soil or in the fall on heavy soil after the leaves have dropped.
Root Injury: Roots do not become dormant in the winter as quickly as stems, branches and buds, and roots are less hardy than stems. Roots of most trees and shrubs that grow in Minnesota are killed at temperatures at or below 0-10 degrees. These plants survive in Minnesota because soil temperatures normally are much higher than air temperatures and because soil cools down much more slowly than air temperature. Snow cover and mulch act as insulators and keep soil temperatures higher. With newly planted trees, cracks in the planting hole backfill will allow cold air to penetrate into the root zone, reducing fall root growth or killing newly formed roots.
To encourage fall root growth and to reduce root injury, mulch new trees and shrubs with up to eight inches of wood chips or straw. If the fall has been dry, water heavily before the ground freezes to reduce frost penetration. Check new plantings for cracks in the soil and fill them with soil.
Wildlife Damage: Protecting trees from rabbits, mice, voles and deer is another major winter concern. Mow or remove tall grass to reduce mice and vole damage. If the bark is removed or severely damaged around the tree, it will die. Protective physical barriers such as tree tubes, hardware cloth or fencing can be done when practical.
Odor, taste and visual repellents can repel many wildlife species, but may have inconsistent effectiveness. Human hair, soaps, garlic oil, hot sauce and animal repellents can be applied to branches and foliage to discourage browsing. Weather, application frequency, animal population and feeding pressure affect the success of repellents. Alternate the repellents since some animals become desensitized to them.
Fall is also a good time to plant trees; water them until the soil freezes. The best time to prune trees is during the dormant season from January to March. Flowering shrubs can be pruned in the summer after flowering.
On the web ■ Recommended trees by Minnesota region: http://z.umn.edu/rectrees. ■ Diagnosing tree problems: http://z.umn.edu/diagnose ■ Preventing wildlife damag: http://z.umn.edu/critters ■ Tips for reducing winter damage: http://z.umn.edu/winterdamage