Minnesota's harsh climate can cause severe damage to landscape plants.
Winter sun, wind and cold temperatures can
- bleach and dry out evergreen foliage
- damage bark
- injure or kill branches, flower buds, and roots.
- Snow and ice can break branches and topple entire trees.
- Salt used for deicing streets, sidewalks and parking lots is harmful to landscape plants.
- Winter food shortages force rodents and deer to feed on bark, twigs, flower buds and leaves, injuring and sometimes killing trees and shrubs.
Here are steps you can take to protect trees and shrubs and minimize injury.
Causes of cold damage on plants include:
- Lack of plant hardiness and its inability to survive extreme cold
- A majority of Minnesota is located in USDA cold hardiness zones 3 (-40 degrees F) and 4 (-30 degrees F)
- Extreme winter conditions such as ice storms, wind and prolonged sub-zero temperatures
- The weight of snow and ice can break branches
- Wind can dry out plants, especially evergreens
- Lack of snow
- Snow cover insulates plants from wind and sub-zero temperatures
- Environmental stresses
- Dry conditions going into the winter can make plant tissues more susceptible to cold damage, especially on evergreens
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 die at temperatures at or below 0 and up to 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.
Many factors influence soil temperature.
- Moist soil holds more heat than dry soil, so frost penetration will be deeper and soil temperatures colder for sandy or dry soils.
- Snow cover and mulch act as insulators and keep soil temperatures higher.
- With newly planted trees, cracks in the planting hole allow cold air to penetrate into the root zone, reducing fall root growth or killing newly formed roots.
Reducing root injury
- Cover roots of newly planted trees and shrubs with 3 to 4 inches of shredded wood mulch.
- Create a "donut" of mulch by pulling the mulch away from the trunk about 6 inches.
- This will prevent adventitious roots from forming and ultimately girdling the tree.
- 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.
- Frost heaving
- Repeated freezing and thawing of soil in fall or spring causes soil to expand and contract, which can damage roots and heave shrubs and new plantings out of the ground. A 4- to 6-inch layer of mulch will prevent heaving by maintaining more constant soil temperatures.