Minnesota winters can be snowy, frigid, warm, dry or any combination of the above.
Weather extremes and variability due to climate change add to the uncertainty, and sometimes Minnesota gardening can feel a little like watching a volleyball match.
Cold damage to plants can be caused by the following:
- Lack of plant hardiness
- Extreme winter conditions
- Lack of snow cover and insulation
- Dry soil and drought conditions
- Branch breakage from heavy snow and ice
- Environmental stresses such as de-icing salt.
Plants are evaluated by breeders and growers for many things, such as flower bloom, disease resistance, size, growing requirements and plant hardiness.
Plant hardiness is determined by a plant’s resiliency to varying temperatures.
In Minnesota, plant labels indicate the cold hardiness of a plant by its hardiness zones, established by data collected by the U.S. Department of Agriculture.
We have the following zones in our state:
- 3a – Hardy to -35 to -40º degrees
- 3b – Hardy to -30 to -35º degrees
- 4a – Hardy to -25 to -30º degrees
- 4b – Hardy to -20 to -25º degrees
- 5a – Hardy to -15 to -20º degrees.
In our area, here in Hubbard County, we traditionally were in Zone 3b.
Recently, the Plant Hardiness Zones have been updated. While most of Hubbard County is still in Zone 3a, there are areas in the southern and eastern parts of the county that are now considered Zone 4a.
“Winter dieback” is a term used when a plant or parts of a plant are killed by cold temperatures. Plant shoots, and especially flower buds, are most susceptible to winter dieback due to lack of hardiness, early emergence from dormancy or late frosts. It is possible for a tree or shrub to survive cold temperatures, but its flower buds do not.
Not much can be done to protect trees and shrubs from dieback during the winter and no plant is 100% resilient, especially when we experience extreme winter cold, ice storms and lack of snow cover. But gardeners can minimize the potential from winter dieback by selecting plants that are labeled for their cold hardiness zone.
Do not prune, fertilize or overwater plants in late summer or fall, as it prompts the plant to put on new growth late in the season. Late season new growth usually will not become successfully dormant and will be killed by cold temperatures.
If you choose to experiment with marginally hardy plants, locate them in sheltered locations.
Lack of snow cover and dry, droughty conditions negatively influence winter soil temperatures and cause root injury.
Snow cover and mulch are good insulators and help keep soil temperatures warmer. Snow is 90-95% air and the air pockets in snow are 32 degrees.
Soil moisture is important to root and plant health. Minnesota frost depths range from 42 to 60 inches. Sandy or dry soils are colder and have deeper frost levels than moist soil. Moist soil holds more heat than dry soil, preventing deeper frost penetration.
Why are roots easily damaged by cold?
Roots are less hardy and become dormant more slowly than a plant’s stems, branches and buds, and will die at temperatures below 10 degrees.
Roots survive winter because soil temperatures are normally much warmer than air temperatures and soil cools down more slowly than air.
Root damage occurs when cracks in the soil allow cold air to permeate and harm roots, slowing their growth, and when we experience “frost heaving.” Frost heaving is the repeated freezing and thawing of the soil in fall or spring. This causes soil to expand and contract, pushing shrubs up from the ground and exposing roots to cold.
Heavy snow and ice can bend and break branches. Trees with more than one central leader, narrow crotches and included bark are most susceptible. Upright evergreens and clump trees also hold snow and can be damaged.
Proper pruning to eliminate multiple leaders and weak branch attachments starting when the trees are small is important to reducing breakage.
Hiring a professional arborist is always a good investment. They can offer pruning as well as advice on how to protect mature trees with form issues.
De-icing salts (chloride)
Cold weather means ice and snow on roads, driveways and sidewalks. De-icing salts are important when ice occurs, increasing the likelihood of people falling and injuring themselves.
But they can have negative effects on the environment – water quality, and salt spray and run-off, especially, can cause and exacerbate winter injury to plants. De-icing salts can build up in soil resulting in root injury, and they can be absorbed by plants, killing stems and foliage.
Fortunately, Minnesota companies and municipalities are using more alternatives to salt for managing ice and snow.
How to prevent salt damage
- Clear snow from hardscape as soon as possible to reduce slippery areas.
- Use alternative de-icing salts (calcium chloride and calcium magnesium acetate). Follow application instructions. Apply the minimum amount needed and only where needed. Avoid spreading salt on grass and in ditches where water collects.
- Keep plants healthy throughout the year through best practices in watering, pruning, fertilizing and pest management.
- Avoid planting woody plants where salty runoff collects and close to streets where vehicles can throw salt spray onto plants. Create burlap barriers to protect plants from salt spray.
- No species is completely tolerant, but choose more salt-tolerant plant species near walks and roads where salt may be applied. Ohio buckeye, Austrian pine, Gingko, honey locust, white polar, black cherry, white oak, etc.
Although the beginning of this winter has been unusual so far, being prepared for whatever Mother Nature throws our way will help our lawns and gardens in the long-term.
Tarah Young is Hubbard County University of Minnesota Extension educator in agriculture, food and natural resources. If you have any questions about this topic or any others, contact her at 732-3391. If information about agriculture, gardening and natural resources interests you, consider signing up for the Hubbard County UMN Extension Agriculture, Gardening and Natural Resources E-newsletter at z.umn.edu/HCExtensionNewsletter.
