How a 'nice' winter can be deadly for outdoor plants
A "nice" winter means different things to different people. Absence of record-cold temperatures is nice. Reduced snowfall is nice for those who hate shoveling. Plentiful snow is nice if you're a skier, a child, or a perennial plant.
What is the ideal winter if you're a flower, tree or shrub? Ironically, a mild winter for humans can be deadly for plants.
Why can a mild winter be dangerous?
- Mild winter weather can damage plants, especially perennial flowers, when temperatures rise above 32 degrees during the day and then fall quickly at night. The alternate freezing and thawing of exposed dark soil can easily injure roots.
- Fluctuating temperatures are most damaging to perennials in exposed soil without a layer of protective mulch or adequate snow cover.
- Winters without plentiful snow can leave ground bare, or poorly insulated. If frigid weather follows, extreme cold can penetrate uninsulated ground deeply. The root systems of plants, even those well-adapted, are less tolerant of cold temperatures than the above-ground branches. Winter cold-injury occurs more frequently in non-snowy winters.
- Sunny, mild days can damage trees, especially fruit trees and thin-barked shade trees like young maples and lindens. The bark on a tree's south and west exposures can thaw during the day and then freeze dramatically at night, causing a rupture of cells in a damage called winter sunscald.
What type of winter is best for plant survival?
- Most trees, shrubs and perennials adapted to our region survive average winter temperatures fine. Extreme, below-average cold can easily damage plants in bare ground that lacks snow cover, and trees, shrubs and perennials that are borderline in hardiness. Even adapted plant material has a cold threshold below which they can be injured.
- Average or above average snow cover is important, because snow is a great insulator. Plants can survive cold much better if well-insulated with snow. Snow prevents soil from dangerous freezing and thawing during mild weather fluctuations by keeping the soil consistently frozen, and prevents extreme cold from penetrating as deeply into the ground.
- Consistent temperatures are preferred without either January thaws or record lows.
Can anything be done in December to increase plant survival?
- If perennials weren't mulched in fall, they benefit from a 12- to 24-inch layer applied now, especially where snow is lacking. Locating a source of mulch in winter can be difficult, but straw bales or shredded bark can sometimes be found.
- Install tree wraps around the trunks of fruit trees and thin-barked types.
- Mother Nature has built-in safety measures for plants if winter weather becomes too mild. Most trees, shrubs and perennials adapted to northern regions have an internal clock that measures how many hours of cold weather have occurred. A certain quantity of "chill hours" must be met before spring growth can begin. That's what prevents northern adapted plants from budding out during winter warm spells, and keeps them dormant until the proper time.
Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler's Greenhouse in Fargo. Readers can reach him at firstname.lastname@example.org.
He also blogs at growingtogether.areavoices.com.