The winter of 2024/2025 did not see very much snow cover in Southern Wisconsin. Believe it or not, the amount of snow accumulation and the presence or absence of snow is significant to winter tree health. In the upper midwest it is common knowledge that trees go dormant during the winter season, but the fact that trees are still metabolizing during the winter (though at a much slower rate) is not widely known. This is important because it highlights that roots need protection year-round from environmental factors that threaten root functionality in trees.
So why is snow cover significant to winter tree health? Well, think of it this way: snow cover is a type of ground covering that is present in our region for several months out of the year. Ground coverings affect how water and oxygen are delivered to roots and provide insulation during temperature extremes. Generally speaking, most tree roots are found in the top 18”-24” of soil, with a few exceptions for species with specific root adaptations. For this reason, soil that is not covered by mulch or a heavy leaf layer (bare or nearly bare soil) are more vulnerable to cold, dry air in the atmosphere above the soil. If temperatures dip below freezing, roots left uncovered can begin to die off.
When snow is present it creates a layer that is made up of frozen ice crystals and the pore spaces between the crystal particles. This structure is similar to other ground coverings like wood mulch where there are also particles and pore spaces between them where oxygen is found. The pore spaces between snow allows it to hold a layer of warmer air just above the soil surface, which acts to insulate roots below. This insulation effect endures even if temperatures are extremely cold above the snow layer and the thicker the snow layer, the greater the insulation effect. The added bonus of snow covering is that snow melt delivers water to roots resuming the growing season, when water is needed to push new buds.
There are other issues roots face without winter snow. Soil near the surface can thaw and refreeze rapidly, causing water to melt within the soil. This results in water that sits around the roots, unable to infiltrate due to soil freeze below and ultimately leads to root rot. Root rot causes root dieoff and is a significant stressor for trees.
What can you do to help? While we can’t control the weather, we can help by covering roots with wood mulch year-round. Up to 3” is best, especially if done within the tree’s dripline or beyond. Any mulch around trees is beneficial. Our arborists are happy to help answer any questions you have about mulching your trees.