It may sound strange, but as we approach the winter months, we need to remember that trees still need soil moisture. This is particularly true of evergreens such as spruce, pine, juniper and arborvitae.

Why? Tree roots will continue to be active until soil temperatures approach 40 degrees F — known as biological zero. And like all plant parts, roots need water. Rainfall may provide the moisture, but if your soils are still dry from the fall, watering may be needed.

The catch? Be sure not to overwater! Evergreens do not like wet feet, so be sure the soil drains between watering. Flooding the soil with excess water will damage roots and expose them to ice when the ground does freeze.

Evergreens will continue to transpire, or lose water to the atmosphere, on cool, breezy, sunny days, especially in February and March when we have those typical spring thaws. If the water in the soil is frozen, plants cannot take up moisture and will begin to desiccate, or dry out. This is called physiological drought or winter burn. It’s not unlike what we see during summer droughts.

For plants situated along driveways or roadways where deicing salts are used, the salt spray and runoff just adds insult to injury. Salt dries out foliage and roots, which impacts soil structure. We all know what it is like to eat salty foods and then crave water. It is the same with plants.

One of the best things you can do for plants is to apply mulch. Ideally, it is best to mulch out to the dripline of the tree, but minimally apply mulch at least 3 to 4 feet out from the trunk. Mulch provides a number of benefits, including preserving soil moisture, providing organic matter, improving soil structure, keeping weeds down, preventing lawn mower or weed eater “blight,” allowing for good water infiltration, and helping moderate soil temperatures. This is especially important in winter.

Bare soil will freeze much deeper compared to soil covered with mulch, sod, a cover crop or crop residue. The deeper the soil frost, the longer it will take to warm up in spring. Protect the roots by applying 3 to 4 inches of mulch evenly under the tree canopy.

A word of caution: do not pile mulch up on the trunk, known as “volcano mulching.” This is not beneficial and will cause tree problems later.

Can’t water all your trees? Here’s a “triage list” that may help in deciding which trees need water more than others:
• recently transplanted trees and shrubs, because they have reduced root systems
• trees that have received root damage or any other type of damage
• species susceptible to borers and bark beetles
• sensitive species (i.e., sugar and Norway maples)
• flood plain species (i.e., pin oak, river birch, red maple)
• trees growing in planters or restricted rooting spaces
• trees growing outside their normal range
• highly valued or favored trees

You cannot do much about the winter, how cold it gets, or how much snow you receive, but you can do your best to make sure plants are as prepared as possible. Watch the weather, pay attention to precipitation and be prepared to water as needed.

Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at [email protected].