These ecologists describe root activity as periodic, with maximum growth in early summer – especially in deciduous species – and pulses of additional growth occurring occasionally in early fall. And complicating things further, they indicate that not all roots grow at the same time. Even within a single tree, some roots may be active while others are not. However, by all accounts, tree roots in our region are thought to spend the winter in a condition of dormancy. This means they are not dead but rather they overwinter in a resting phase with essential life processes continuing at a minimal rate. Full-on root growth resumes in spring, shortly after soils become free of frost, usually sometime before bud break.
But unlike the aboveground parts of most trees that pass the winter in a prolonged dormancy – marked by unbroken inactivity until spring – tree roots seem to maintain a readiness to grow independent of the aboveground parts of the tree. That is, roots remain mostly inactive but can and do function and grow during winter months whenever soil temperatures are favorable, even if the air aboveground is brutally cold. While roots tend to freeze and die at soil temperatures below 20°F, minimum temperatures for root growth are thought to be between 32 and 41°F. So, if soil temperatures warm to or stay above this minimum, winter roots can break dormancy and become active.
Control and Prevention
The most effective way to reduce the possibility of root injury and disease is to keep the tree healthy and vigorous. A healthy root environment consists of adequate growing space for the root system, well-conditioned soil 16 inches to 24 inches deep, and sufficient water and oxygen. To check the water and soil condition of the root environment, dig a hole outside the dripline of the tree and determine if the soil is dry, wet or compacted. If you can’t get the shovel in the ground, the soil is dry. Soil moisture is adequate if the soil can be madeinto a ball with little pressure. Long, deep watering over the entire root system with time for the soil to dry between watering is better for trees than frequent light watering. Watering once a month during a long, dry winter also is helpful.
Avoid any practice that injures the roots. This includes: soil compaction, soil depth changes, mechanical injury, and improper watering and fertilization techniques. However, if these practices cannot be avoided, try to minimize damage.
To minimize soil compaction, remove compacted soil and replace it with noncompacted soil. Provide adequate drainage before planting. Use 2 to 3 inches of organic mulch (peat moss, wood chips, tree bark) around the base of a tree to improve soil aeration and water availability. Adding new mulch every three years or so will be needed as the mulch decays and improves the soil structure.
Avoid fertilization damage by applying nitrogen fertilizer to established trees immediately after spring leaf expansion, not in the late summer and fall.