Root-pruning may at times be necessary, but it is always risky. Roots that
may be interfering with foundations, walkways, or other important features can
sometimes be removed without harming the tree. If a tree is already in good
health, and the root pruning is done cautiously and correctly, the tree may
remain stable enough to survive the pruning.
When to Prune Roots
If it is necessary to trim the roots of a tree, it is imperative not to prune
them in the spring after the buds have opened (budbreak). To do this would
deprive your tree of nutrients and water during its greatest need for both. In
order to prevent dehydration and the eventual failure of the tree, try to only
prune your tree’s roots either in late winter or the early spring.
Cutting the Roots
The roots that grow horizontally from the tree are structural roots and
supply the balance and anchoring that a tree requires to withstand winds. To
compromise the structural root system would be to compromise the safety of the
tree, especially against strong winds. In order to maintain the integrity of
the structure, assess the diameter of the trunk, multiply the diameter by 3 or
5, and cut at least that far from the tree. For instance, a tree with a 2.5
foot diameter trunk would require that you cut at least 7.5 to 12.5 feet away
from the trunk of the tree. Avoid cutting too many of the smaller feeder roots
as these provide the food source for the tree. Never remove more than 25% of
the root system of a tree.
Once the roots have been pruned, it is important to cover the root area in
order to maintain moisture. The soil should be kept moist, though not soaking.
Keep an eye on the tree’s health and look for signs of wilting, fungi, or bug
infestations as the tree will be more susceptible to these hazards after
pruning. Signs of malnutrition or infestation should be taken seriously and may
require that the tree be removed safely by experts. Pruning should not be done
more often than 2 years apart, lest the tree not have time to recover.