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Trenching and digging in the soil near trees can cut roots, and this can damage the tree resulting in tree decline or the tree falling over. This can cause liability and safety concerns. Root pruning is more injurious to old mature trees than it is for younger more vigorous trees. Cutting roots greater than about one inch diameter during trenching and digging can mean problems for the tree. In some cases roots of one to three inches diameter represent the major structural roots holding the tree upright.
The impact from pruning roots depends on several factors (see table below). Damage typically increases with more cuts, bigger cuts, and cuts made closer to the trunk. Root pruning, trenching, and other construction activities close to the trunk result in more injury on shallow, compacted soils or on soils that drain poorly than on well drained soils. This is due to the shallow roots common on sites with shallow soils or high water table. Trees that are leaning are poor candidates for root pruning. Prune roots only with sharp tools to avoid tearing behind the cuts.
Well, the answer appears to depend on who you ask. For mature trees, some experts recommend not cutting roots closer than 6 to 8 inches from the trunk for each inch in trunk diameter. That means stay at least 10 feet away from a 20 inch tree! Others are more realistic and state that we should root prune no closer to the trunk than a distance equal to 3 times the trunk diameter, preferably 5 times the trunk diameter. Dr. Tom Smiley at the Bartlett Tree Research Laboratory in Charlotte showed that roots on one side of very young trees can be pruned off completely at a distance equal to 5 times the trunk diameter without any impact on tree stability. Which ever rule-of-thumb you decide to use, do so knowing that pruning roots on trees can result in trees falling over or dying. While root pruned large trees on well drained soil may not fall over because of deeper sinker roots under the trunk, they can and have. There are fewer deep roots holding the tree up on poorly drained and compacted soils.