Large trees often have roots protruding out of the ground around the base of the trunk (such as the ones that can push up sidewalks next to the tree). When is it safe to cut those roots (that might be a hazard to people running in the area near the tree), and when should you be worried about that causing damage to the tree?
- Measure the trunk diameter,
- multiply by 18
- don't touch anything within that distance from the tree.
E.g. a trunk of diameter 20cm(8") would expect a 3.6 meter(12 foot) radius of healthy undisturbed roots at bare minimum.
There is some guidance here
It is difficult to say how much of a tree's root system can be removed. I prefer to talk about how close to the tree you can cut roots. I teach a concept called "Critical Root Radius (CRR)" which recommends not cutting roots inside the CRR. This Critical Root Radius (radius, not diameter, in feet) is calculated by measuring the diameter (not radius or circumference) of the trunk (in inches, not feet) at four and one-half feet from the ground and multiplying by 1.5. A tree with a five-inch diameter trunk (at the four and one-half foot level) will have a CRR of 7.5 feet. The CRR provides a general guideline.
Balance between the tree’s crown (top) and root system is important for maintaining healthy trees. When roots are lost for any reason, the imbalance creates stress. A tree usually has 4 to 7 major roots. Cutting just one of them within a few feet of the trunk can remove up to 25 percent of the root system.
Removing large or major roots from a tree is going to have a detrimental effect on the health of the tree i.e.
- Weaken it from a structural point of view.
- Put it into shock, stress.
- The wound(s) caused by the removal of such roots can allow disease to enter.
Any of those could easily lead to the tree dying.
Removing "sucker" roots, if done properly, should have no long term negative effects on the health of a tree, assuming they weren't sent up as a sign of stress (see below), even then they can be removed as the tree has much greater problems that need addressing (if possible, feasible to-do-so)...
Some trees by their very nature are prone to send up "sucker" roots eg
Other times "sucker" roots can be a sign a tree is in stress, and those roots have been sent up (above ground) as a survival mechanism eg
Underground roots have been damaged
Overlay compacted soil
Drought (lack of moisture in the soil)
To remove "sucker" roots, use an appropriate sized pair of hand pruners or a pruning saw, make sure the pruning tool is shape and the cutting area is clean (use something like rubbing alcohol or a mixture of one part household bleach to 10 parts water). It's good practice to clean your pruning tool after working on each tree (plant, shrub, etc), as doing so helps prevent the spread of diseases if present. If you're really concerned about diseases being spread, you can of course clean your pruning tool after each cut.
Remove the "sucker" roots via a clean cut, either at:
The soil line (or as close as possible to the soil line)
Just below the soil line, accomplished after careful removal of the surrounding soil to allow such a cut to be made. The soil can be returned to the area once the cut has been made.
Do keep in mind, just because you remove "sucker" roots once does not mean the tree will not send up more, in fact you might find the tree sends up even more...