Issue: April 6, 1998

Cutting problem tree roots

I have a large fruitless mulberry tree in my yard. The roots are right on the surface, sticking up well above the soil and causing problems when I walk around the yard. The shade from the tree is important to me so I don't want to hurt the tree, but I don't want the tree to hurt me. If I remove those roots which are on the surface, will the deeper roots be enough to take care of the tree, or should I just put new soil over the offending roots?

If you cut the roots, you may kill or at least severely injure the tree. It depends on how close to the trunk you cut the roots. If you put soil over the roots, you may see the same results.

Trees such as the mulberry are genetically programmed to have shallow roots, and cultural conditions often exaggerate the problem. So, there are no, or at least few deeper roots to support the tree if you cut the roots; however, that does depend on how close to the trunk you cut the roots. As you follow a root from the trunk, you will find that the root produces "sinker" roots which go downward and provide anchoring for the tree. They are not primarily intended for water and nutrient absorption as are the shallow, "lateral", roots. Also, as you go away from the trunk, the lateral roots become smaller and more capable of regenerating new roots if they are cut. The mulberry is a tree which seems to have a great capacity for surviving such root injury if you cut roots at a distance from the trunk equal to the dripline. So, without knowing where you want to cut the roots, I would advise against it unless you cut the roots beyond the dripline.

Another factor to consider when cutting roots is that these lateral roots are important in the stability of the tree and its ability to resist blowing over in the wind. Cutting roots can increase the likelihood of the tree blowing over and damaging your house, vehicle, or people.

Putting soil over the roots can cause problems in most trees as well. Tree roots need oxygen. By putting a layer of soil over the roots, you will change the oxygen concentration in the soil air spaces and can cause death of the roots. Some trees are more tolerant of this, while others are very intolerant. Even those which can tolerate some added soil will have a limit as to how much depth of soil and oxygen concentration change they will tolerate. The age and vigor of the tree will also determine how well it can tolerate added soil.

So, either cutting the roots or burying the offending roots can cause problems. Be careful.

Wood ashes in the garden

Now that winter is over, I have a lot of wood ashes from the fireplace that I want to spread on the garden. My wife tells me that this is not good, but I've read many garden books which say that wood ashes are good for the garden. Please settle this argument that I am having with my wife.

Sorry, I must take your wife's side in this debate. Here in the Southwest our soils are very alkaline and contain excess salts. Wood ashes add to this problem. In parts of the country, for which many garden books are written, the soils are acid and deficient in some nutrients which are present in wood ashes. So, in the East and other areas with acid soils, ashes are beneficial. In much of the West, especially here in the Southwest, wood ashes can cause major problems when added to the soil (or to the compost pile).

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!