Ask Mr. Smarty Plants

Yep, that's what oak roots do. Your tree is not in trouble, but your lawn probably is, and we can't guarantee that planting anything else under there will be successful, either.

Given the choice between having great trees and having ground cover under them, we would vote for the trees every time. And it's necessary to remember that most tree roots occur in the upper 12" of soil, and may extend three times the dripline of the tree. Oak roots also form a mat beneath the soil, which discourages any other plant. Planting within the dripline can damage the roots of the tree, and the understory plant probably doesn't have a chance of competing anyway. The Lady Bird Johnson Wildflower Center recommends only plants native to North America and to the area in which they are being grown, as these are adapted by millions of years of experience to the soil, annual rainfall and temperatures of that area. We still can't guarantee that any plants you might choose would even survive beneath your trees, and you may decide to simply cover the bare ground with a good quality shredded hardwood mulch. This will have to be replenished from time to time, but as it decomposes, it will add nutrients to the soil, and improve the soil texture, as well as protecting the tree roots from heat and cold.

There is also the problem of the shade the tree casts. We consider full sun to be 6 or more hours of sun a day, part shade 2 to 6 hours of sun, and shade less than 2 hours of sun a day. Finally, there is the question of allelopathy. Various studies have demonstrated that oaks can have allelopathic affects on surrounding plants. Allelopathy is the production of plant inhibiting chemicals by one plant to regulate the growth of others in its vicinity. One important group of chemicals produced by oaks is tannins. They are produced in leaves and litter and also directly by root systems in soil. Tannins are inhibitory to many organisms. Salicylic acid and other organic acids are also produced by oaks and are toxic to other plants. Allelopathy is species specific for the oak in question and the species that is inhibited.

So, bottom line, no, don't try to mow over those roots. In fact, keep machinery away from your tree, remembering that damage to the bark can cause sap to leak out, which is an invitation to the nitulidid beetle, a carrier of the oak wilt fungus.