Cottonwood Trees Invading our Lawn
Hello. I live in the city of Gresham. Our house borders a city owned green space that contains several cottonwood trees. Over the years, the roots have gone under our fence and surfaced in our yard. The city paid for an arborist to assess the situation. They recommended severing the roots, installing a 4 ft. root barrier, or just adding more topsoil to our lawn and re landscape. Which method would you recommend?
Multnomah County Oregon
Containing cottonwood tree roots is a common problem, as many a homeowner faced with sidewalk replacement can attest.
Your question is a little unclear as to what is actually proposed. I presume the first option is to sever the roots (presumably on City property), and install a root barrier. The exposed roots in your lawn will die, and eventually decompose; if you want to remove them and fill the divots, note that adding more than 1/2 inch of soil / sand / mix will require reseeding those areas. See below.
Just adding soil, apparently to bring the existing lawn level up higher than existing exposed roots has two limitations. First, unless the roots are severed and a barrier installed, the roots are going to continue to grow both in length and in diameter, soon popping up through the new lawn level. Second, not more than 1/2 inch of soil, sand, or mix should be added during any season. More than that will kill much or all of the grass. So to do major leveling you are looking at the serious project of putting in a new lawn, and all that entails. Of course, if that work is done well, the result is a stunning new lawn. (And if you go that route, DO take the opportunity to install an in-ground sprinkler system!)
In other words, your situation may require a two-part solution. It definitely includes a physical solution to containing the cottonwood roots. It MAY also include work to restore your lawn to acceptable aesthetic standards.