What to put — and not to put — under your trees

09/27/2013 2:53 PM

09/28/2013 9:22 AM

A very common question that Jason Griffin fields as director of the K-State research station in Haysville is what to do when tree roots break the surface of the soil, making it difficult to mow around the tree.

The answer to that question is a good guide to which plants and materials to use and to avoid under trees – surface roots or no surface roots.

“As a tree guy I’m always biased on the side of the tree,” Griffin, director of the John C. Pair Horticultural Center, said. “I never like to see those pretty little rings under trees – those little landscape timbers and pavers and little flower gardens. I think about somebody every year digging around their trees ... That’s not the best thing for trees.”

Instead, mulch and ground covers planted between the roots are the answer to keeping mowers and people’s footsteps from disturbing the roots. Extension agent Bob Neier advises killing out the grass under the tree using glyphosate, assuming the roots don’t have any cuts on them. Or you can simply remove the grass by hand. Put some wood chips – no more than 2 or 3 inches deep – around the tree. Before putting down the mulch, you can plant ground covers between the roots.

Never cover the roots with soil, because it reduces the oxygen content to roots and reduces the lifespan of the tree, Neier said. That goes for trees that don’t have surface roots as well; soil, rocks and landscape fabrics should not be placed around the base of trees.

Even grass is not a friend to trees. There may be a chemical in fescue and other grasses that holds back growth of newly transplanted trees, or it could be the weed-suppressing properties that have been bred into turfgrass varieties that have unintended results.

A study done at the John C. Pair Center a few years ago showed that seedling trees grew much better in mulch or simple bare ground than in grass that came anywhere near their roots. Fescue was the worst for holding trees back, but bluegrass and Bermuda inhibited growth, too.

Much of the advice for removing grass from under trees with exposed roots applies here, too. You can pull out any grass by hand or apply glyphosate to the grass around young or newly transplanted trees – the poison shouldn’t hurt the trees as long as you don’t apply it directly to the trunk and are careful around thinner-barked new trees. And then mulch, no deeper than 3 inches, and keep the mulch from resting directly against the trunk.

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