Q: Soil and grass keep washing away around a large oak in our lawn, exposing more tree roots plus creating more of a downward slope. A friend says to pile up mulch to cover it and create a volcano look. A neighbor says to build a wall around the outer limits and fill in with soil. I say I am not sure what is the correct method to stop further erosion. Any suggestions?
— A reader in Ashburn, Va.
A: Having exposed roots doesn’t necessarily hurt a tree, but mowing over those roots certainly can. The mower blades inevitably hit the roots, creating wounds that allow disease organisms, usually fungi, a pathway into the tree.
Adding soil to cover the roots and then planting more grass might seem like a good solution. But the soil is likely to either erode, getting you back to the situation you have now, or compact, which would reduce the oxygen that the roots need. And grass will not grow well because the tree will shade the area. Adding a wall and filling it with enough soil to create a bed for shade-loving plants could also be a problem. The soil level would be even deeper.
End of carouselHere’s what to do instead.
- Arborists recommend covering the exposed roots with mulch. Use a light, fluffy material that still allows air to get to the roots. But avoid having any mulch over the root flare — the area where the bark of the tree transitions to roots. The tree needs to take in oxygen there through pores called lenticels, and piling mulch over them or keeping them continually wet cuts off the gas exchange and can kill the bark tissue. A Rutgers webpage explaining the issues says that mulch should generally be kept 3 to 5 inches away from the stems of young plants or shrubs, and 8 to 12 inches away from mature tree trunks.
- On your tree, you might need an even wider no-mulch zone. Go by how the tree looks. Do not cover the big root flares. Do mulch where little forked roots are popping up. If in doubt, stay further away. Remember that having exposed roots doesn’t really hurt the tree — you are just trying to deal with the aesthetics, trip hazards and lawn-mowing complications.
- Keep the mulch layer relatively thin, no more than 3 inches, and taper it down to nothing at the root flare area. The mulch helps soil stay moist, so extend it a considerable way out from the trunk — even past the drip line (the outermost reach of the branches), if you want. At the very least, mulch where the lawn struggles because of shade, and where the roots are popping up.
- Install the mulch over bare dirt to keep weeding to a minimum. You can use a string trimmer if there are large patches where no roots are visible. Pulling by hand is safer.
- Don’t use weed killer — it could injure the tree. A pre-emergent weed preventer can be used around most trees, but it won’t kill existing vegetation — it just keeps weed seeds from sprouting. If you want to consider using this kind of product, read labels completely. Preen Weed Preventer recommends applying it on top of mulch, not as part of the preparation steps, contrary to what you’ll see recommended on some YouTube videos.
- Don’t use weed cloth, which will become a maintenance nightmare. Weeds will grow into it and, on a slope, mulch will slide off.
- Add plants that will naturally grow into living mulch. Yes, mulch keeps soil loose, so any weeds or lawn remnants that do grow through it will be easier to pull, but choosing your plants carefully will mean you don’t have to replenish the mulch and pull as many weeds each year.
- Get starter plants in 2-inch-wide pots, and use a trowel to create holes that are only as big as necessary. If your trowel hits a tree root, move over a few inches and try another spot. Ask a local garden center for advice about what would do well in shade.
- For mulch, you can use wood chips, bark, compost or shredded leaves, which aren’t as likely to blow around as whole leaves. (Shred them by running a lawn mower over piles of leaves at least twice.) The shredded mixture of branches and leaves that arborists often give away if they are working in a neighborhood also works well, but make sure the material isn’t from a tree that was being worked on because it was diseased.
- Install edging along the lower area you are mulching and curve it partway up the slope on both sides to keep the mulch from washing down the slope. On a steep slope, you could have multiple edging strips, creating a terraced effect.
Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.
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