Garden Detective

Garden Detective: Missing tree leaves sunken spot in lawn

About two years ago, a large oak tree had to be removed from our backyard. The stump was ground and, within a few months, a new lawn was laid in the whole area. Now, however, the spot where the tree stood has sunk several inches, although the sod in the dip looks perfectly healthy. What would be the best way to correct the dip? My husband is thinking along the lines of adding soil over the dip and reseeding, while I’m thinking of lifting a flap of grass the size of the dip, adding soil, and replacing the flap. Also, I’m wondering if the ground-up material should be removed first to prevent further sinking?

Kathie Marynik, Granite Bay

If left in place, the ground-up stump and tree roots will continue to decompose, causing the area to continue to sink, according to UC master gardener Carol Hunter. Removing the material and filling the area with soil is recommended.

It is possible that either of your ideas will work, however both have potential problems:

As for the suggestion to lift a flap of grass the size of the dip, it won’t be easy. After two years, the grassroots should be well developed and lifting the sod will tear the roots.

As for adding soil over the dip and reseeding, covering the sod will smother the existing grass and roots. As it decomposes, the remaining sod may impact drainage and root development of the new grass. So, the reseeded lawn may not match the surrounding sod in either look or vigor.

Your “sod lifting” approach may be the best. If the low spot is more than 1 inch deep (as is your case), lift the sod, fill in the area under the sod evenly with soil about 1 or 2 inches higher than the surrounding area, replace the sod, and water thoroughly.

For lawns with shallow sunken spots, it’s possible to fill the hole without digging up or lifting the sod. If the depression is less than 1 inch deep, add a thin layer of planting soil on top of the turf so that the grass can grow up through the layer.

Either procedure should be done when the grass is growing vigorously. When mowing the lawn, be careful not to scalp the fill area.