Weak under Trees

Trees, especially the Oaks, have surface roots that can form a thick twisted interwoven mat several inches deep. This is great for the tree: the tree can explore and mine nutrients and minerals from just about every nook and corner of a yard. But the thick mat of roots form a barrier to grass roots reaching the soil below and a physical barrier to water and nutrients. It is like growing grass on a sheet of plywood.

It dries out fast, has little nutritional value and at best can only sustain a few sprigs of hardy grass.

Tree Roots are Random

To make matters more complex, the tree roots are randomly matted in a yard – a thick mat here, not roots there and another thick mat over there (get the picture). This is very confusing because an area under an Oak tree can have everything from thick grass to bare soil

To simplify, weak grass under trees is a soil problem – or even better the lack of soil. The tree roots so dominate the area that there is no soil available to sustain the turf. Most people attack the tree canopy thinking weak turf is caused by shade but in most cases it is a soil problem, hidden beneath your feet.

Solution

The solution is not as simple as adding soil or removing the roots - for that can damage the tree. I suggest two possible solutions:

Solution 1: Consider an alternate ground cover like Liriope, Asiatic Jasmine or mulch. This has great advantage in that you reduce the turf footage and reduce mower damage caused by exposed roots.

Solution 2: Adding one to two inches of good composted organic matter or soil to the weak areas. This will need to be repeated every year or two to maintain healthy turf. This will provide a sustainable soil layer for the turf but because of the high organic matter content will not last very long. The City of St Petersburg offers a perfect solution with a product called "soil builder" and is available through the sanitation department.