The first and most important step for growing grass under trees is to use the right variety of grass for the site. Some grasses grow better in full sun and others grow better in the shade. Grass growing under trees will be shaded all year under evergreens and during the summer growing season under deciduous trees.
Fescue and special varieties of some grass types can tolerate shade more than others. Warm-season grasses like Zoysia and Bermuda don’t do well in the shade. Read the label when choosing a seed mix for under a tree and pick one that grows in the shade (Amazon link) or has a high percentage of shade grass seed in the mix.
Learn what grass types you have in your lawn and which ones are best for your climate. Don’t struggle to try to make a lawn with varieties that barely survive your environment.
It is recommended that grass lawns under trees be mowed a little longer than the grass that is growing in full sun. Grasses growing in shady areas will have thinner blades and will not fill in as well as in sunny spots. Mowing a little longer will keep the grasses healthier and stronger.
Raking and cleaning up
When raking and removing leaves and debris from under a tree use a soft rake and don’t rake too hard. The finer softer blades and thinner patches growing beneath the tree are not as strong and can be damaged more easily than the rest of the lawn. Using a grass catcher on your lawn mower is a good way to clean up without raking.
Fertilizer and feeding
The feeder roots of most trees are growing in the same region as grassroots and can begin about 12 inches away from the trunk of the tree and out past the drip line. Competition for nutrients and water will be the greatest here. Feed the grass beneath a tree using a regularly scheduled foliar fertilizer application. This keeps the fertilizer available to the grass with less competition between the tree and grass roots in the soil.
If the grass is pale or weak it may only need a foliar micronutrient to correct the problem. A yearly soil test will keep you up to date on the needs of the lawn. Take one test under the trees and one in open areas for comparison.
Monitor the water needs of grass under trees more carefully and spot hand water when needed. Even if it rains, rainfall may not reach the ground because a good proportion of the drops are collected by the leaves and branches of the tree before they can reach the ground.
The upper surface of the soil dries out more quickly because of evapotranspiration and grass will suffer more during periods of drought than deeper-rooted plants. The shallower roots of grass often lose the competition with tree roots for soil moisture.
Some species of trees may exude chemicals that inhibit or prevent other plants from growing under them. There are also changes to the pH of the soil under some trees. Identify the plant family and species of your tree and look it up on the internet to see if any of these factors are at play. If you find that your tree affects the soil, then do what you can to counteract the effects or choose grass varieties that can grow under those conditions.
Pruning to increase light
Get professional advice on proper pruning and thinning of the tree to allow more sunlight to filter through to the grass.
If all else fails
Consider placing a ring around the tree and applying mulch to the area that is the most difficult to maintain grass in. Mulch under a tree is a more natural medium for trees and is also very beneficial to the health of the tree. Other landscape structures and ground covers can also be used in difficult areas.