It's tricky to grow grass around tree roots because the roots often compete with the grass for water and nutrients. Some grass varieties might also receive inadequate sun exposure under the tree canopy. This means the grass you selected for the rest of your yard might not be suitable for growing around tree roots. With a little research, you can choose a different type of grass to fill in the bare spots around those exposed roots. It's important to make an appropriate selection and to learn how to prepare the soil without damaging the tree roots.
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Choosing the Best Grass Type
Choose a fine fescue (Festuca spp., USDA zones 3-7) cultivar for planting around tree roots or directly under a tree. Unlike many types of grass, fine fescues tolerate shade and don't need much fertilizer or water. This means they should survive even when the much hungrier and thirstier tree roots absorb most of the moisture and nutrients from the soil.
Despite its reputation as a low-maintenance grass, you should still plan to fertilize fine fescue when planting it around tree roots and to water it during drought-like conditions. Although the tree will steal most of the water and fertilizer, this extra pampering ensures the grass gets what it needs to grow lush and match the rest of your lawn. Apply a balanced NPK fertilizer in the spring and fall, or test the soil to determine which nutrients are lacking.
Note that this is a cool-season variety of grass that may experience heat stress in zones 8 through 11 unless it's kept well-shaded. In addition, fine fescue does not tolerate much foot traffic. If it will be planted in a high-traffic area, consider adding stepping stones to protect the grass. Fine fescue is also a clumping grass, which means it may look a little patchy unless it's overseeded each year.
Preparing Soil Around Tree Roots
Grass seed germinates poorly on compacted soil. Typically, an area should be aerated or tilled before grass seed is scattered, but doing so around tree roots could injure the roots. Serious injuries to roots could kill the tree outright, and open wounds allow disease pathogens to enter.
You can use a garden hoe to scuff up the top of the dirt around the tree roots to get it just loose enough for grass to germinate. But for the best results, the grass needs just a couple more inches of loose, rich dirt to grab onto and to establish a healthy root system. Spread some well-aged compost between the tree roots and sprinkle the fine fescue seeds directly on top. Gardens Alive notes that fine fescue seeds should be sown in mid-August so that it has time to germinate before leaves drop and cover it up.
Alternative Root Covers
Growing grass where tree roots are presents some challenges. If you don't want to invest time, energy and money into a plan that "might" work, consider adding a different type of green carpet: moss. Moss is low-maintenance and can be established via sod rolls for an instant cover-up.
You may also consider establishing a shade garden with plants that tolerate compact and dry soils. Or, for a well-groomed look, add bark mulch around the base of the tree and between all of the roots. Just be sure to pull the mulch away from the tree trunk to mitigate pest and disease problems.