The Grass Won't Grow Under a Large Oak Tree

Grass Degradation

• Growing a healthy lawn is not as easy as rolling out new jute carpet to cover the floors as you do in the house. The lawn is a living collection of plants. While laying fresh, deep green sod across the yard — including under the oak tree up to its trunk — initially looks perfect, shade and dry sod conditions are a death sentence to lawns. Lack of adequate sunlight under the oak tree's leafy boughs thins out the number of grass plants, seen as yellow and tan thatch. Months later, the once-perfect sod is sparse or fully dead, decomposing into humusy compost under the oak tree.

Futile Efforts

• Fertilizing the shaded grass under an oak tree will not overcompensate for the basic lack of sunlight. The salts in fertilizers may actually burn the grass roots, further stressing plants and hastening grass degradation. Watering the lawn under an oak helps the grass — since the large, water-absorbing oak tree roots are making the soil dry — but applying too much water keeps the grass wet and leads to fungal problems. Choosing more shade-tolerant lawn grasses is not an absolute solution; even these plants suffer if the light is too dim.

Growing Grass

• Expect any lawn grass that does not receive at least four hours of uninterrupted sunlight daily to begin to thin out and look sickly. The more daily sunlight, the better. Grass won't grow densely, if at all, in the dense shade nearest the oak tree's trunk. However, grass may perform slightly better the closer it is to the edge of the canopy where direct sun occurs. Mow grass growing in shade at a higher setting compared to sun-grown grass. The grass must be able to photosynthesize light to remain healthy and dense. When the grass is stressed, bare soil is revealed, creating habitat for various weeds to sprout.

Solutions

• Save disappointment and installation resources by not sowing lawn seed or laying sod inside the farthest reach of the oak tree's branches. Use a shade-tolerant ground cover plant under the oak, such as barrenwort, ferns, periwinkle, lilyturf, Japanese spurge or ivy. Alternatively, place a 2- to 4-inch layer of organic mulch across the shaded area under the oak tree. The mulch looks attractive, decomposes to provide nutrients to the oak and prevents weeds. The mulch bed under the oak may be walked or played on. Keep the lawn grass out from under the tree where it will look and perform its best. Your neighbors might even ask how your landscape looks so well-kept.