Grass growing under cedar trees has a double problem: dry shade and the acidic soil created by falling leaves. The cedar tree roots will compete with the grass for nutrients and moisture making it difficult for the more shallow rooted grass to thrive. The cedar tree will also cast considerable shade depriving the grass of the sunlight it needs. Although it seems like a desperate situation, with care and trial and error, it is possible to achieve a beautiful rich lawn even under your cedar trees.
Test the soil. Although cedar trees grow in soil containing lime, if the needles have been allowed to fall, they can increase the acidity of the soil beyond what most grass can tolerate. Grass prefers a pH of 6.5. In extreme situations, add lime to the soil.
Cut the bottom boughs off the Cedar tree to allow more light to filter down to the grass. Prune away any damaged branches to thin the Cedar tree's canopy.
Remove any competing weeds from the soil. Adding compost to the soil will improve the nutrients in the soil and increased the likelihood your grass will thrive. Rake the topsoil smooth.
Choose a variety of grass that will tolerate shade. Grasses in the fescue family such as red fescue or hard fescue do well under shade and are somewhat drought tolerant, making them a perfect choice for under your cedar tree.
Distribute the seed evenly under the cedar tree and water well. Keep the soil moist while the seeds sprout and the grass forms.
Do not mow the grass too short under cedar trees. Not only do you risk damaging roots, the soil will lose its moisture faster and grass under 3 1/2 inches tall has a harder time photosynthesizing, especially in shaded conditions, and is more likely to die.
Rake up and remove any foliage or branches when they fall from the cedar tree as soon as possible to avoid smothering the grass.
Reseed your grass every spring and every fall to keep the grass lush and green and water well.