I apologize for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Getting turf to grow in the shade is a challenge. St. Augustine is probably the most shade tolerant turfgrass to choose for your area. The key is to increase the light intensity so the turf is able to get more light to make carbohydrates to support good growth. Thinning out the tree canopy is an effective option. Remove selected limbs throughout the canopy where they attach to another limb. This way the tree maintains its natural look while increasing the light reaching the turf. You can also remove low hanging limbs around the tree to raise the canopy and allow some reflected light in.
Avoid the temptation to overwater or overfertilize. You can't make up for a lack of light with fertilizer or water, and in fact shady areas generally need less of both than sunny spots.
Set the lawnmower at a higher cutting height. This will increase the leaf blade and allow the grass to capture more light. It will also promote deeper rooting and make the turf look thicker. Finally, try to minimize foot traffic over that area. Weak turf is much less able to stand traffic and recover from the damage it causes.
One more measure to take is to punch some aeration holes about 6 inches deep and 4 inches apart throughout areas where the turf does not want to grow. Then topdress those areas with a thin (1/3") layer of finely screened compost.
If all else fails, the area may be just too shady for turf. You may need to consider a shade tolerant groundcover as an alternative.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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