The easy answer is just don't grow grass there. Grass doesn't like to grow under pine trees, so why force it. The soil is acidic, it can be messy, there is little sunlight and competition for water is intense. Create a bed of mulch or pine straw preferably to the drip line of the tree. It looks good and saves the battle of trying to grow grass where nature insists otherwise.
- Clean the area of needles and debris
- Apply lime to decrease the acidity of the soil
- Remove all tree limbs below ten feet
- Prune/thin the upper limbs to increase available sunlight
- Use fescue seed for it's shade tolerance. In the southern zone, zoysia, bermuda and centipede grasses have been known to do well under pine trees.
If you must have grass under your pine trees there are some measures that can be taken to achieve success.
Growing grass under pine trees is no easy task and will require constant attention. Subsequent applications of lime will need to be put down. Needles will continuously need to be cleaned up. Additional watering may also be needed due to competition from tree roots.