From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156703-537701
Question: We have a huge pine tree in the front yard and the grass
just doesn't seem to grow very well under it. What can we do to
encourage the grass to grow?
Answer: Growing lawns under pines is difficult for several reasons.
The shade under a pine is quite dense, and the canopy of the pine
keeps natural rainfall from reaching the soil. Needles that are shed
make a thick mulch that acidifies the soil, and pine roots are shallow
so they take up most of the moisture -- lawns and groundcovers just
can't compete. Instead of struggling to get grass to grow under the
tree, you might consider placing a bench there so you can enjoy the
tree's shade, perhaps surrounded by self-watering pots or planters of
shade-loving plants (such as impatiens, caladium, coleus).