Answer: Grass always has a hard time growing under pine trees. The roots have to compete with the tree for moisture and nutrients, the shade is often too dense for grass, and the soil acidity (from fallen needles) can overwhelm grass plants. Evergreen trees tend to keep rain from falling down to the soil, too, making the area directly under the tree rather dry. In my yard I use Ajuga (Carpet Bugle) and Vinca minor (Periwinkle) under my pine trees. Both will take everything a pine can throw at it and still thrive. Ajuga will take some foot traffic (or wheel traffic) and come right back. Vinca spreads by vining and would not be suitable if you need to cross over it. Viola (violets) make a great groundcover in shady areas but the leaves may be too tender for constant traffic. Oxalis oregana is another shade-loving groundcover, again with tender stems and foliage. Unless you're constantly running through the area, any of the above suggestions should make suitable (and reasonably carefree) groundcovers.
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