Answer: Oak trees are shallow rooted and very sensitive to changes in the soil depth over their roots. Adding soil to cover the surface roots could kill the tree for this reason. (Other shallow rooted trees such as maples will simply grow their roots right back up to the surface in a short time, so covering tree roots is not very effective.) Grass will not grow in a shady area or in competition with tree roots, so you might consider using a shade tolerant ground cover instead. Pachysandra is a good choice. Or, you could mulch the area lightly with an organic mulch year round.
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