Covering Tree Roots

The following question was sent to the P&PDL diagnosticians here at Purdue University:

Question: There is a very large pine tree near our house. The owners of the tree have covered the roots with a substantial layer of black earth and planted plants on it, to make it look better, but can this cause any problems for the tree?

Answer: Yes, adding soil to the top of existing roots is a problem. It often injures or kills trees by decreasing the oxygen supply to the roots. Inadequate oxygen interferes with metabolic processes and root growth. Once root growth slows down, the supply of water and minerals to the tree is diminished. Lack of root growth and consequent decreases in water uptake lead to water stress in the shoots, thereby reducing photosynthesis but increasing respiration. Decreased nitrification and eventual loss of nitrogen to the tree reduce vigor and growth and may eventually kill it.

-Rita McKenzie