Answer: Mature trees will occasionally grow roots at the soil surface. As you have seen, cutting them usually stimulates new growth. In some cases it is caused by soil conditions such as a compacted hardpan beneath shallow topsoil, in other cases it is simply the way the tree grows in search of both water and (surprisingly) air. Setting the lawn mower to the recommended mowing height of about two and a half inches usually allows the mower to pass over the roots. You might also want to consult with a professionally trained and certified arborist as to the health and rooting patterns of your trees, just to make sure there are no unusual circumstances causing them to grow unnaturally.
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