Recently on GardenLine, we discussed using compost instead of mulch around the base of trees, and I was once again moved to warn about piling it up too high to make a "mulch volcano" – a mound or cone of mulch, a foot or more deep, piled up around trees and shrubs.

Too much compost or mulch can be as detrimental as not enough. The pictures here show what NOT to do.

I’m not sure how this practice developed, but deep mulch can cause considerable damage. I suspect uneducated, untrained and unprofessional landscape companies are the originators. And it seems to be especially pronounced at model homes in new residential communities. I further suspect that naive or first-time homeowners spot the practice and decide to copy it.

Professional landscapers get paid to fill their customers' requests, and when homeowners order (as they should) fresh mulch twice a year, several inches sometimes get piled up in spots that might need just one inch. As a result, it accumulates to cover the base of tree trunks and the very important root flare.

That bottom section needs air and light. With excess mulch, it's forced into darkness and subjected to moisture. Bark that's too moist for too long will rot! And rotted bark cannot protect the tree from diseases. In fact, diseases grow better in the dark moisture of the mulch, and then the trees become susceptible to insects.

Some trees, such as maples, have shallow roots. If mulch is piled high around their trunks, the roots will start to grow into the mulch and encircle the trunk.