Too much mulch can strangle trees and woody plants

Scripps-Howard News ServiceJuly 26, 2013 

Failing to pay attention to little things can bring down a massive old tree or spell the demise of a new one. I know this to be true. It was my job to keep them alive, and some didn’t make it.

It’s not just true for trees, but also shrubs like rhododendron and vines such as wisteria. All have woody parts and bark that extends down to the soil line.

Use a fingernail to nick young bark and see the green layer underneath. This is where you’ll find the cambium that serves as a freeway for nutrients and water. It extends down the trunk to where the bark transitions to root tissue at the soil line, known as the crown.

The crown is uniform in nearly all woody plants – except for a few water lovers like willows and poplars. Bark must remain above the soil, roots below. Changes to soil levels against the trunk, whether due to grading, transplanting or mulching, risk damage to this important part of the plant.

Living bark that gets covered up by leaves, soil, weed-barrier fabric or mulches can’t breathe, and that allows the organisms that cause rot to flourish, ultimately damaging the cambium. When the ground is raised even an inch around the trunk, a ring of cambium layer dies, which quickly kills even a very large, old tree.

This is why it’s vital to plant a tree at the exact depth it was before, whether in a container or in the ground.

Also beware of too much organic matter or compost mixed into soil at the bottom of the planting hole. In a short time, that organic matter will decompose, and the root ball could drop. Therefore, it’s critical that the bottom of the hole be well-compacted natural soil, resistant to settling.

My worst experience was the year I failed to rake up the autumn leaves around my large old wisteria vine. The leaf litter built up around the trunk, rain compacted it, and a perfect environment for rot developed. The following spring the wisteria bloomed and leafed out, then the foliage suddenly wilted. That happened when the cambium layer failed as warming temperatures fostered rot organisms and girdled the vine.

To avoid such heartbreak in your garden, use care when spreading organic mulches and keep at least 2 inches clear around the trunk. Beware of weed-barrier fabric lying against it, too. Weather also may play a part. Heavy rains can cause fine bark or wood-chip mulches to accumulate on the upstream side of the trunk.


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