Planting, mulching rules have shifted

In my 30 years as a horticulturist, advice on the proper ways to plant and care for trees has changed considerably. New technology informs research, and this leads to better practice, or so it is supposed to go.

Our understanding of tree roots, for example, has benefited from advances in the equipment and techniques available for studying the landscape below ground. I recall a lecture in which the speaker, a root researcher, used the juxtaposition of a wine glass and saucer to illustrate the relationship between the canopy of a tree and its root system. He placed the saucer on top of the wine glass and said, “This is how we have viewed this relationship. Notice that the base of the wine glass, representing the tree roots, extends out to the edge of the saucer, representing the drip-line of the tree’s canopy.” This made sense to all of his listeners – we were in the habit of advising gardeners to mulch their trees out to the drip line.

But then the speaker replaced the saucer with a dinner plate and turned the “tree” upside down. “This is how we should view the relationship between roots and canopy. We now know that roots extend two or three times as far from the trunk as the canopy drip line. And most of those roots are in the top 12 inches of the soil.”

It has been over a decade since this new understanding, yet many professional landscapers still pile a ring of mulch around the base of a planted tree that doesn’t even make it to the drip line. The mulching looks like an afterthought, window dressing devoid of purpose. Many homeowners copy this approach in their landscapes, thinking it must be the correct procedure if the professionals do it.

There is a prevailing disconnect between what we see in practice and what should be done. Could this be a reason the average life expectancy for trees in urban landscapes is seven to 10 years?

Planting and care tips

Plant small trees, 1 to 2 inches in trunk diameter.

Dig the planting hole at least twice as wide as the tree’s root ball. Do not dig it deeper than the root ball; the loosened soil under the tree will settle, and so will the tree.

Your tree should show an obvious trunk flare at the true soil surface, and this should still be visible after planting. Before planting a ball-and-burlap tree, remove any soil that was piled up on top of the root ball during harvest and wrapping.

Do not amend the backfill soil with organic matter; use only the native soil that came out of the hole. The only exception is when the native soil is almost totally sand or clay, when up to 25 percent by volume of organic matter should be added to the backfill.

Mulch at least out to the drip line, starting a few inches from the trunk. Do not pile mulch at the base of the trunk! Extend the mulched area as the tree grows.

Healthy root growth depends on chemicals transported from an abundance of leaves to the roots. At planting, remove only dead, damaged and diseased branches, delaying other pruning until after the establishment period (one year for each inch of trunk diameter).

Do not fertilize your tree at planting time. Excess nitrogen pushes growth at the expense of resistance to stress, insects and disease.

Do encourage slow, healthy growth. Even after establishment, do not fertilize a tree unless growth completely stops or the foliage shows signs of nutrient deficiency. Then have the soil tested to make sure the problem is lack of nutrients in the soil.

Make sure your tree is watered every week during the establishment period, one inch of water for each inch of trunk diameter.