Gardeners Kill Their Trees With Kindness

Green Piece

October 14, 1990|By Miriam Mahowald | Miriam Mahowald,Contributing writer

Fall has arrived. The department stores have stacked their gardening tools, sprinklers, mowers, sweepers and fertilizers into a corner and marked them for clearance. Garden centers announce that it's time to plant and mulch.

It's true that fall is a good time to plant spring flowering bulbs, perennials, and balled and burlapped trees and shrubs. It is time to fertilize your lawn and patch up small areas of the turf with sod. It is time to keep the fallen leaves raked so they won't smother the grass. It is time to take a closer look at how well your plants have survived the summer.

And it's time to learn more about mulch before adding another forkful.

Mulch is a wonderful, organic, natural protection for trees, right?


Most of the mulch you see throughout Howard County's landscapes is killing or causing stress to the plants it surrounds because it is usually too deep. Any amount over 2 inches is too deep. Mulch applied this deeply reduces or eliminates oxygen from the roots it is meant to protect. Then the roots die.

If many roots die, the tree may send out new roots into the upper part of the mulch, where the oxygen supply is adequate. But these roots, exposed to extreme weather conditions, dry out, or get too hot or too cold. And then they die. When some of the tree roots die, the tree becomes weak and susceptible to the stresses caused by heat, cold, dryness, and insect and disease problems. But when many roots die, the tree dies.

But mulch conserves water and keeps the soil moist, right? Wrong again.

Mounds of mulch around trees usually shed water. Dig into the middle of a pile of mulch during a drought and you will usually find it to be dry.

Set a sprinkler to water the tree for a half hour and check again -- it will still be dry.

Mulch controls weed growth, right? Right, but only when it's too deep.

Because mulch usually does not contain weed seeds the first year, it smothers the roots of most existing weeds. But when weed seeds, especially those of perennials such as dandelions, germinate in a pile of mulch, they also grow. Take a good look at some of those piles of mulch. They often resemble miniature green mountains.

But professional landscapers mulch heavily. And many homeowners emulate them. As a result, many trees are unnecessarily killed. Most often, the trees persist for a couple of years after they are planted. Sometimes they even survive this torture to grow into beautiful trees. But it is much better to avoid the cause of the problem.

Mulching has to be a natural benefit to trees. What about the many trees in the woods whose leaves are left, year after year, where they fall? It is a mulch; that's true. The trees thrive on this type of mulch, formed by loose layers of leaves that gradually decompose into a leaf mold. The leaves are spread over a very large area and oxygen can penetrate this material to reach the roots of trees.

To mulch properly, you have to understand a little about the location of tree roots. The roots of a tree extend beyond the "drip line." For example, roots of a 20-foot tree probably extend over a circle with a 40-foot diameter, or about 1,257 square feet. Equally important, the vital roots of a tree are in the top 18 inches of soil. This means that a big pile of mulch placed against the trunk of a tree, extending out a couple of feet, is not going to help a tree very much.

The mulching problem usually begins at the time of planting. A few years ago, we were taught to dig extra-large, deep holes for trees, to amend the soil with lots of organic material, and to set the trees at the same depth that they had been growing in the nursery. Many of the trees died because the large, deep holes we prepared often acted like a bucket to hold so much moisture that the roots were not able to get oxygen. This bucket-like condition was especially severe where clay soils prevailed. The clay sealed the holes at the sides and bottom. Because the amended soil was so loose, many of the plants sank too deeply into the hole, again causing roots to rot and trees to die.

Now we have new planting guidelines. The new rules warn that the holes should not be too deep, and that the plants should be set higher than they grew in the nursery. Hurried landscapers take this one step further. They scoop out a shallow hole and set the tree in it, leaving the root ball half exposed above the ground. Then they cover the burlapped roots with mulch, making a huge mound around the tree. If the tree lives, another pile of mulch is added the next fall or spring. This practice, which forces the newly planted trees to establish an entirely new root structure, has become the standard planting procedure.

Mulch around a tree looks nice and keeps the lawn mower from skinning the bark. Make the mulch an inch or so deep, keep it from direct contact with the tree bark, and extend it out from the tree a bit farther. Instead of adding a new layer of mulch, use a rake to fluff up the mulch that is there. If you've already put on too much mulch, remove it. It's time to stop the mulch wars. Your trees will be much happier and healthier.

Green Piece features local gardening tips and profiles of county gardeners every Sunday. It is written by Miriam Mahowald and Mary Gold, two county residents blessed with green thumbs.

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