Putting flower beds under trees can hurt the tree

Putting flower beds under trees can hurt the tree

Bill Sevier: Ask a Master Gardener

Saturday, October 24, 2015

Q: Grass will not grow under a tree in my lawn, and I am planning on putting in a flower bed around the tree. The edging will be bricks, and I need to add some soil. Will this hurt the tree? J.T., Tulsa

A: The trees in our landscape are beautiful and add value and functionality to our homes and usually require little care. However, there are significant issues when attempting to grow a lawn or create a garden bed under their canopies.

Any disturbance to the soil over the roots — either adding or removing soil — may have serious consequences for a tree. OSU, in its fact sheet, “Site Disturbance and Tree Decline,” outlines the hazards to a tree from change of grade. OSU horticulturists feel that adding soil to the root zone of mature trees should not be done.

Even temporary piling up of soil often done during construction of a home may be harmful.

To understand the problems, one must understand what makes up the root system of a tree.

For most mature trees, the large majority of roots are in the top 18 inches of soil, and the tree’s root system often extends out two to three times the distance from the trunk to the drip line. A tree’s root system includes the larger roots, which support the tree, and the smaller roots, which absorb water, nutrients and oxygen (plants get their oxygen from roots, not leaves). The smaller absorptive roots are generally in the top 3-6 inches of soil.

If one adds soil to the area under a tree, as is often done when creating a flower bed, this blocks the flow of oxygen into the upper layers of soil, resulting in injury due to suffocation. It may also promote diseases.

The addition or removal of soil over roots will often cause a tree to go into a gradual decline, a decline which may be fatal over several years. This is manifest by smaller leaves, less linear growth of limb tips, early fall leaf coloration and leaf drop. The decline may then progress to die-back of twigs and then death of larger limbs in the canopy (top) of the tree. Once trees manifest these symptoms, they usually die in a few years.

If a turfgrass with some shade tolerance, such as fescue, will not grow under a tree and a garden bed is desired, shade-tolerant plants and loose mulch may be the answer. When planting something such as a ground cover or other shade-tolerant plants, one should minimize the ground preparation in the tree’s root zones.

Then add a layer of a loose mulch but no more than 3-4 inches. This will allow air to circulate into the soil without posing a risk to your trees.

Garden tips

§  Remove green fruit from tomato plants when frost threatens. If they are green but full sized, they will ripen indoors. They do not need to be in sunshine to ripen indoors.

§  The average first frost date (temperature below 32 degrees) for our area is Nov. 3.

§  Use a cold-frame device to plant spinach, lettuce and various other cool-season crops for production most of the winter.

§  Take tropical water garden plants indoors when the water temperature reaches 50 degrees. Also, stop feeding fish in the pond at this water temperature.