Thursday, March 30, 2023
March 30, 2023

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Gardening with Allen: Keep grass away from new trees

The Columbian

When I recently purchased two new trees to plant in my lawn, I was advised to keep the grass from growing next to the tree trunks. Is there some reason for this? I like the look of grass growing under trees. I have a line trimmer so I can keep it trimmed.

There are good reasons for keeping grass and weeds away from tree trunks. You have mentioned the most important one. Line trimmers can cause extensive damage to tree trunks. The speed and force of the plastic lines can cut through the bark, especially with repeated use. The inner bark layers contain tubes that transfer food made by the leaves to the roots. When bark is damaged by mowers and line trimmers, some of these tubes are cut. This reduces food transfer to the roots and stunts tree growth. If bark is cut or damaged all the way around the trunk, trees will die.

The growth rate of a young landscape tree can be doubled by providing an area around the tree without competition from grass or weeds. You can start with a 3-foot-diameter circle with a newly planted small tree. As the tree grows, expand the diameter to 6 feet or more.

Grass has a shallow fibrous root system that is very competitive for water and nutrients. In addition, grass roots produce a growth inhibitor that affects nearby plants. The combination of these two factors effectively doubles the growth rate of young trees without grass competition.

Weed- and grass-free circles around trees can be maintained in several ways in addition to pulling or cultivation. Edging barriers such as bender board, steel or plastic can be placed at the outer edge of circles. Edging prevents grass from growing into the circle. Edging can be moved outward as trees grow larger. Concrete edging can also be used, but it is difficult to enlarge. So grass-free areas should be made larger in anticipation of tree growth.

The best way to prevent weed and grass growth within the circle is to apply black landscape weed-barrier fabric. Landscape fabric is woven so it contains holes for air and water to pass through. The fabric is not only a physical barrier, but prevents light from reaching the soil beneath. Plants cannot grow without light. Landscape fabric is usually covered with bark dust or other mulch to improve appearance. After a year or two weed seeds will blow into the mulch, but they are easily controlled. Do not use ordinary black plastic as a weed barrier. It does not have holes so air and water can reach the roots.

Even without landscape fabric, a 2-inch bark dust mulch will prevent 90 percent of weed sprouting.

Weeds can also be controlled by spraying weed killers such as Roundup (glyphosate). Granular weed preventers such as Casoron or trifluralin (Preen) can also be used. Corn gluten, an organic weed preventer, is usually available only from full-service nurseries and garden stores.

Flowers planted around trees will also be competitive for water and nutrients, but not to the same extent as grass and weeds. I like to keep flower plantings at least 2 feet away from tree trunks.

Allen Wilson is a Vancouver gardening specialist.