KENNEWICK -- One of the most common lawn problems brought into our local Washington State University Extension Master Gardener Plant Clinic is creeping bentgrass.
Lawn owners note that patches of this grass seem to die or turn brown in the middle of summer but then come back again in the fall. The owners also note that when they pull on patches of the grass, it comes up much like a carpet or a loose piece of sod. We like to see samples of the grass to confirm its identification, but the offending grass is usually creeping bentgrass.
Creeping bentgrass is a low-growing, perennial cool-season grass that spreads out horizontally via above ground stems (stolons). These stolons can root where ever they touch the ground, resulting in circular patches of grass that stick out like sore thumbs in the typical Kentucky bluegrass lawn. That's because the bentgrass has a finer texture and a blue green color that makes the spots obvious, especially when the bentgrass goes dormant and brown during the heat of summer.
Walking on large areas of creeping bentgrass feels like you're walking on a soft carpet. That's no doubt why another common name for creeping bentgrass is "carpet bentgrass."
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Creeping bentgrass is an invasive grass that often shows up in older lawns, especially ones that are watered and fertilized heavily and mowed too short.
Bentgrass can get started via seed in irrigation water or even as a contaminant in the original lawn grass seed. Because creeping bentgrass also reproduces via pieces of the stolons, it can be spread by mowers and other lawn equipment used on bentgrass infested lawns.
When it comes to control of bentgrass, Jenny Glass, WSU plant diagnostician, notes that "there are no magic-bullet herbicides available for removing unwanted grass species from the desired types without hurting the wanted turf."
Because creeping bentgrass has very shallow roots, one approach is to cut small patches out of the lawn using a spading shovel, digging down one to two inches to remove the grass and roots. The resulting hole is filled with clean soil and reseeded with grass that matches the rest of the lawn. However, one is seldom able to remove all pieces of bentgrass with digging, and it will eventually return in that spot.
The other alternative is treating the bentgrass patches with glyphosate. Glyphosate, the active ingredient in Roundup and other nonselective grass herbicides, kills perennial grasses including the desirable lawn grass.
When trying to kill a patch of creeping bentgrass in a lawn with a glyphosate product, the patch, as well as an area six inches beyond the patch, should be treated. The effective time for treatment is when the grass is green and actively growing in spring or fall, not when it's brown and dormant in mid-summer. Once the grass in the treated area is dead, rake it out and reseed the spot.
If a creeping bentgrass infestation involves large areas of lawn, not just a few patches, you will need to renovate the entire lawn by killing all the grass and starting over ... or you may decide to just live with it.
As with many yard and garden problems, your best defense against a bentgrass infestation is keeping the lawn in a healthy condition with deep, infrequent irrigation, proper fertilization, and mowing the lawn regularly at a height of 2.5 inches.
Need help with a lawn or garden problem?
Call the WSU Extension Master Gardener Plant Clinic at 736-2726 Monday through Friday from 9 a.m. to noon and from 1 to 4 p.m. You can also stop by their table at the Pasco Farmers Market on Saturday mornings.
* Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.