COLUMN: Managing unwanted grasses in the home garden

Garrett Ford

Drive along any roadway in Northeastern Oklahoma and you will likely see large swaths of Johnsongrass - sorghum halepense - covering the shoulders and ditches. Johnsongrass is a warm-season grass that forms massive colonies and can take over areas in a single season.

One medium-sized clump can create up to 60 feet of rhizomes in a single year. From these rhizomes, large, finger sized stems will emerge that can tower over 6 feet tall and are topped with a panicle of flowers that produce a red/purple seed head, capable of producing two pounds of seed. So many rhizomes and seeds make Johnsongrass a nightmare for a row-cropping farmer or home gardener.

The fable of how Johnsongrass became one of the nation's top 10 most noxious weeds dates to the early 19th century when an imported grass from the Mediterranean had taken over the plantation of South Carolina Gov. John Means. Later, Alabama planter Col. William Johnson acquired some of Gov. Means' seeds and took them to his plantation near Selma. He began to freely share seeds with other southern aristocrats and thus, "Johnson's grass" was born.

Today, Johnsongrass plagues the fields of farmers and home gardeners while simultaneously worrying ranchers because of its tendency to store nitrates and produce cyanide whenever stressed, both of which can kill livestock. Johnsongrass now runs amok throughout the southern U.S. and we mostly tolerate it. However, when it continually pops up in your garden bed, you probably wonder, "What can I do to get rid of this?" when you should be asking, "How much do I have to do to get rid of this?" The answer is "a lot."

Managing unwanted Johnsongrass will be extremely difficult and intensive. A sound, integrated strategy must be used to: prevent new seed import, deplete the existing seed bank, and deplete the energy stores of existing rhizomes. It is recommended that you employ mechanical, biological, and chemical pest management to be effective.

Prevent new seed import by being informed about your animals. Livestock who may have consumed Johnsongrass should remain idle for at least a week to avoid spreading seed through excretion. Deplete the existing seed bank and the energy stores of existing rhizomes by manually removing young shoots, but be sure to do so when the ground is softened to remove a good portion of the rhizome.

Scalp, or mow, the area repeatedly to decrease the photosynthetic capability of the grass, which will weaken the plant's rhizomes. Grazing can decrease the shoot count - similar to mowing, but care must be taken to not allow the animals to eat stressed Johnsongrass as it can be fatal.

You can use soil solarization plastics to cover large areas for a season to damage the rhizomes energy stores. You may elect to utilize tillage as a means of management. Tilling can be effective when used in an integrated program, but tilling alone will likely do more harm than good by fragmenting the rhizomes and multiplying the issue. Furthermore, proper tillage is hard to do in most of Cherokee County. Chemical methods should be used appropriately to gain effective control. Use a systemic post-emergent herbicide at the proper time for the best control.

Grass-selective herbicides - fluazifop, clethodim, or sethoxydim - should be applied when plants are 8 to 18 inches tall and non-selective herbicides - glyphosate - should be applied when plants are 12 to 24 inches tall. In either case, the most damage can be inflicted on the rhizomes when the plants are flowering but not seeding because the rhizomes are growing vigorously at that point.

Grass-selective herbicides will damage sweet corn and grain crops. Properly use everything at your disposal to begin to fight Johnson's grass and call the Cherokee County OSU Extension Office for assistance.

Garrett Ford is the agriculture educator for the Cherokee County OSU Extension office.

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