Advice on Controlling Johnsongrass Weeds

Advice on Controlling Johnsongrass Weeds

As the winter season gives way to warmer temperatures, homeowners are facing the problem of weeds invading their yards. While your grass may still be dormant, weeds have already started growing, and they have developed a special resilience to harsh conditions. One such example is Johnsongrass, which is an invasive weed species that can quickly take over your entire yard.

Although you may try regular weed control measures, you will need a different approach to get rid of Johnsongrass and reclaim your yard. This weed is expert in propagating and resisting any attempts to limit its growth. It seeds aggressively and sends out thick and fast-growing roots called rhizomes, which makes it difficult to control its spread.

The best way to tackle this weed is to kill the entire plant, including its roots. Weed poison, such as Roundup Ready-To-Use Weed and Grass Killer III ($15 at Home Depot), is particularly effective at penetrating the soil and infiltrating a weed’s root system, which kills the plant before it can spread further. However, you must use weed killers with care, as they can also eliminate other nearby plants they come into contact with. So, it is essential to wear the right protective gear to avoid any skin contact.

Know thy enemy

Overgrown weeds in yard

Mark van Dam/Shutterstock

If you hope to beat Johnsongrass for good, a brief history is called for. Sorghum halepense, dubbed “Johnsongrass” in the United States, is also known by the names Aleppo grass or Aleppo milletgrass, as it emerged in Mediterranean regions of the world, such as southern Europe (though it also has roots in India), hundreds of years ago. When brought to North America in the early 1800s it was introduced to the U.S. as a forage crop, but it quickly grew out of control. Today it is nearly universally despised as this invasive plant spreads quickly and its hardy characteristics make it difficult to decimate.

Johnsongrass grows up to 8 feet tall and has developed the ability to be both cold-resistant and fairly drought-tolerant. It easily grows in nutrient-lacking soil and is highly resistant to many diseases that afflict other plants, making it a sinister nemesis of farmers’ crops; the weed has even been able to develop resistance to many herbicides (currently 21 types, and counting). Incredibly bothersome for homeowners and hobby gardeners, Johnsongrass can also pose a deadly risk to cattle populations, as it produces toxic prussic acid when stressed (such as during a freeze), which can kill grazing cows. Johnsongrass is so harmful that in 2016 The University of Georgia received a $5 million grant to study it and develop better control methods.

More tips for preventing Johnsongrass

Person mowing overgrown yard


Once it has taken hold, the best option to get rid of Johnsongrass is to poison the entire plant, but being proactive can be an even better approach. In the spring, before the grass has had an opportunity to take over your yard, applying Roundup Landscape Weed Preventer ($23 on Amazon) can help to create an inhospitable environment to Johnsongrass and other weed seeds.

If you want to avoid using weed killers on your lawn, the best option is to regularly mow the grass — monthly, or even more frequently — which has been shown to disrupt the roots’ energy reserves and slow the spread of Johnsongrass. Although many weeds respond well to being pulled by hand, the aggressive root system of Johnsongrass means that significant enough remnants of the rhizomes may remain in the soil, leading to a false sense of accomplishment on your part and continued growth of the grass. While not a quick fix, mowing Johnsongrass regularly for 2 years can result in weakened roots, which bodes well for your yard. When taking the mowing route, remember to cut the grass before the arrival of visible seeds, which will further help to keep it in check.

Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.