Controlling Invasive Cogongrass

By Daniel J. Leonard

Over the last decade or so, the Panhandle has been overrun, and I don’t just mean by the summer beach traffic.  Rather, by an aggressive, exotic perennial grass that quickly displaces all native species.  Cogongrass is not useful as a forage for wildlife or livestock, can spread by roots or seeds, and has no natural enemies.  If you own property, or spend any amount of time on the roads, chances are you have become acquainted with this worst of the invasive grass species, Cogongrass (Imperata cylindrica).

A native of Southeast Asia, cogongrass was introduced into the US in 1912 around Mobile, Alabama as a hitchhiker in orange crate packing.  Then in the 1920s, and 30s, it was intentionally introduced from the Philippines into other Gulf Coast states, including Florida, for soil erosion control.  Since then, cogongrass has become one of the most economically and ecologically important invasive species in the US and worldwide, infesting nearly 500 million acres and is now found on every continent.

Cogongrass in a Calhoun County natural area encroaching into a pine plantation. Photo courtesy of Daniel Leonard.

Cogongrass is easily identified in late spring, when the grass flowers and then has fluffy, white-colored seedheads above the mats of grass beneath.  Additionally, patches of cogongrass are almost always noticeably circular in nature, radiating out indefinitely from the initial infestation.  A closer inspection of the grass will reveal light green leaves up to 4 feet in length, with an off-centered, silvery colored midrib (the primary leaf vein that runs from the base of the leaf to the tip) and serrated leaf edges.  Underground, cogongrass exhibits a dense underground root system that can reach as deep as 4 feet.  This feature is the primary reason cogongrass out-competes other plants, withstands any drought, fire, or soil condition thrown at it, aids in its resistance to herbicide activity, and generally makes it very difficult to manage.

The first step in managing cogongrass is prevention.  If your property, or the property you manage doesn’t have cogongrass, do everything you can to keep it that way.  While the species can spread some distance through wind-borne seed dispersal, it is much more frequently moved around by fragmented rhizomes hitching a ride on equipment.  If you, or a contractor you’ve hired are working in or around an area with cogongrass, avoid disturbing it with equipment, and be diligent in monitoring the site for outbreaks following the job’s completion.

If you find cogongrass on your property, effectively eradicating it requires patience, persistence, and several years’ worth of correctly timed herbicide applications.  Currently, of the numerous herbicides available for purchase, only two active ingredients have been proven to be very effective at controlling cogongrass, impazapyr (Arsenal, Stalker, etc.) and glyphosate (Roundup, Cornerstone, etc.).

• Imazapyr is an extremely effective non-selective, residual herbicide that controls a wide variety of weed species, including cogongrass. Just one or two applications of imazapyr can provide 18-24 months of effective cogongrass control, with follow up treatments required as needed after that.   However, Imazapyr has one major downside that limits its use in many settings.  Because it is a non-selective herbicide with significant soil residual activity, it cannot be used around the root zones of desirable plants.  Oaks and other hardwood trees are especially sensitive to imazapyr.  This herbicide is best limited to use in fields, waste/fallow areas, and monoculture pine plantations.
• The other option, glyphosate is a non-selective herbicide with no soil residual activity. It is often the better option where severe injury or death of desirable hardwood trees cannot be tolerated.  However, due to its lack of residual soil activity, glyphosate applications on cogongrass patches will need to be repeated on an annual or biannual basis for several years for eradication of the infestation.