For those keeping track of invasive weed species around the world, cogongrass is a villain. Now on every continent save Antarctica, cogongrass was first brought to the United States from Asia in the early 20th century as packing material.
Not long after, it was planted in Mississippi and Florida as a potential livestock forage and erosion control. Both experiments were failures, but by the time they were deemed so, the perennial grass, facing no chemical obstacles, had begun a march across the South.
“Cogongrass is a major threat to agriculture, forests, right-of-ways, and natural areas,” says Charles Bryson, a USDA-ARS research botanist stationed in Stoneville, Miss. “Over the past decade, this invasive grass has moved north in Mississippi. At least one infestation of cogongrass has now been detected in 75 percent of the 82 counties in Mississippi.”
Even so, until this summer, cogongrass had largely been absent in the Delta region of the state. Over the years, two small infestations were found and quickly eradicated.
Then, this past July, a large cogongrass infestation was discovered in Holmes County. “This discovery was important because it was from the Mississippi-Yazoo Delta region and it is one of the first reports of cogongrass in a hardwood plantation. Subsequent surveys have yielded sites in Carroll and Leflore counties along upper tributaries of the Yazoo River, in cottonwood plantations, in oak-ash plantings, along roadsides and on and inside the levee of a creek.
“These infestations are poised to threaten additional natural communities and agricultural land. The largest of these cogongrass infestations are only a few miles from three national wildlife refuges. Unless these cogongrass populations are eradicated, additional spread is inevitable.”
Bryson says there are several ways the grass could have been introduced into the Delta. “We don't know where it came from. Maybe it washed down a creek. The important thing is this is the first, large infestation we've found in the Delta. Farmers there need to be on the lookout for it — especially on edges of fields, in CRP or WRP land.”
It isn't only farmers and academics who are concerned with the grass. Last spring, the Arkansas Plant Board banned the sale of Japanese blood grass — a type of cogongrass once popular with landscapers.
“Japanese blood grass was grown as an ornamental for years. The Mississippi Bureau of Plant Industry put a stop to its sale four to five years ago. And they were right to stop its sale.”
The wild, aggressive type of cogongrass can be put under stress in lower temperatures and the leaf color will turn red.
“If someone stressed the ornamental and sold the wild type as Japanese blood grass, it would revert back to the green, aggressive type in the field. That would be devastating for the environment. That's one reason why all sales were stopped in the state.”
Prior to the ban, the ornamental grass was frequently seen in landscaped patches in front of businesses.
“A couple of falls ago, I was invited to speak at a conference in Arkansas on invasive plants. I had planned to talk about prickly nightshades and cogongrass.”
Unable to park at the building where the meeting was, Bryson ended up about three blocks away. While walking to the meeting, he passed a bank. There, planted in the landscaped median, was Japanese blood grass.
“(Japanese blood grass) looks nice, but landscapers don't understand its aggressiveness. I mentioned that during my talk. It surprised folks and, before lunch, some of them walked over to check it out.”
Bryson believes there are several logical avenues the recently discovered cogongrass traveled.
“It was probably introduced upstream and washed down south of Greenwood. With either mowing or maintenance of that levee, the grass was spread. There's also a bunch around a bridge. That could be explained if field soil with seed or rhizomes was dumped there.”
As for the cogongrass found in the hardwood plantation, “it was probably dragged off the levee into the field where the hardwoods were planted. The oaks and ashes aren't more than four or five years old.”
Bryson and colleagues spend much time riding back roads to scout for odd, potentially problematic plants. This winter, they'll especially be on the lookout for cogongrass.
“An aerial survey during winter is much more productive. Cogongrass will be easier to find once leaves come off the trees. Preferably, we'd like to use a helicopter. That way, if there's something suspicious, we can set the copter down and walk into an area and ground-truth it.”
There are three cogongrass control options with cogongrass.
- Repetitive tillage.
“But if there's any seed reserves or if you've got plants flowering that can produce seed the next spring, there's always a possibility of re-infestation. The seedlings of cogongrass aren't very competitive if something else can be established.
- Options two and three: glyphosate and/or Arsenal (imazapyr).
“Both (products) are non-selective — spray and you'll kill everything. That will lead to bare soil that needs a (wanted) crop to be established. Arsenal is a bit better with residual activity on cogongrass. But there are a lot of crops or native vegetation that can't be planted following an Arsenal application. There are a few legumes you can use to re-establish vegetation to the sprayed area.”
The best treatment is a mixture of glyphosate and Arsenal.
“After application, you need to go back and check the site for several years. There are areas where cogongrass has been sprayed with Roundup repeatedly over several years before finally eliminating it.”
Why is the cogongrass so tough?
“It has an unbelievable root system. Cogongrass in south Mississippi forms solid stands, huge masses of foliage. But that's on the surface. Over 75 percent — sometimes as much as 85 percent — of the plant is underground in rhizomes. Those rhizomes are tougher than johnsongrass and harder to kill because of the volume of biomass they hold.
“The secret of dealing with cogongrass is to get as much chemical as you can into the plant so it will translocate into the rhizomes. That's why you have to spray glyphosate so many times.”
The reason the Delta has largely been spared an infestation of the grass is the abundance of row crops.
“There's a lot of tillage and chemicals going out. That's helped keep cogongrass at bay and we've got to keep it that way.”
Bryson is also working with deep-rooted sedge by tracking and studying the biology and control options of the South American plant. So far, the primary corridor for the sedge's distribution — within 40 to 60 miles — has been I-10. There are two exceptions to this: a site is in southern Florida and another in Tunica County, Miss.
The botanist suspects the sedge is being moved around on construction and mowing equipment. The Tunica County plants are along Highway 61 in front of an outlet mall.
“Two years ago, I found about a dozen plants there and I assume (they) came in with a construction crew traveling from south Mississippi.”
Deep-rooted sedge is an aggressive plant that produces huge clumps — anywhere from 10 to 100 flowering spikes per year. A single, healthy plant can produce 500,000 to 1 million seeds annually.
“Billions and billions of seeds are produced across the South every year.”
Luckily, there are plenty of options to deal with deep-rooted sedge.
“There are a number of sedge herbicides that work pretty well. The best option is glyphosate, although it often takes repeated sprayings.”
The use of herbicides along highway right-of-ways in Mississippi has probably prevented deep-rooted sedge from spreading as fast as it could. Populations of the sedge are much more frequent, and much larger, in Georgia, Louisiana and Texas.
Deep-rooted sedge is much hardier than its relatives, purple and yellow nutsedge. The nutsedges die back after the first fall frost. Deep-rooted sedge remains green much longer.
“It likely evolved in the temperate regions of South America. I've gone out in fields where we've grown it in a containment area checking for winter survival. Even after we've gotten a killing frost and cold snaps in December, they won't completely die back like the nutsedges. I've dug around the plant bases and found green material.”
One of the good things is, at Mississippi's latitude, the sedge doesn't begin producing seed or flowers until late May or June. Because it doesn't flower early, “you can spray to prevent deep-rooted sedge from producing seed. Mowing it every two or three weeks can prevent seed production too. But that doesn't kill the plant, obviously. So it's always lurking.
“Like cogongrass, deep-rooted sedge threatens agricultural and natural ecosystems. In Texas, this weed has invaded vast expanses of national wildlife refuges, roadsides and CRP or WRP lands.”