Research by K.C. Olsen at Kansas State University holds promise for helping ranchers defeat one of the great enemies of the endangered tallgrass prairie ecosystem — the invasive species sericea lespedeza.

For four years, starting in the grave drought years of 2012 and 2013, Olsen has been compiling the results from an experiment, after spending the better part of a decade looking for the money to fund it. His research looks at controlling sericea with prescribed burning in the growing season as opposed to early-spring dormant season burning. The results: The best promise for actually defeating the noxious weed that researchers have been able to unlock in a decades-long fight.

Sericea now infests about 700,000 acres of pastures in the Flint Hills. It can be killed with herbicides, but getting full coverage can be challenging.

"Broadcast spraying for sericea also affects desirable plants," Olsen told ranchers attending the winter conference of the Kansas Forage and Grasslands Council in December. "If you try to spray sericea plants one at a time, you miss too many plants. It's virtually impossible to find them all."

Herbicides are also expensive, and repeat applications may be necessary.

For thousands of years, spring fire has been routine on the tallgrass prairie, which once covered most of the American Midwest and now has dwindled to about 4.5 million acres, with 3.5 million of that in the Flint Hills.

TYPICAL FIRE: Most Kansans are familiar with the practice of burning the tallgrass prairie in the spring. The traditional burn leaves the pastures flat and black, but a regrowth of prairie plants occurs quickly.

Ranchers have routinely burned from mid to late April or early May, just before the summer grazing season, following the tradition of Native Americans, who burned in the spring to attract the bison.

Spring fire, researchers determined, actually helps sericea lespedeza by stripping off the hard seed coating and making it easier for new seed dropped in the fall to germinate in the spring.

Olsen wondered what would happen if you did the prescribed burn in midsummer to early fall — July 15 to Sept. 3 — before sericea flowered and when it would be vulnerable to vegetative damage.

"It took me a decade to get the money to do this study," he said. "But I think it may be the most important work of my career."

What he learned was that growing season burning is a powerful, low-cost weapon against sericea lespedeza.

"We learned that the prairie does burn in the growing season, but the fire pattern is vastly different," Olsen said. "It burns much, much slower. It burns the litter on the ground at the bottom of the growing plants and moves so slow that you can easily walk faster than it moves.

"The risk of losing control is much less in a growing season burn," he said. "It burns cooler, and most of the energy of the fire is expended in releasing water vapor. The smoke is more than 80% water vapor. After the burn, you still see a lot of standing, green foliage as opposed to the flat black of a spring burn. A few days later, you will see dead foliage, but a week after that you will have explosive regrowth of grasses and forbs."

In fact, Olsen said, a burn conducted in early September produced strong regrowth by Oct. 10 and available fall and winter forage comparable to wheat pasture. It also produced better regrowth of nectar-bearing wildflowers and forbs the following spring. Fall burning is also comparable to spring burning for elimination of undesirable woody species such as eastern red cedar.

"What we feared was that late-summer burning would send us into winter with bare soil and vulnerability to erosion," Olsen said. "It didn't happen. In three years of data, we went into winter with the same or better vegetation."

In fact, desirable forbs increased in population, and overall prairie health was at least as good as and possibly better than with spring burning.

"Some of the desirable woody plants such as leadplant and New Jersey tea also saw better growth with a growing season burn," he said. "Those plants and forbs provide habitat for birds, butterflies and other pollinators."

Olsen said the studies proved that fall burning is the most effective — and least expensive — control for sericea lespedeza.

He said the sericea does come back after a late-summer burn. But its clock has undergone a reset. The late-summer burn destroys plants that are nearing the beginning of their reproductive cycle. The ones that re-emerge after the burn are juveniles, and they don't have time to reach maturity before freeze. That means little to no seed production.

In a typical spring-burned prairie, you get sericea that produces about 700 seeds per plant, he said. With a midsummer burn (July), you can reduce that to 33 seeds per plant. With a late-summer burn, Aug. 15 to Sept. 1, you can cut seed production to less than 0.5 seed per plant — virtual elimination of reproduction.

"That is victory," he said. "What we have been looking for all this time is way to stop the reproduction process. If you can prevent seed, you can eliminate the pest."

How the study was done
K.C. Olsen's growing season burn research is a four-year study funded by the National Fish and Wildlife Foundation. Statistics are available from years one, two and three of the study.

The experiment is being conducted on 125 acres of native tallgrass prairie that is heavily contaminated by sericea lespedeza.

The site was divided into nine replicate units of 14 acres each. Each unit has a permanent 100-yard transect. Yearling steers or cows are grazed during the winter or spring.

Following removal of the cattle, units are burned at one of three times — midspring, midsummer and late summer.

Key measurements in the study included soil cover and plant species composition, forage biomass pre and post-treatment, basal and canopy frequency of sericea lespedeza, and stand vigor and seed production of sericea.