Growing Season Burns for Sericea

By Kaleb Baker, Land Steward, Illinois Audubon Society

Soil disturbance from maintenance along a natural gas pipeline gave sericea (Lespedeza cuneata) an opportunity to establish and proliferate throughout a recreated prairie at Bremer Wildlife Sanctuary in Hillsboro, IL. Illinois Audubon Society volunteers and contractors have been spot-spraying the sericea, but it is very labor intensive to find all the plants, especially through multiple years of unburned growth. In 2020, we decided to try a growing season burn to: (1) top-kill sericea, preventing new sericea seed that year, (2) encourage the sericea seedbank to germinate, hopefully reducing the number of years herbicide treatment will be required, and (3) remove old prairie plant material, making the sericea easier to find. Mowing would have top-killed many sericea and made them easier to find. However, sericea that would be run over by tractor tires may not get mowed and could still produce seed. Mowing also doesn’t encourage seed bank germination.

We carved out a 3.4-acre prairie burn unit. Since our burn equipment was limited on site, we ended up mowing the area to keep flame lengths low. This was my first growing season burn. Mowing occurred on September 1, 2020, just as the sericea flowers began to fade and before seed was formed. Two weeks later, September 15th, we burned the unit.

2020 Burn Unit Map

The day of the burn, air temperatures reached 78F with reported ESE winds reaching 7mph. RH dropped to 44%. The mowed material had mostly cured and provided a healthy thatch layer. However, the thatch was still slightly damp underneath, in part because it was shaded by the surrounding woodland. Burn progress was slow and smokey. Winds were light, so the thick smoke rose relatively vertically as opposed to blowing sideways. Lots of drip torch fuel was used to keep the fire going. Most of the unit was burned by stripping off 5- to 10-foot sections, following the mowed windrows. Despite an RH of 44, the slightly damp thatch kept fire temperatures very low.

Click photos to enlarge

The unit had about 85% burn coverage with no resprouting of sericea observed that fall. Success!

85% burn coverage

In 2021, we continued the growing season burn treatment and expanded the unit to 7.1 acres with half being mowed and half unmowed. Half was mowed for two reasons: (1) to test the difference between mowed and unmowed growing season burn, and (2) to create enough thatch to carry fire since this same area was burned the previous year. The unmowed portion had not been burned since March 2019, so there was a healthy thatch layer. We also had better burn equipment borrowed from another site, giving us greater confidence in our capacity to control the burn. 

2021 burn unit map
The monitoring point before the burn.
Monitoring point before the burn

Firebreaks were mowed about 12- to 15-feet wide and thatch was removed with a backpack leaf blower the week before the burn. Bare ground firebreaks are strong and the fluffy windrow created by leaf-blowing created a good fuel source for ignitions, given the dry conditions.

Click photos to enlarge

Winds were light and variable with 0-5mph winds observed at the unit. RH low was 41 with air temperatures reaching 90F. This was my second growing season burn. There were no injuries, no escapes, and a lot of top-killed sericea. Success! We will be monitoring the success of the burn treatment through the end of the year and into next year. We will also continue to focus on herbicide control of sericea in the growing season burn unit. 

Lessons learned from the 2021 burn:

  • Growing season burns are very smoky. Wider firebreaks would give more space for smoke to lift and give the crew more space to get out of the smoke.
  • Light winds help lift the dense smoke up, but also make wind shifts more likely. Winds were reported at 9mph ESE, but observed winds were much lower 0-5mph with some light gusts and 180-degree wind direction shifts. Despite smaller flame lengths and lower fire temperatures, fire breaks should be very wide to accommodate shifting wind directions.
  • The mowed area burned well, but still required a lot of internal ignitions. Stripping was about done in 15- to 20-foot sections in 2021 compared to 5- to 10-foot sections in 2020. We had a couple dry weeks prior to the burn in 2021 which yielded drier thatch and lowered internal ignition requirements. We also rotated our ignition pattern to be perpendicular to the windrows.
  • Growing season burns are hot for crews. While the fire burns cooler, air temperatures reached 90F this time. Crews should bring lots of water, take their time, and create opportunities for water breaks, and get out of the smoke.
  • Scheduling growing season burns can be tricky. In 2021, we burned a little too late due to wet weather in early September. Burn date was 9/29/2021. One sericea was found on site with hard green seed on 9/23/2021. Hopefully, the burn consumed any sericea seed that was just forming.

Overall, I think growing season burns are a useful management tool. Good for localized invasive species control, but can be tricky to get the timing right. These burns create nice structural heterogeneity in the prairie. This particular growing season burn unit is located entirely within a larger dormant season burn unit. Since the growing season burn unit has time to produce some vegetation still this year, but not enough to carry fire very well; it could possibly serve as a refugia for some insects or possibly herps when the larger dormant season unit is burned. 

Visit the Illinois Prescribe Fire Website:
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• Browse or submit prescribed fire equipment reviews.

Kaleb Baker is the Lands Steward for the Illinois Audubon Society. He earned his bachelor’s and Master’s of Science degrees in Biological Sciences from Northern Illinois University. For four years, Baker worked with The Nature Conservancy’s Nachusa Grasslands as a crew member, crew leader and volunteer, and he developed a volunteer stewardship committee with Franklin Creek Conservation Association.