By: Barbara Sherwood and Julianne Mason, Forest Preserve District of Will County, IL

Key Takeaways

• A late-August prescribed fire killed all mature sericea lespedeza (Lespedeza cuneata) plants where the fire burned with at least moderate intensity.
• Herbiciding the base of sericea plants with a mixture of Triclopyr 4 herbicide in bark oil during early September killed 100% of treated mature plants while causing minimal collateral damage to neighboring native forbs.
• One year after treatment, seedlings were present near previously occupied sericea locations, indicating that follow-up monitoring and treatment will be necessary to exhaust the soil seed bank.

Background

At one of our prairie preserves near Braidwood, Illinois, we have incorporated late-August prescribed burns into a rotating program of primarily dormant season burns. The summer burn timing is intended to benefit the Rattlesnake Master Borer Moth (Papaipema eryngii), an endangered insect in Illinois.  By late August, the larvae have tunneled belowground into the roots of rattlesnake master (Eryngium yuccifolium), where they are protected from fire. In contrast, dormant-season burns may consume eggs that have been deposited in the  duff layer. This vulnerability is shared by many prairie insects, both common and rare, reinforcing the importance of burning only portions of a site in any given year and rotating burn units over time.

Growing-season burns also have potential ecological drawbacks. Consequently, we limit these burns to areas where rare reptiles and amphibians are absent and burn relatively small units, minimizing the risk of direct mortality to species that may be active aboveground during the growing season.

Although an interesting topic, the potential benefits and drawbacks of growing season burns isn’t the point of this article.  After the growing season burn last year on August 26, 2025, our crew did a post-burn basal bark treatment to unwanted woodies in the burn unit.  This is a highly effective management technique which we strongly endorse – see here – but it’s also not the point of this article.

Treatment Methods

Now we’re coming to the point of this article.  The unit burned in August 2025 is a 10-year old prairie restoration that has scattered sericea lespedeza (Lespedeza cuneata), a highly invasive perennial legume. While the crew was doing the post-burn herbicide treatment to woodies on September 2, 2025, they also “basal barked” or base sprayed several dozen mature sericea plants.  This involved using a backpack sprayer to treat the base of the sericea plants with 15% Triclopyr 4 herbicide mixed in bark oil. The lower two inches or so of the sericea were sprayed, both front side and back side, with the herbicide mixture. At the time of treatment, the sericea plants were just starting to flower.

To evaluate treatment effectiveness, pin flags and GPS coordinates were used to mark about a dozen individuals in three treatment groups:

1) Sericea plants that received the prescribed burn only.

2) Sericea plants that received the prescribed burn followed by base spray herbicide treatment.

3) Sericea plants in an adjacent unburned area that received base spray herbicide treatment only. 

Fire intensity varied across the burn unit. Many sericea stems remained standing following the burn, exhibiting varying degrees of scorching from light to moderate. For each marked plant, the degree of scorching and whether green tissue remained one week after the fire were recorded.

Results

One year after treatment, all plants that had received the base spray herbicide application were dead, regardless of whether they occurred in burned or unburned portions of the site. This represents 100% mortality among the treated plants.  An additional benefit of the base spray technique was the confined amount of overspray. Damage to adjacent native forbs was minimal and substantially less than would typically be expected from a foliar application of either triclopyr ester (T4) or triclopyr amine (3A).

Prescribed fire alone also proved highly effective. Every mature sericea lespedeza plant exposed to at least moderate scorching was dead one year after the burn. The single surviving plant experienced only very light scorching and retained green foliage one week after the burn.

In a previous Grassland Restoration Network post, Jeff Horn (Illinois Department of Natural Resources) documented sericea lespedeza treatments that included summer prescribed burning. At his site, some sericea plants resprouted following fire. However, because summer burns can be patchy and vary considerably in intensity, tracking individual plants can help establish the relationship between burn coverage, fire intensity, and plant mortality.

At our site, all tracked sericea plants that experienced at least moderate scorching were dead at the one-year evaluation. These results suggest that where summer fire achieves moderate or greater intensity at the plant level, it can provide substantial control of mature sericea lespedeza. However, these observations are based on a single burn at one site and should be interpreted accordingly, as noted in the disclaimer at the end of this article. It will be valuable to determine whether these results are consistent across multiple sites and years, particularly under varying weather and fire conditions.

One year after the treatments, numerous small legume seedlings were present around many of the locations formerly occupied by mature sericea plants. Although these seedlings may be sericea, some may be sweet clover or native legumes and follow-up monitoring will be needed to verify species identifications. 

Management Implications

It will take continued monitoring and treatments to the next generation sericea plants to deplete the seedbank and reduce the invasive sericea population.  In the ongoing effort to control this invasive perennial, it’s helpful to know that both late-summer burning and base spraying with triclopyr 4 herbicide in bark oil are effective treatments. We intend to alternate these treatment methods on an annual basis as we continue to address the sericea at this preserve.

Disclaimer: This was an impromptu field study, not a formal experiment.  Sample sizes were small and not replicated across multiple sites.  However, by marking treated individuals and tracking their fate, we can get useful information on treatment effectiveness. Our collective understanding is strengthened when we monitor diligently and share observations and results from across our various sites. Even informal field observations can contribute valuable information when considered alongside results from other practitioners and locations.