Text and photos by Jake LaVigne, Arboretum Gardener, Natural Lands
As spring turns to summer, the Bennett-Johnson Prairie at the Arboretum begins blooming in shades of blue and white. Wild white indigo and northern bedstraw, with their bright white blooms, stand in stark contrast to the greenery around them while the cooler blues of Ohio spiderwort provide more subtle embellishment.
Ohio spiderwort (Tradescantia ohiensis) provides cool blue blooms to the prairie.
The vast variety and diversity of life supported by prairies is the reason many of us who work in such environments draw such satisfaction from our work. However, as in any worthy undertaking, there are significant challenges to the maintenance of a healthy prairie. Invasive species can reduce the variety of species found in a prairie or, in extreme cases, overwhelm an area altogether.
Crown vetch is an invasive species that must be controlled in the Bennett-Johnson Prairie at the Arboretum.
Too good at covering ground
One of the most easily recognizable invasive species this time of year is crown vetch. Minnesotans can see this pink-blooming groundcover blooming along roadsides in large swaths in late June and early July.
Crown vetch spreads quickly and aggressively through both prolific seed production and rhizomatous growths below ground. For this reason, it was used as an agricultural cover crop and a soil stabilizer on hillsides. Once introduced into a prairie setting, however, it can quickly overtake native vegetation, forming dense patches that shade out the plants below.
In addition to its detrimental effect on the diversity of prairie species, it can also pose threats to local pollinators. While bees and other pollinators do forage upon crown vetch, its potential to dominate an area outweighs its temporary usefulness as a source of food.
A robust and diverse prairie will provide forage for a vast array of pollinators all season long.
By contrast, a robust and diverse prairie will provide forage for a vast array of pollinators all season long, as well as winter shelter for a variety of insects. A field of crown vetch will provide a temporary feast, followed by a lengthy famine once their flowers have faded. Furthermore, a healthy prairie will support all manner of specialist pollinators with very selective tastes.
Wild lupine, for example, is the only host plant for the caterpillars of the endangered Karner blue butterfly. The generalist pollinators that crown vetch could support will thrive just as easily upon native flowers. Therefore, the removal of crown vetch is a net gain for the pollinators that depend upon prairies for their survival.
Wild white indigo with its bright white blooms stands in stark contrast to the nearby greenery.
Sporting rhizomes and seeds
From a management perspective, crown vetch poses additional challenges. Beyond its ability to spread quickly it can be remarkably difficult to remove once established. Because it flowers early in the summer, it must be dealt with quickly before it has the chance to produce viable seed. However, it also spreads through rhizomes, making it extremely difficult to remove mechanically. Any rhizomatous growths that remain in the soil have the potential to become new plants. Because of these challenges, the application of herbicide is the most effective tool we have to eliminate crown vetch in our prairies.
Using herbicides and surfactants
Once the decision to undertake an herbicide application has been made, there are a host of other considerations to take into account. Which chemical to use? What conditions will be most favorable? How much potential for non-target damage exists? The use of herbicide is only valuable when its benefits outweigh the potential drawbacks. We therefore take steps to ensure the maximum efficacy of our applications while at the same time minimizing potential risks.
For starters, we select products that will selectively harm crown vetch. Broadleaf specific herbicides will kill broadleaf plants while minimally affecting grasses. This will allow native prairie grasses to quickly fill the space left by the dying vetch plants. We also use adjuvants, products that modify our herbicide mixture, to further maximize the effectiveness of our applications.
Surfactant, one type of adjuvant, helps the spray mixture stick to the target plant and minimize runoff. Drift control agents reduce the risk of tiny droplets traveling on the wind and landing in undesirable places. Indicator dyes provide a visual record of where we’ve sprayed to reduce the risk of us walking through treated areas or unnecessarily treating an area twice.
Crown vetch wilts after treatment with herbicide in the Bennett-Johnson Prairie at the Arboretum.
Spot spraying carefully
Once we’ve decided upon the chemicals to be used, we mix them with water in portable backpack sprayers and head out into the 20-acre prairie. Spot-spraying, as this type of application is called, allows us to be as precise as possible when applying herbicides. We also choose times of day that minimize contact with foraging pollinators and will avoid spraying on windy days or when impending rain is likely.
If you see us out in the prairie, backpack sprayer and bug-net equipped, surrounded by a cloud of mosquitoes and deer flies, give a wave. We’re doing our work as carefully and thoughtfully as possible, and we do it for a good reason. Through the removal of crown vetch and other invasive plants, we can maintain and enjoy the rich variety of plant and animal life that prairies support for many years to come.
Jake LaVigne is a Gardener at the Minnesota Landscape Arboretum who has worked with the Natural Lands team since 2017. He helps to maintain the Bennet-Johnson Prairie, the Dayton Woodland Wildflower Garden and the Johanna Frerichs Garden for Wildlife. When he isn't helping native plants to thrive, he strives to keep invasive species at bay. In his spare time, Jake writes fiction, plays board games, and is teaching himself taxidermy.