Invasive Case Study: Birdsfoot Trefoil

Sometimes I wonder – when I will learn some important ecological lessons? Lately at Wellfield, we’ve been doing a lot of weeding; during that time, I have noticed a familiar weed called Lotus corniculatus, or birdsfoot trefoil. I became conscious of this Eurasian/North African species several years ago while touring a prairie in Illinois which the Nature Conservancy manages. The guide spoke about dealing with this dense mat-forming legume. I returned from the trip on alert.

Birdsfoot trefoil was originally introduced, like many exotics, for agricultural applications and erosion control. As a matter of fact, I even found a university extension website still touting it as a great forage species! However, Birdsfoot can form such dense, ground hugging matts, it can choke out the native seedlings attempting to get a foothold in the soil.

Birdsfoot trefoil is another classic case study in invasive species epidemiology. When I see a “weed” or other “pest” in my garden, my first reaction must not be to kill, kill, kill in blind rage, but rather to step back and ask questions. First, what do I have before my eyes? Second, why is it there in the first place, and third, do I really need to do anything about it? It is the why question I want to address in this article. When a weed shows up on my humble soil plot, I need to ask myself why it is there. More importantly, what ecological niche is it filling? Nature hates a vacuum, even in my border bed.

Some present day invasive species, native or exotic, were present in the North American landscape for extended periods without showing any epidemic patterns, as I discussed at some length in a blog article earlier this month (Orchids in my yard?). Like other opportunistic species, birdsfoot trefoil loves disturbance. It thrives where man or nature have left bare, open ground. It thrives in many different soils and withstands drought conditions, as I can testify to, while pulling out well rooted young seedlings. 

So how to deal with this baby? This is where the what and why questions inform the how. What niche is Lotus corniculatus filling? I am not sure I have a full answer, but my guess is it is filling a couple of important ecological roles. One, it is a legume, fixing nitrogen in the soil, from which other species and soil biota benefit. Second, it is a soil stabilizer. Ecosystems are designed to conserve soil, nutrients and energy flows. Birdsfoot being utilized as an erosion control measure speaks to this truth. It does a great job of covering disturbed, bare ground, holding soil in place. Knowing these two facts can inform a control strategy. For existing stands, whatever removal method is chosen must be coupled with replacing native species appropriate for that ecoregion and role. Small populations can be removed by hand while limiting further disturbance. I would avoid chemical applications if possible, as you may not know what collateral damage it might do to both soil species and other surrounding plants. I do not know how well it responds to organic burn down products, but it might be worth a try. If time is on the landowner’s side, there might be other non-chemical options. Once the stand is under management, one might try Chamaecrista fasciculata, or partridge pea. It is a legume, fairly quick to establish, and I have seen it growing well on a dry sandy slope.

A square foot of prevention is worth an acre of cure in my book. The greatest tool a land steward has at their disposal is creating, full healthy, biodiverse habitats where the ecological holes are filled. Sorry, there are no vacancies in this neighborhood; you will have to look for some real estate down the street at the Jones’.

Josh Steffen, Horticulture and Facilities Manager

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