Sweet Clover
It has been said that every plant has its year and that every year has its plant. In that case, I would call this the year of sweet clover. Sweet clover (yellow: Melilotus officinalis and white: Melilotus alba) is native to Europe and was brought to the U.S. in the late 1600s. It was, and still is used as a forage crop and soil enhancer throughout in the Great Plains and Upper Midwest. In an agricultural setting, its ability to fix nitrogen in the soil (it’s a member of the legume family) and its aggressive growth make it invaluable. In our prairies, sweet clover grows just as aggressively and shades out native sun-loving plants and creates a monoculture.
We have always had a few pockets of sweet clover, but in early June of this year I began to notice new populations popping up in many of our prairie restorations. Neither the age of the prairie, the soil type, nor whether it was dominated by grasses or flowers seemed to matter. Then I realized one thing the infested prairies have in common – they were all burned in 2008.
Sweet clover is a biennial; it grows low and green in its first year, then in its second season it flowers, sets seed and dies. First year plants remain inconspicuous. Second year plants in flower are generally 3-5 ft in height, although I can personally attest to seeing plants taller than the cab of the tractor.
I prefer to manage sweet clover at the Belwin Conservancy mechanically rather than chemically. Because sweet clover is a biennial, if we interrupt its life cycle by mowing it down before the plant is able to reproduce, the plant will die without producing any offspring (seed). Sweet clover can be hand-pulled, although the deep taproot on even the smallest of plants could cause you to throw your back out. No, we usually we mow this plant with a tractor or brush cutter.
The trick is getting the timing exactly right. If you mow too early, the sweet clover has enough time to regroup and flower again. If you mow too late, it may have already set seed. Depending on how long the population of sweet clover has been established, there may be seed present in the soil that will germinate in future years. This means any population must been managed for several years in a row, until the seed source is depleted.
Why has sweet clover shown up in so many of the prairies that we burned two years ago? It turns out that fire actually stimulates the germination of sweet clover by activating its seed. The first year, plants germinate after the fire and thrive in the flush of nutrients released. In 2009, it was the flowering second year plants that got our attention.
The benefits of prescribed burning still far outweigh the potential costs of a potential sweet clover outbreak. For one thing, many invasive species are negatively affected by a prescribed burn. The key is to recognize the potential for an outbreak and manage appropriately.
However, I’m left with a couple of nagging questions. Where did this seed come from in the first place? I’ve read that sweet clover seeds can remain viable for 30 years. Could the wind have blown in that much of it? Was it present in the soil even before the fields were planted to prairie? If that’s the case, why didn’t we see it when these prairies were burned in previous years?
We may never really know the answers to these questions. For now, we’ll be content staying on top of the sweet clover management in our prairies and continuing to learn about the complex interactions of weather, land management practices, and landscape dynamics that contribute to outbreaks of invasive species.