Many prairie restorationists are at the moment in the throes of eradicating sweet clover, one of the most persistent and annoying invasive plants. There are two species, Melilotus alba
(white flowers) and M. officinalis
(yellow flowers). The latter flowers about a week before the former, and both species can be found at a single site. It is a biennial, but it is only the second year plants that are visible enough to eradicate easily.
Sweet clover was introduced into North America in the 17th century, and large acreages have been planted for pasture. Beekeepers also planted it. Although no longer common as a field crop, it persists widely in natural areas. (It is less of a problem in planted prairies because their sites have been prepared by extensive herbicide spraying.)
The two sites that Kathie and I manage, Black Earth Rettenmund Prairie and Pleasant Valley Conservancy, both have lots of it. Despite detailed control efforts over the past six or seven years, it still remains a major problem for us. The photo below shows our interns and employees working on a particularly large patch at Black Earth Rettenmund Prairie. We spent the whole afternoon here and are not even close to finishing.
A major reason that sweet clover is so troublesome is that its seeds persist for many years in the soil. Interestingly, fire plays a major role in breaking seed dormancy. Many natural areas that have not been burned for some years suddenly become heavily infested in the second year after a burn.
Unfortunately, the method of choice for control of sweet clover is hand pulling. It is best pulled when the soil is moist, and it is essential that the whole plant, root and all, be removed. A two-handed pull is best, grasping the stem near the base and slowly pulling the whole plant out of the ground. If the whole root is not removed, dormant buds will grow. If the ground is too dry for easy hand pulling, a shovel must be used to cut the root well below the surface. The photo to the left show what the root system looks like. This photo also shows that many sweet clover plants are multi-stemmed.
Cutting with a brush cutter can be used for patches of sweet clover too large for hand pulling. However, timing of cutting is critical. It is essential to wait until the patch is in full flower, at which time most of the energy is in the stems. However, if the patch is cut too late, mature seeds may already have been made. Further, flowers in their later stages can continue to make mature seeds even after being cut.
Because the goal of eradication is to prevent new seed formation, it is essential that every flowering plant be removed. This requires careful work; persistence is essential.
Unfortunately, sweet clover also exhibits fall regrowth, and most of these plants flower. They are much smaller than those seen in the summer, and have fewer flowers, but they still must be dealt with. At Black Earth Rettenmund Prairie we usually see these plants when we are collecting seeds of good plants. At times, we spend more time pulling fall-grown sweet clover than collecting seeds.
So far, I have never worked on a site where sweet clover has been completely eradicated.