2017-07-14 / Columnists

Nature Notes

Crown Vetch Takes Root on Island
By Patricia Martin

At a gathering a couple of weeks ago, a friend came up to me most upset because the owners of a property near hers had planted a hillside of crown vetch (Securigera varia or Coronilla varia). She has some reasons to be concerned. I rode by the area in question and the crown vetch is growing quite well.

Crown vetch, also known as purple vetch or purple crowned vetch, is a perennial legume in the pea/legume family (Fabaceae or Leguminoseae). (By the way, it is sometimes referred to as axseed, axwort, hive-vine, or trailing crown vetch). It is a low-growing vine native to North Africa, Asia, and Europe, which was brought to North America from Europe in the 1950s. It is used to control erosion, as a roadside planting, and for soil rehabilitation. Its deep tenacious, complex root systems do help in erosion control and its thick, fern-like leaves make a good ground cover; however, because of its long germination period, it does not create full coverage until two or three years later. It also is used to help with nitrogen fixing in soils that have been over-farmed and are depleted. It is now found in most of Canada and the United States.

Crown vetch is a vine that forms large clumps from creeping stems. Individual stems can be up to six feet long. The underground rhizomes can grow up to 10 feet long, which allows the plant to spread rapidly. A single plant may fully cover 70 to 100 square feet within a four-year period. The leaves of the plant are compound with 15 to 25 pairs of oblong leaflets, giving it a fernlike appearance. The flowers are typical “pea” flowers, generally pinkish, ranging from pinkish-lavender to white, and are clustered in umbels at the end of long stalks. This plant blooms from May through August. The flowers develop into narrow, flattened pods and the seeds contained therein are considered poisonous. It spreads by both seeds and vegetatively through rhizomes. The seeds can lie dormant in the soil for more than 15 years. It generally prefers open, full-sun areas, but will also grow in partially shaded areas.

There are several reasons to be concerned about crown vetch. It is a serious management threat to natural areas because of its seeding ability and rapid vegetative spreading by rhizomes. This aggressive exotic is now widespread along roadsides and natural areas in the United States. It becomes a problem when it invades natural areas such as grassland, prairies, and dunes, where it works to exclude native vegetation by fully covering and shading out native plants. It can climb small trees and shrubs, and eventually form large, single-species stands. It is difficult to eradicate once it has been established. It has been reported to be invasive in Michigan and many other states. The United States Department of Agriculture considered it a useful, but overused erosion control, especially in places where it can get in to natural areas.

The second reason to be concerned about crowned vetch on Mackinac Island is that it is toxic to horses and other non-ruminants because of the presence of nitroglycosides. If consumed in large amounts, it can cause slow growth, paralysis, or even death. Signs of poisoning in horses include weight loss, depression, and ataxia (lack of muscle coordination). The good news is that some sources indicate that horses generally do not care to graze on it; but if you know horses, if they are hungry enough, or if it is baled into hay, they may very well eat it. If you have horses, it’s a good plant to remove from your property. For ruminants such as cattle, goats, and sheep, it is not poisonous, as the nitro compounds are degraded in ruminant digestions and so do not affect these animals.

Crown vetch is tenacious once it has been established. There are several methods for controlling it. In small areas, pulling it repeatedly by hand, if it has just begun to be established, can be effective. In fire adapted communities, prescribed burning in late spring can be an effective control, but it must be repeated for several years. Late-spring mowing for several years can help to control this species. Another technique is to mow in June and again in August, corresponding to successive leaf-out periods. Chemical controls include application of glyphosate, a broad-spectrum herbicide that can be applied to the leaves in a 1% or 2% solution in the spring, but it is nonspecific and care should be taken to avoid non-targeted plants. Follow-up applications may be needed in the fall or the following spring. There are other herbicides that may be used. Combinations of burning/mowing and chemical control can be effective.

Of course, the most effective way is not to plant it in the first place, or if it volunteers on your property, remove it before it can be firmly established. There are native plants that could act in the same way for erosion control and can replace crown vetch. Some of these are: spreading dogbane (Apocynum androsaemifolium), Canada milk vetch (Astragalus canadensis), Canada tick trefoil (Desmodium canadense), and round-headed bush clover (Lespedeza capitata). One of my favorite alternative suggestions is to plant our native and locally ubiquitous Virginia creeper (Parthenocissus quinquifolia), the wonderful vine that has five leaflets and turns a brilliant red in the fall. I spend part or all of my gardening time trying to remove it from growing up trees and fences on my property. It also makes a good cover for chain link fences, and it sure isn’t hard to grow.

Trish Martin is a year-around resident of Mackinac Island, has earned a master’s degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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