Benefits of Crownvetch as a Living Mulch



Benefits of Crownvetch as a Living Mulch

Crownvetch (Coronilla varia L.) is a perennial legume adapted to the temperate climates of the northern two-thirds of the United States. It is native to central and southern Europe, North Africa, and the Near East. Crownvetch was most likely introduced in the late 1800’s and early 1900’s as a contaminant in alfalfa and other legume seed. From these chance introductions, the variety Penngift was named from a selection in eastern Pennsylvania. Crownvetch is a hardy, deep-rooted plant that has proven its value for soil stabilization and erosion control for roadsides, steep banks, strip mine sites, and other non cropland areas.

Contrary to information found on the internet and espoused by plant ecologists, state and governmental experts and local activists, crownvetch is not invasive by my definition. To be invasive, a plant has to spread on its own, establish itself in cropland and non-cropland environments with relative ease, and be able to compete with other native and non-native vegetation already established at the site. After growing crownvetch for use as a living mulch for corn and agronomic crop production for 30 years, I have never seen it spread more than 10 or 20 feet on its own. This spread was by growth of underground root stocks and not by seed. Crownvetch seedlings are so non-competitive that they can only survive where there is little or no competing vegetation. Once established, crownvetch is competitive and this characteristic along with its ability to spread and fill in via growth by underground root stocks is why it is ideally suited to soil stabilization and erosion control for roadsides, steep banks, strip mine sites, and other non cropland areas. Unfortunately, it's not quite competitive enough to totally prevent invasion of other weeds and woody perennials but it is still one of the best and one of the prettiest ground covers presently available for sites that commonly have such poor soil that little else will grow. Its use as a living mulch for cropland use is only possible with the help of herbicides to control competing vegetation and for suppression of crownvetch itself so it won't compete with the primary crop.

Establishment is very slow but after 2 to 3 growing seasons, you should have a good stand. Initially glyphosate, atrazine, simazine, dicamba, or 2,4-D cannot be applied to crownvetch without killing it. Crownvetch can never be moldboard plowed or it will be lost. Even min-tillage with a chisel plow or heavy offset disk more than once every 2 or 3 years will severely thin the crownvetch stand. Crownvetch recovers from winter dormancy late in spring and only begins to make significant growth by corn planting time, so there is little early competition. After it is well established, crownvetch can be chisel plowed, disked and harrowed every second or third year with minimal damage because it recovers from buds on the rootstocks. All tillage operations should be accomplished in a week or less to prevent excessive thinning of the crownvetch stand however. Mechanical suppression normally is not enough to prevent it from competing with corn, soybeans or small grains, so additional suppression with herbicides will be needed. Herbicide treatments listed in the following tables can be applied to crownvetch for suppression and for weed control.

A perennial crownvetch living mulch reduces water runoff, soil loss, and pesticide loss from sloping land by 98% or more. By increasing the amount of water that permeates into the soil, a crownvetch living mulch often improves moisture availability. In 1983 and 1988, both dry years, corn was found to suffer less from moisture stress when planted into crownvetch and as a result yields were greater. However, when a drought starts in April, as was the case in 1991 and 1997, there is greater crownvetch competition for moisture because the corn never has a chance to develop a root system deep enough to tap into the subsoil moisture. If the drought does not begin until two or three weeks after corn planting, corn root development gets below the crownvetch and taps into subsoil moisture. As a result, corn in crownvetch usually yields more than corn planted without crownvetch or corn following rye (see Figure 9-2).

Figure-9-2. Corn yields with a crownvetch living mulch and following winter rye compared to no cover

Figure-9-2. Corn yields with a crownvetch living mulch and following winter rye compared to no cover. The 1988 and 1991 growing seasons were very dry.

The summer of 1995 and 2002 proved an exception to this theory in many parts of Pennsylvania. Moisture availability was very good until early July but then came a period of prolonged drought lasting 6 to 12 weeks. The top 6 feet of soil were depleted of moisture so the depth of rooting did not save the corn from severe drought stress. Under these conditions, a living mulch competes for moisture and can reduce corn yields up to 20% or more. During the drought of 1999, in some parts of Pennsylvania corn was stunted so bad that farmers mowed off the cover crop and corn, put it into a silo or made round bales for hay in mid-summer before the corn dried up completely.

The real problem is to fine-tune the management system to suppress the crownvetch and weeds enough to avoid important competition with the row crop while retaining the crownvetch in viable form so it will come back late in the season and make good fall and spring growth. Herbicides are the key elements in the strategy. Research in Pennsylvania shows that with proper herbicide use it is now possible to obtain corn yields on no-till crownvetch comparable to those on no-till corn residues (Figure 3).

Figure 3. No-till corn yields with and without a crownvetch living mulch at different levels of nitrogen in 1984.

Figure 3. No-till corn yields with and without a crownvetch living mulch at different levels of nitrogen in 1984. (Mayer and Hartwig, 1986)

A crownvetch living mulch aids soil fertility by stopping nutrient loss in surface runoff. In addition, because crownvetch is a legume, it fixes nitrogen from the air. Research at Penn State indicates that yields of 100 bushels per acre are possible without any added nitrogen for corn planted into fields that were crownvetch seed fields or conservation reserve acres having a crownvetch cover. For each additional bushel of potential yield, add about 1 pound of nitrogen per acre in the form of manure or fertilizer. Some farmers have gotten good yields with no additional nitrogen, but 10 to 20 pounds applied in the row at planting time is usually required to obtain maximum yields. This row fertilizer gives the corn a faster start, enabling it to compete better with the crownvetch. When small grains or alfalfa and/or forage grasses are planted into crownvetch, they usually follow corn in the rotation, and normal fertilization required for that crop should be followed.

Crownvetch doesn't contribute any nitrogen the first year when seeded into corn and probably doesn't contribute more than 40 to 50 pounds of nitrogen per acre to second-year corn. Even so, this amount represents a savings of $10 to $12.50 an acre when nitrogen is 25 cents per pound. By the third and successive years, crownvetch contributes about 40 pounds of nitrogen each year. Fertilization with phosphorus or potassium beyond what is needed to produce a good corn crop is not necessary unless the crownvetch is pastured or removed for haylage.

Nitrogen Contribution from Crownvetch

Removing the crownvetch and crop residue for feed or bedding does not reduce its soil-erosion-controlling benefits since the root system is still intact and the ground cover is regenerated from root stocks the following growing season. A surface residue of 500 lb/A is all that is needed to control soil erosion. This is about what you get after a 95% suppression from tillage and/or herbicides. If crownvetch is allowed to make any more growth than this, it starts to compete with the primary crop. When suppressed sufficiently, crownvetch will never flower. If crownvetch starts to flower, it’s an indication it has not been suppressed enough and if possible it should be treated immediately with Basis plus Banvel. If suppressed to late, the corn will likely suffer from crownvetch competition from which it will never totally recover. Most of the suppression should be accomplished from treatments at or before planting time.

Crownvetch Roots

Crownvetch Roots

The presence of crownvetch will help control escape weeds and may prevent or slow the invasion of new weeds that otherwise might become a problem. Yellow nutsedge has been controlled very nicely by competition from a crownvetch ground cover. It appears that competition from well established crownvetch will reduce dandelion problems as well. The idea of using crownvetch or some other ground cover as a “designated weed” and learning to live with it is very appealing compared to the constant battle of fighting an ever-changing weed spectrum.