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Crown Vetch

Securigera varia

Summary 7

Securigera varia, commonly known as crown vetch or purple crown vetch, is a low-growing legume vine. It is native to Africa, Asia and Europe and is commonly used throughout the United States and Canada for erosion control, roadside planting and soil rehabilitation. It has become an invasive species in many states of the US.

Management considerations: control 8

More info for the terms: cool-season, cover, fire management, invasive species, litter, natural, nonnative species, prescribed fire

Like most nonnative invasive plants, successful control of crownvetch is most likely in the early invasion stages (review by [139]). Because crownvetch is often restricted to disturbed sites and other habitats primarily dominated by nonnative species [32,70], nontarget effects of control may be reduced, but a lack of native plants and seeds may require active revegetation on control sites. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [88].

In all cases where invasive species are targeted for control, no matter what method is employed, the potential for other invasive species to fill their void must be considered [16]. In the Lost Mound Unit of the Upper Mississippi River National Wildlife and Fish Refuge in northwestern Illinois, Kentucky bluegrass (Poa pratensis) increased dramatically when crownvetch cover was reduced by herbicide treatments. The study area was dominated by little bluestem and sand dropseed (Sporobolus cryptandrus) with monoculture patches of crownvetch. One year after herbicide treatments, crownvetch cover was less than 1% on treated sites, significantly less than that of untreated sites (P<0.0001). Available soil nitrogen was significantly (P<0.0001) greater inside than outside crownvetch patches but was not significantly affected by the herbicide treatment. Kentucky bluegrass cover was greater than 50% on treated plots and averaged 14% on control plots. Native species cover was not significantly different on treated and untreated sites. Kentucky bluegrass may have utilized high nutrient levels better than the native species [134].

Fire: For information on the use of prescribed fire to control crownvetch, see Fire Management Considerations.

Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities 88,116 and by monitoring several times each year [62]. Discontinuing the use of crownvetch in the revegetation of roadsides and mine sites could be key to preventing crownvetch establishment in areas where it does not yet occur (review by [22]).

Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [51]. Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [146]. See the Guide to noxious weed prevention practices [146] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural Seeding or planting native plants in crownvetch stands may provide some control. Grover (2001 personal communication cited in [139]) suggests planting cool-season native grasses, such as switchgrass, into sites invaded by crownvetch or on sites where crownvetch was treated. In tall fescue and crownvetch stands along roadsides in West Virginia, establishment and growth of seeded native species were best when stands were herbicide treated or tilled before seeding [118].

Physical or mechanical Small crownvetch populations may be controlled by hand-pulling, digging, or heavy shading by cloth or mulch [44]. For larger crownvetch populations, mowing may be a more useful mechanical control method.

Hand-pulling or digging of crownvetch is time consuming, labor intensive, and requires the removal of all stems, roots, and rhizomes. Sites should be monitored for seedlings and sprouts in successive years (review by [139]). Mowing to control crownvetch spread may be most effective in the late spring or at flower bud stage (reviews by [119,121,123,139,151]), although Henson [47] reported that crownvetch recovery was slow when plants were cut for hay at the full bloom stage. Repeated mowing is necessary (reviews by [119,121,123,139,151]). A review recommends mowing twice a year, first in June and again in late August [22]. Another review suggests that mowing treatments should avoid native or desirable vegetation [123]. While mowing may not be feasible in wildlands, controlling crownvetch in adjacent public use areas may limit its spread into wildlands.

Repeated mowing may be necessary to deplete the carbohydrates stored in crownvetch roots. Immediately following clipping, total available crownvetch root carbohydrates generally decreased but recovered quickly as aboveground stems grew. Researchers found that clipping crownvetch early in the growing season (before 1 June) or late in the growing season (August-October) allowed other "weedy" species to invade crownvetch stands. Clipping also limited seed production [77].

Biological Crownvetch has insect predators in North America and may be reduced by livestock grazing. During a 2-year survey of insects in Pennsylvania in the early 1970s, Wheeler [161] collected 125 phytophagous arthropods in crownvetch stands. Differential and redlegged grasshoppers were the most destructive crownvetch pests, but alfalfa plant bugs, potato leaf hoppers, clover stem borers, pollen beetles, and glechiid moths also injured or stressed crownvetch plants [161]. In Kansas, researchers found the striped willow flea beetle using crownvetch as a host plant, suggesting it may be useful as a biological control in areas where crownvetch is invasive. However, the striped willow flea beetle has other known hosts, primarily willows (Salix spp.) and cottonwoods (Populus spp.) [108]. Several studies indicate that crownvetch abundance is typically lower on grazed than ungrazed sites. Livestock may reduce crownvetch abundance by removing aboveground biomass, compacting soils, or both. For more information, see Grazing.

Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [149,165] and the Weed control methods handbook [140] for background information and important considerations for developing and implementing biological control programs. Because crownvetch is still planted for erosion control and in pastures, release of a biological control is unlikely (review by [139]).

Chemical While herbicides are often effective in gaining initial control of a new invasion or a severe infestation, they rarely provide complete or long-term weed control [18]. See the Weed control methods handbook [140] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

For information on herbicides and applications useful for crownvetch control, see [139]. Applying herbicides before crownvetch flowers and before it develops a thick vegetative mat (late April or early May) may be most effective (Walters 2001 personal communication cited in [139]). To kill crownvetch plants, complete coverage of aboveground material is necessary. Prescribed fire to remove litter may improve herbicide coverage (reviews by [119,121,139]).

Integrated management: An integrated weed management plan for heavily infested crownvetch sites may include cutting or burning a site, treating with an herbicide, then actively revegetating and managing to encourage dense growth of native vegetation (review by [139]).

Management considerations: impacts 9

More info for the terms: allelopathy, cover, density, forbs, natural, phase, restoration, shrub, shrubs, succession, tree, warm-season

Descriptions of crownvetch's weedy behavior range from "minor" [34] to "highly invasive" [144]. Crownvetch is often, but not exclusively, described as weedy and problematic in the Midwestern and eastern United States. Although considered aggressive in some North American locations [42,107,144], in many areas, crownvetch is restricted to disturbed sites and considered a minor threat to native vegetation [25,27,32,34,70,126,150]. Whether or not crownvetch is more invasive in the Midwest and eastern United States because of site conditions, more plantings, and/or greater time since first introductions is unknown. Oregon's Native Plant Society found crownvetch in the Willamette Valley in 2008 and suggested that crownvetch may be in the lag phase of colonization but may, in time, develop into a "high impact" species capable of modifying native habitats and/or altering ecological functions and processes [99].

Although more common in the eastern and Midwestern United States, descriptions of crownvetch's aggressive growth habit and potential to negatively impact native vegetation occur nearly throughout crownvetch's nonnative North American range. At the Konza Prairie Biological Station in Kansas, crownvetch is "aggressively invading" tallgrass prairie [42] and is considered "a serious management threat to prairies and savannas" (review by [121]). In a survey of plant and natural area experts in Wisconsin, crownvetch ranked 13th when 66 nonnative invasive plants were evaluated for their impacts on native plant communities [107]. The Eastern Region of the Forest Service ranks crownvetch as "highly invasive" [144]. As of 2001, crownvetch was considered a "significant" threat in Tennessee [135] and as of 2008, was considered a severe threat in Kentucky [69].

Introduction and spread: In some cases, where crownvetch has been used in revegetation it has displaced native plants. In a restoration handbook for the West, researchers cautioned that crownvetch is "very competitive" and often dominates revegetated sites to the exclusion of other vegetation [129]. Colorado systematists indicated that crownvetch produces dense cover rapidly and "thrives in shade" [159]. As of 2002 in Missouri, crownvetch had invaded and/or disrupted native plant communities in more than 10 counties [95]. In Michigan, crownvetch was heavily promoted as a rapidly growing ground cover good for erosion control. Crownvetch populations spread extensively along roadsides, waterways, and fields in that state [154]. For more about crownvetch spread, see Local distribution changes.

In several areas, crownvetch populations persist as monocultures. In Pennsylvania, crownvetch likely arrived as a contaminant in alfalfa seed. It was discovered in an alfalfa field in the early 1900s, and 60 years later a dense crownvetch stand remained in the field. Except for a few sumac shrubs (Rhus spp.), few other species had established in the crownvetch stand [92]. Crownvetch was used for revegetation throughout the Appalachian coal region [137]. In southeastern Kentucky, it was used to revegetate a coal mine site where, 24 years later, observers described crownvetch growth as "especially aggressive". Crownvetch spread from the original planting area into adjacent vegetation. While crownvetch has persisted on harsh and continually disturbed sites, it did not persist on Lee Canyon ski slopes in southern Nevada. Crownvetch was seeded on the slopes between 1970 and 1980 but was not found in a 1999 survey of ski runs and adjacent forests [138].

Displacement of native vegetation is the most commonly described impact in crownvetch-invaded habitats, but delayed succession and reduced reproduction were also reported. Crownvetch may limit the development of some native plants by increasing available nutrients ([134], review by [158]) or through allelopathy [90,131]. However, nutrient increases were not directly measured on invaded sites, and results from allelopathy studies are mixed. In some areas, crownvetch invasions may reduce insect diversity.

Impacts on vegetation:
Native grasses and forbs: Rapid, dense crownvetch growth can displace native plants. Experimental studies and observations suggest that impacts to native vegetation are greatest after crownvetch is established and producing substantial vegetative growth. Crownvetch growth has been described as a dense mat that "gradually chokes out more or less persistent weeds" [162]. Land managers have associated crownvetch spread with the displacement of native plants in prairies in Minnesota's Ottawa Bluffs Preserve (McGuigan 2002 cited in [139]) and in Iowa's Broken Kettle Grasslands and Ames High Prairie Preserve (Moats 2001 cited in [139]). In shale barren communities of the Allegany National Forest in Pennsylvania, crownvetch has excluded native plants from open woodlands (Keech 2002 cited in [139]). In Kentucky's Rolling Fork/Salt River Drainage area, crownvetch was associated with reductions in the abundance of tall warm-season grasses, native forbs, and 2 threatened species: Tennessee gladecress (Leavenworthia exigua) and Eggert's sunflower (Helianthus eggertii) (Mazyck 2002 cited in [139]). However, when seeded into plots with established vegetation in western Iowa, crownvetch seedling emergence and growth were "uniformly poor". Established vegetation included a bluestem (Andropogon spp.) monoculture, a mixture of tallgrass prairie species, and a mixture of tall and shortgrass species [85].

In Kentucky, crownvetch occurs in communities with Short's goldenrod (Solidago shortii), a federally endangered species. During a field experiment in Robertson County, Short's goldenrod seedlings established and flowering increased by about 3 times in plots where associated vegetation (dominated by crownvetch) was removed. The number of nonflowering Short's goldenrod ramets increased each year associated vegetation was removed, and Short's goldenrod seedlings occurred in 9 of 10 treated plots but not in control plots [156]. In a later vegetation survey of Harrison County, Kentucky, researchers found a Short's goldenrod population in a brush prairie gravel wash community where crownvetch also occurred. Impacts were not evaluated in this study [54].

Crownvetch can limit success in prairie restoration. On the Walnut Creek National Wildlife Refuge in Iowa, crownvetch occupies 1,000 of acres and is considered one of the most problematic species in restoration projects (Shutte personal communication [37]). On a research farm in Monoma County, Iowa, researchers attempted to restore a smooth brome (Bromus inermis) pasture to prairie. In 2002, the pasture was herbicide treated, plowed, and planted to prairie species. In 2003, crownvetch was present but only in trace amounts. By May 2005, crownvetch covered 46% of the restoration area, and the project was abandoned. When the project began, crownvetch was restricted to a ditchbank adjacent to the pasture (Wilsey unpublished data cited in [85]).

Woody plants: On some sites, crownvetch may limit shrub and tree seedling recruitment and delay succession. In a revegetation guide for coal mine spoils in the eastern United States, Vogel [152] recommended against planting crownvetch with tree seedlings, and a review reports that crownvetch often shades out shrub and tree seedlings [44].

Crownvetch plantings in Kentucky, Pennsylvania, and Maryland restricted the establishment of woody species. Along a road in Campbell County, Kentucky, crownvetch dominated the understory within and outside 8- to 10-year-old staghorn sumac (Rhus typhina) clumps. Researchers found few tree seedlings in the area and suspected crownvetch was limiting tree establishment, because in other succession studies, tree seedlings were often present in 8-year-old staghorn sumac stands [86]. On 51 sites in Pennsylvania and 12 sites in Maryland seeded with crownvetch at least 10 years earlier, woody plant density generally decreased as crownvetch cover increased. On sites with no crownvetch cover, there was 1 woody plant/160 feet² (15 m²). When crownvetch cover was 80%, there was 1 woody plant/1,540 feet ² (143 m²) [115].

On sites in West Virginia and Ontario, however, crownvetch growth did not limit the establishment of woody vegetation. Within 2 years of seeding crownvetch on a roadside in West Virginia, smooth sumac (Rhus glabra), sycamore (Platanus occidentalis), and sugar maple (Acer saccharum) were "naturally encroaching" in dense crownvetch cover [100]. For up to 5 years after crownvetch was seeded on a utility right-of-way near Tobemory, Ontario, tree regeneration was not restricted [17].

Allelopathy: Results from studies designed to uncover crownvetch's allelopathic potential are mixed. Leachate from crownvetch seeds did not significantly (P<0.05) reduce the germination of a variety of field, forage, turf, weed, flower, and vegetable species, but often seedlings were abnormal. Roots and shoots of seedlings emerging in crownvetch seed leachate were shorter than those of seeds germinated in water [90]. Studies conducted at the Ohio Agricultural Research and Development Center showed that growth of northern red oak (Quercus rubra) was inhibited when seedlings were watered with crownvetch foliage extracts (Larson unpublished data cited in [153]). In another study, crownvetch extracts stimulated germination of quackgrass (Elymus repens) and crownvetch but inhibited germination of smooth brome and meadow fescue (Schedonorus pratensis). Extracts were made from live crownvetch shoots collected from an abandoned field in Argonne, Illinois. An extract made from decomposing crownvetch roots or shoots rarely affected the growth of other plants. Often the extracts from decomposing crownvetch roots stimulated the growth of crownvetch, quackgrass, smooth brome, meadow fescue, and timothy (Phleum pratense) [131].

Impacts on insects: Several studies related crownvetch invasions to unbalanced insect herbivory and reduced insect abundance and diversity. In tallgrass prairie at the Konza Prairie Biological Station in northeastern Kansas, total leaf area reductions due to natural enemies and/or abiotic stresses were significantly greater for a native slimflower scurfpea (Psoralea tenuiflora) than for crownvetch [42]. When bee populations were compared in "restored" prairies and "weedy" roadsides in Kansas, average bee richness and abundance were significantly (P<0.05) greater in restored than weedy vegetation. Nonnative forbs including crownvetch, sweetclover (Melilotus spp.), and common teasel (Dipsacus fullonum) covered more than 50% of weedy roadsides. Restored prairies likely provided a greater diversity of insect foods than the roadsides. Before the study, prairie and roadside vegetation was mowed multiple times each year, and roadside vegetation was treated with herbicides. Effects of prior management on bee populations were not discussed [55]. In western Wisconsin and eastern Minnesota, researchers found that Melissa blue butterfly (Lycaeides melissa melissa) populations have expanded their range and are using crownvetch as a larval host. This range expansion puts Melissa blue butterfly populations near endangered Karner blue butterfly (L. m. samuelis) populations. Although it is unknown whether these species can hybridize and produce viable offspring, hybridization could threaten Karner blue butterfly as a distinct taxon [23].

Sources and Credits

  1. (c) Jerry Oldenettel, some rights reserved (CC BY-NC-SA), http://www.flickr.com/photos/7457894@N04/2768375538
  2. (c) Bas Kers (NL), some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/21933510@N07/3890367209/
  3. (c) Jerry Oldenettel, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/jroldenettel/4694267281/
  4. (c) Joshua Mayer, some rights reserved (CC BY-SA), https://www.flickr.com/photos/wackybadger/4315475240/
  5. (c) Tom Potterfield, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/tgpotterfield/14456427103/
  6. (c) zimbabim, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/simbalee/3990532763/
  7. Adapted by Kate Wagner from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Securigera_varia
  8. Public Domain, http://eol.org/data_objects/24628296
  9. Public Domain, http://eol.org/data_objects/24628295

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