Securigera varia
(crown vetch)

S. varia is a perennial herbaceous vine from the pea family that has been introduced into various countries as an ornamental, for erosion control, ground cover, soil improvement, as a cover crop and as fodder/forage for livestock (GISD, 2016, US Forest Service, 2016).

The species is considered a threat in the USA, for its rapid vegetative growth resulting in monocultures and its detrimental effect on the native vegetation (Molano-Flores, 2014; Invasive Plant Atlas of the United States, 2016). The multi-branched rhizomes can extend up to 3 m long; and the stems can extend up to 2 m long; plants can fully cover 20-30 square metres in four years (Molano-Flores, 2014; US Forest Service, 2016). Luneva (2009) cites it as a dangerous weed that is a problem in rye, oat, summer and winter wheat crops and in vineyards; seeds littering the cereal grains, being hard to separate. There are no invasive behaviour reports for the species in western Europe (Alien Plants of Belgium, 2016).

It is reported as invasive in many states of the USA, and in British Columbia in Canada (Molano-Flores, 2014; Casals, 2016; E-Flora of British Columbia, 2016; US Forest Service, 2016). It is included in a list of noxious plants of concern in Mexico, with the recommendation of doing a country assessment for its eradication or containment (Sánchez-Blanco et al., 2012). It is also included in the Global Invasive Species Database of the IUCN (GISD, 2016). An invasiveness assessment carried out for Alaska gave it an invasiveness rank of 68 on a scale of 1-100 (Alaska Natural Heritage Program, 2016).

S. varia is cited as native to Europe, Asia and North Africa (GISD, 2016), although FAO (2016) considers it as native only to Central Europe. While the GISD (2016) and some American sources list it as native to North Africa, no specific country records could be found for this region.

The species is found in Asia, Africa, North America, the Caribbean, South America; Europe and Oceania (See Distribution Table for details). It is recorded in all states of the USA except for North Dakota (US Forest Service, 2016; USDA-NRCS, 2016).

S. varia is a perennial vine that is considered as a minor threat in some countries and as highly invasive in others (Gucker, 2009). The risk of introduction is in temperate areas. Due to its aggressive vegetative growth, the species impacts native habitats by reducing the biodiversity, and by causing changes in ecological and successional processes (GISD, 2016). It is also reported as impacting threatened species (Gucker, 2009; Molano-Flores, 2014). The species could be easily introduced to new areas as it is available over the internet and locally to be used for ornamental purposes, erosion control, soil improvement, as a cover crop and as a fodder/forage for livestock (GISD, 2016; Minnesota Wildflowers, 2016; US Forest Service, 2016).

In countries where it is reported as introduced, S. varia is found on mesic prairies, weedy meadows, drainage ditches, roadsides, railway embankments, gardens, open fields, waste areas, dry grassy places, utility corridors, forest margins, secondary dunes, banks of rivers and gravel bars along streams (Encyclopedia of Life, 2016; GISD, 2016; PFAF, 2016). In its native range, the species is reported to occur in meadows, grasslands, savannas, and disturbed areas (Gucker, 2009).

Genetics

The reported chromosome number for S. varia is 2n=24 (Löve, 1980).

Reproductive Biology

The primary form of reproduction in S. varia is vegetatively though the production of abundant spreading rhizomes, and regeneration from stem and rhizome fragments (Losure et al., 2009; Gucker, 2009). A single plant can cover 70-100 m² in four years and form dense colonies in some areas (Gucker, 2009).

S. varia is almost dependant on pollinators for seed production (Encyclopedia of Life, 2016). In controlled experiments the number of seeds produced from selfing was extremely low and when insects were excluded from plants in the field, no seeds were produced (Gucker, 2009). The nectar and pollen of the flowers attract long­tongued bees, including honeybees (Gucker, 2009; PFAF, 2016). Megachile addenda and Megachile campanulae have been also been reported as pollinators of the species (Encyclopedia of Life, 2016). Seeds are produced in the second year of growth. Fruits take 6 to 10 weeks to mature after pollination (Gucker, 2009).

Seed remain viable in soil for less than a year to 5 years (Encyclopedia of Life, 2016; GISD, 2016), but are also reported as viable by Gucker (2009) for up to 15 years. Germination period is long, of about 6 months (ZipcodeZoo, 2016). Seed production is low, and is affected by various factors, including high precipitation, low light, low temperatures and availability of pollinators (Gucker, 2009). The germination improves with scarification of the seeds and exposure to warm temperatures of up to 24°C (Encyclopedia of Life, 2016). Losure et al. (2009) report that seeds are important in establishing new populations, but once plants are established the reproduction is primarily vegetative; and in some areas reproduction by seeds is almost non-existent.

Physiology and Phenology

S. varia produces flowers from May through November (GISD, 2016; ZipcodeZoo, 2016). Flowering and seed production occurs throughout the growing season, having open flowers and mature fruits on plants at a time (Gucker. 2016).

Longevity

Gucker (2009) reports the species as "long-lived" up to 20 years; with some plants surviving at the same location for over 50 years.

Associations

S. varia forms associations with root noduling nitrogen-fixing bacteria (PFAF, 2016). It is reported to be compatible with the following non­aggressive grasses: smooth stalked meadow grass (Poa pratensis), cocksfoot (Dactylis glomerata) and timothy (Phleum pratense) (Burnham, 2016).

Environmental Requirements

S. varia will grow on well drained dry to moist, coarse to medium texture soils, including sand, rocky soils, loams, clay and silts. It grows best in soils that are pH 6 and above, but will tolerate a pH in the range of 4.8 to 7.2 (PFAF, 2016).  The species is not tolerant to saline and alkaline soils, but will grow in soils with low fertility (GISD, 2016).

S. varia can withstand droughts and heavy precipitation (up to 165 cm annual), but cannot tolerate flooded soils or anaerobic soil conditions. It will not grow in shade but will tolerate sparse shade in edge of forests (Southeast Exotic Pest Plant Council, 2016). The minimum temperature tolerated is -2°C (US Forest Service, 2016).

Seeds of S. varia are reported to be predated in Europe by Bruchidius pusillus beetles (Delobel and Delobel, 2006). Other insects that feed on the foliage of the species are: the Bean Leaf Beetle (Cerotoma trifurcata), a flea beetle (Disonycha punctigera), a leaf­miner fly (Liriomyza trifoliearum), the butterflies Orange Sulfur (Colias eurytheme) and Melissa Blue (Lycaeides melissa melissa), the Wild Indigo Duskywing (Erynnis baptisiae), the Alfalfa Plant Bug (Adelphocoris limbolarius), the Marmorated Brown Stink Bug (Holcostethus limbolarius), the Differential Grasshopper (Melanoplus differentialis), and the Red­legged Grasshopper (Melanoplus femurrubrum) (Illinois Wildflowers, 2016).

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Erosion control or dune stabilization
• Revegetation
• Soil improvement
• Wildlife habitat

Materials

• Oils
• Pesticide

Medicinal, pharmaceutical

• Cosmetic
• Traditional/folklore

Ornamental

• garden plant
• Potted plant

S. varia can be confused with Vicia spp., Lathyrus spp., Chamaecrista fasciculata and Lotus corniculatus. The differences, as discussed by Burnham (2016), are:

Vicia spp. leaves have semi-sagittate to lanceolate stipules, terminating in a tendril; S. varia leaves have oblong stipules and bear a terminal leaflet and tendrils are absent. Vicia inflorescence is a raceme of 1-many flowers, and legumes are not segmented; S. varia inflorescence is an umbel of usually more than 10 flowers, and legumes are segmented.

Lathyrus species differ in having a terminal tendril instead of a terminal leaflet, a raceme inflorescence, and non-segmented legumes.

Chamaecrista fasciculata leaves have an even number of leaflets, yellow caesalpinioid flowers, the stamens are free, and the legumes are not segmented; S. varia leaves have an odd number of leaflets (terminal leaflet present), and the papilionaceous flowers are whitish-pink to purple with diadelphous stamens.

Lotus corniculatus leaves have only 5 leaflets, 2 resembling stipules. The flowers of L. corniculatus are yellow, legumes are dehiscent and non-segmented; S. varia leaves have at least 7 leaflets, and the legumes are indehiscent.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Published reports, videos and factsheets for the general public are available in the USA through Federal and State agencies (US Forest Service, 2016). It is on regulated and/or invasive species lists in the USA in various states, such as Illinois, Kentucky, Tennessee, Georgia, South Carolina and New Hampshire (Invasive.org, 2016).

Control

Physical/mechanical control

Pulling out the entire plant; mowing and prescribed burning are recommended to slow the spread of the species or act against the seedlings (US Forest Service, 2016). All pieces of the roots need to be removed and populations must be monitored for several years, as seed stored in the soil seed bank may germinate for up to 10 years (GISD, 2016). Follow-up treatment with herbicide is required to control any surviving stems or new seedlings (Gucker, 2009; US Forest Service, 2016).

Chemical control

The use of herbicide is the most effective means to control large infestations (Gucker, 2009; US Forest Service, 2016). The species can be controlled using general use herbicides such as glyphosate, triclopyr, or clopyralid. Multiple applications will be required over a period of several years to prevent regeneration through dormant seeds (Gucker, 2009). The species is tolerant to imazethapyr and imazapic (GISD, 2016).

Ecosystem restoration

The best management approach for the species is one that integrates manual, mechanical, chemical controls, and active habitat restoration efforts (GISD, 2016).

Gucker (2009) discusses the need for research related to fires occurring on sites where dense patches of the species occur.

21/08/2016 Original text by:

Jeanine Vélez-Gavilán, University of Puerto Rico at Mayagüez