Oxalis pes-caprae L.
(Bermuda buttercup, soursob, sour grass)
Oxalis pes-caprae, although widespread as a garden weed throughout much of California, has never before been observed invading natural areas in North America. It is now reported as invading native coastal dunes in northern California. Due to its extensive occurrence in lawns and gardens, O. pes-caprae has the potential to rapidly spread via the production of bulblets and contaminated soils into adjacent natural areas in California and elsewhere.
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Oxalis pes-caprae is a small perennial herb in the Oxalidaceae (Oxalis family). Shoots of O. pes-caprae arise from a short vertical stem that is attached to a pale brown underground bulb. Each bulb (usually smaller than 2.5 cm in size) is capable of producing over 20 small whitish bulblets each year. The trifoliate (clover-like) leaves arise from an enlarged basal stem tip, and are arranged in a loose basal rosette. Petioles are usually less than 12 cm long. Each leaflet measures less than 3.5 cm in length and are often spotted with hairy lower surfaces (Hickman 1993).
Flowers of O. pes-caprae are bright yellow and are arranged in umbel-like inflorescences. These inflorescences generally have fewer than 20 flowers each, with peduncles shorter than 30 cm in height. Sepals of O. pes-caprae are green, less than 7 mm long, lanceolate to oblong in shape, and the tips often have two orange or yellow tubercles. The yellow petals are clawed, less than 2.5 cm in length. Each flower has 10 stamens (Hickman 1993). In North America, O. pes-caprae generally flowers from late fall to early summer, but fruits and viable seed have not been observed in California (Ornduff 1987).
Scientific and Common Names:
Synonyms for Oxalis pes-caprae L. include Oxalis cernua Thunb. and Bolboxalis cernua (Thunb.) Small (USDA, NRCS 2001). The genus name Oxalis is derived from Greek meaning sour, referring to the sour-tasting oxalic acid present throughout the plant. The species epithet pes-caprae means goat's foot, perhaps referring to the cloverleaf/cloven shape of the leaves. Bermuda buttercup is the common name most frequently used in the U.S., and refers to its yellow-flowered appearance and likeness to the buttercup family. Soursob is the common name most frequently used for O. pes-caprae in Australia, and refers to its sour taste.
O. pes-caprae is a garden weed in many regions, and can easily spread throughout a garden into nearby lawn and turfgrass areas (Elmore & Cudney 2002). It can form dense mats on the ground, outcompeting native plant species for light and space, and also works to inhibit the germination of native species (Brooks 2001). Additionally, it may cause oxalate poisoning in livestock if eaten in large quantities.
Andrea Pickart of the Humboldt Bay National Wildlife Refuge in northern California reports that O. pes-caprae has within the past two years, started to spread explosively into native dune habitats. It is now encroaching upon the endangered plant areas maintained by the Bureau of Land Management (BLM) and by the City of Eureka. She also reports that O. pes-caprae is extremely aggressive, and that it the only plant that has been observed surviving and coexisting with iceplant (Carpobrotus edulis), another prolific invasive exotic species of coastal dune habitats. Pickart adds that O. pes-caprae often forms dense mats in any area with open space, and that is very successful at competing with and excluding native plants. Even when it does not exclude natives, it has detrimental impacts in open unvegetated areas, as it causes soil enrichment and stabilization of semi-stable areas, altering ecosystem nutrient cycling.
O. pes-caprae is native to the Cape Region of South Africa.
Range As An Invader:
O. pes-caprae is most often associated with Mediterranean climates, but also occurs in subtropical and semi-arid regions (Peirce 1997). In Australia, it is widely invasive throughout the cooler regions, and is pestiferous in all states and territories. In the Mediterranean region, O. pes-caprae is invasive in Italy, Greece, the Iberian Peninsula, as well as in North Africa (Brandes 1991; Damanakis & Markaki 1990). It is often seen in disturbed areas and can occur in all soil types. It tends to do best on heavy, well-drained fertile soil, especially in cultivated garden areas.
In North America, O. pes-caprae has been recorded from Arizona, California and Florida (USDA, NRCS 2001). Doria Gordon of TNC-Florida reports that she has not seen it in Florida, however, and notes that O. pes-caprae is not listed in the Florida flora. In California, it is a naturalized agricultural and horticultural weed that occurs in disturbed habitats from 0 to 500 meters (1640 feet) in elevation. It was first noted in California in the San Francisco Bay region in the 1960s, especially along the coast (Semmes 2002). It is currently present in most coastal counties and several inland counties in California (CalFlora 2000), but has not been observed invading intact natural areas until now. Along the coast, O. pes-caprae often grows in areas in full sun. Inland, it prefers shaded areas (Elmore & Cudney 2002). The California Exotic Pest Plant Council's 1999 Exotic Plant List names O. pes-caprae as a "Need More Information" species, indicating that current information does not adequately describe the nature of threat of this species to wildlands.
Recently, Andrea Pickart of the Humboldt Bay National Wildlife Refuge in northern California, reported that O. pes-caprae is rapidly invading native dune habitats in Humboldt County. She has also seen it invading natural areas in Monterey County (central coast of California) and notes that it is usually restricted to invading only ruderal sites. However, since dunes are naturally disturbed habitats, O. pes-caprae is able to invade these native areas as well as into open, unvegetated sites.
Pickart adds that she has observed O. pes-caprae in Humboldt County for at least five years, but that it was only present as an occasional garden weed. However, populations of O. pes-caprae exploded in 2000, when BLM executed a large annual grass removal project in their endangered plant area using heavy equipment. Apparently, soils were disturbed and soil contaminated with O. pes-caprae bulblets were introduced at that time, and flowered the following spring along with another invasive exotic species, calla lily (Zantadeschia aethiopica).
In the following year, 2001, BLM attempted to hand-remove the bulblets, but a greater abundance appeared in 2002. O. pes-caprae is now spread over a 1.6 ha (4-acre) area, mixed mostly with dense iceplant (Carpobrotus edulis), lupine (Lupinus spp.) and other weeds, and is invading northward toward City of Eureka's endangered plant area. Pickart notes that while the bulk of these infestations are in already degraded dunes, O. pes-caprae has the ability to move into undegraded high quality dunes. It does well in degraded areas with more nitrogen, litter and other weeds.
Reproduction and Methods of Dispersal:
O. pes-caprae spreads predominantly through the dispersal of its numerous bulblets (Brooks 2001). Each bulb is capable of producing more than 20 bulblets per year, which are often then spread by the transport of contaminated soils or garden refuse. Heavy earthmoving equipment and cultivation techniques may also disperse bulblets (Brooks 2001). Bulbs and bulblets can be moved by soil disturbance, wind, vehicles, water (bulbs and bulblets float), and may be dispersed by birds (Brooks 2001; Pickart 2002). Fruit and seed production has not been observed for O. pes-caprae in North America, and it is assumed to be sterile (Ornduff 1987).
Little information is available for the successful control of O. pes-caprae. The best control method is prevention, and if new infestations are spotted and controlled early, it is possible to eradicate small populations. Large populations are difficult to control, and will require multiple years of control efforts.
Manual & Mechanical Methods
Small infestations may be controlled by repeated manual removal of the entire plant. Gluesenkamp (2002) reports from northern California, that repeated pulling will deplete bulb reserves, but these control efforts must be repeated for several years to be successful. Repeated mowing will also eventually deplete the carbohydrate reserves in the underground bulbs, but may not kill the bulb (Elmore & Cudney 2002). The soil from which these plants were removed, should also be carefully examined or sifted to remove all bulbs and bulblets. This method is extremely time and labor intensive, as many tiny bulblets are easily missed. Brooks (2001) recommends removing the whole plant early in the season before bulblet formation and sifting soil to remove all bulbs and bulblets. Pickart reports that after 3 hours of manual removal and sieving soil for bulblets, only a 1 square meter area was weeded, and that a 5-gallon bucket was almost full of bulblets!
Solarization, or covering the infestations with a clear or black plastic may work to kill O. pes-caprae. Infestations must be covered for at least one full growing season to be successful (Brooks 2001; Elmore & Cudney 2002). Covering infestations with stiff cardboard, then covering the cardboard with a thick layer of weed-chip mulch may also kill O. pes-caprae. Semmes (2002) recommends leaving this mulch on the infestation until the mulch and cardboard has rotted, then planting competitive native species into the soil-mulch mixture. The bulblets should be weakened at this point, and incapable of competing well with other plants.
Herbicides can kill O. pes-caprae, but the timing of these chemical treatments is crucial for successful control. In San Diego, California (southern coastal California), Mike Kelly (2002) reports that he applies glyphosate (RoundUp) in a 2% solution on O. pes-caprae just before flowering, and achieves a 95% kill rate. He adds that it is not difficult to kill O. pes-caprae using herbicide, but that new plants are able to germinate from bulblets and mature so rapidly, that within a few days or weeks, new plants are flowering and control treatments must be done again.
In Australia, Brooks (2001) also recommends spraying just before flowering, when the old bulb is most exhausted and new bulbs have not yet developed. Brooks (2001) uses metsulfuron methyl (Ally or Escort) applied at 1 g in 100L for spot spraying in Australia, and has sometimes mixed metsulfuron methyl with glyphosate for good control.
There is currently no biocontrol agent available for the control of O. pes-caprae in North America. Kluge & Claassens (1990) reported a potential biocontrol agent using Klugeana philoxalis, a larval feeder on shoots of O. pes-caprae, but no other information on this species is available.
Brandes, D. 1991. Sociology and ecology of Oxalis pes-caprae L. in the Mediterranean region with special attention to Malta. Phytocoenologia 19(3): 285-306.
Brooks, K. 2001. Managing weeds in bushland: Soursob, fingerleaf & four o'clock. The Environmental Weeds Action Network (http://members.iinet.net.au/~ewan/oxalis.pdf). [Accessed: April 2002]
CalEPPC. 1999. California Exotic Pest Plant Council's Exotic Plant List. Available: http://www.caleppc.org. [Accessed: April 2002]
CalFlora: Information on California plants for education, research and conservation. [web application]. 2000. Berkeley, California: The CalFlora Database [a non-profit organization]. Available: http://www.calflora.org/. [Accessed: April 2002]
Damanakis, M. and M. Markaki. 1990. Studies on the biology of Oxalis pes-caprae L. under field conditions in Crete Greece. Zizaniology 2(3): 145-154.
Elmore, C.L. and D.W. Cudney. 2002. Pest Notes: Creeping woodsorrel and bermuda buttercup. University of California Agriculture and Natural Resources, Publication 7444 (http://www.ipm.ucdavis.edu/PMG/PESTNOTES?pn7444.html). [Accessed: April 2002]
Gluesenkamp, D. 2002. Resource Management Fellow, Audubon Canyon Ranch, Marin and Sonoma Counties, California. Personal communication.
Gordon, D. 2002. State Ecologist, The Nature Conservancy of Florida. Personal communication.
Hickman, J.C. (ed.). 1993. The Jepson Manual: Higher Plants of California. University of California Press, Berkeley.
Kelly, M. 2002. California Exotic Pest Plant Council (CalEPPC). Personal communication.
Kluge, R.L. and M. Claassens. 1990. Klugeana philoxalis Geertsema (Noctuidae: Cuculliinae), the first potential biological control agent for the weed Oxalis pes-caprae L. Journal of the Entomological Society of Southern Africa 53: 191-198.
Ornduff, R. 1987. Reproductive systems and chromosome races of Oxalis pes-caprae L. and their bearing on the genesis of a noxious weed. Annals of the Missouri Botanical Garden 74: 79-84.
Peirce, J.R. 1997. The biology of Australian weeds: 31. Oxalis pes-caprae L. Plant Protection Quarterly 12(3): 110-119.
Pickart, A. 2002. Humboldt Bay National Wildlife Refuge, Lanphere Dunes Unit. Personal communication.
Semmes, T. 2002. Travis Semmes' "Native Guide": Oxalis pes-caprae's yellow blooms deceiving. Santa Cruz Sentinel, Online Edition. http://www.santacruzsentinel.com/archive/2002/February/28/style/stories/12style.htm. [Accessed: April 2002]
USDA, NRCS. 2001. The PLANTS Database, Version 3.1 (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA. [Accessed: April 2002]
--Mandy Tu/Wildland Invasive Species Team; September 2002, updated February 2003