Yellow woodsorrel

Oxalis stricta L.

Closeup of yellow woodsorrel seedling
Yellow woodsorrel flowering plant
Closeup of a yellow woodsorrel's seed pods

Images above: Upper left: Yellow woodsorrel seedling (Antonio DiTommaso, Cornell University). Upper right: Yellow woodsorrel flowering plant (Antonio DiTommaso, Cornell University). Bottom: Yellow woodsorrel seed pods (Randall Prostak, University of Massachusetts).

Identification 

Other common names:  common yellow woodsorrel, common yellow oxalis, lady's sorrel, lemon clover, sheep's clover, sheep sorrel, sheep sour, sourgrass, tall wood sorrel toad sorrel, upright wood sorrel

Family:  woodsorrel family, Oxalidaceae

Habit:  Short, much branched perennial herb, commonly behaving as an annual in agricultural fields.

Description:  Cotyledons of the seedling are round to oblong, green to pinkish, hairless, and 0.12-0.24” (0.4-0.6 cm) long by up to 0.2” (0.5 cm) wide.  True leaves are alternate, green to occasionally purplish, and are divided into three heart-shaped leaflets.  Leaf edges are smooth.  The leaflets are smooth on the upper surfaces and have short, scattered hairs on the lower surfaces and a fringe of hairs along the edges.  Mature plants are 2-15” (5-38 cm) tall and unbranched or branching at the base.  Stems are green to purplish, and covered in upward-facing, flattened hairs.  Leaf shape and hairiness are similar to the seedling.  Mature leaves are 0.5-1.1” (1.3-2.8 cm) wide, and individual leaflets are 0.25-0.5” (0.6-1.3 cm) wide and long.  Leaf stalks are up to 2.5” (6.4 cm) long.  The root system is fibrous, but the plant also produces shallow, spreading, white to pinkish rhizomes.  Flowers grow in clusters of 2 to 6 from the leaf axils on stalks which reach up to 1” (2.5 cm) long.  The flowers are yellow, have 5 notched or rounded petals and 5 pale green sepals, and are 0.28-0.43” (0.7-1.1 cm) wide.  Each flower is replaced by an upright, 5-sided, pointed, cylindrical seedpod.  The seedpods are hairy and 0.4-0.6” (1-1.5 cm) long.  When ripe, the seedpods split and eject the seeds up to 6.5 ft (2 m) from the plant.  The seeds are flattened, red to brown, ridged transversely, and 0.04-0.06” (0.1-0.15 cm) long.        

Similar species:  Creeping woodsorrel (Oxalis corniculata L.), a frequent weed in greenhouse and nursery culture, has a more prostrate, spreading habit than yellow woodsorrel, and has aboveground runners rather than underground rhizomes.  Slender yellow woodsorrel (Oxalis dillenii Jacq.) is very similar to yellow woodsorrel, but is generally smaller and has a taproot rather than rhizomes.  Clovers (Trifolium spp.), black medic (Medicago lupulina L.) and other trifoliate legumes have similar leaves to yellow woodsorrel.  Legume leaflets are not heart-shaped, however, and the leaves often have a pair of small bracts at the base of the stalk.

Management

Yellow woodsorrel is relatively non-competitive, but it is sufficiently prolific to make itself a problem in vegetable crops.  Even when its density is too low to decrease yield, its rapid, upright growth causes harvest problems in herbs and leafy greens.  In addition, it may serve as an alternative host to several diseases (Puccinia and Fusarium) of field and vegetable crops including sweet corn and onions (Lovett Doust et al. 1985).  Consequently, rotate these crops to other fields or beds until you have this species under control. 

This weed is most common in untilled crop fields, so tillage is an effective means of control (Anderson 2008).  For dense infestations, flush the seeds out of the soil with repeated shallow cultivations before planting.  Then plant competitive crops like snap beans or short season cabbage that can be repeatedly cultivated shallowly close to the row.  Alternatively, grow an early crop like radish or head lettuce and use a cultivated fallow during part of the summer.  Avoid crops with long, post cultivation periods as these will allow late emerging yellow woodsorrel to go to seed.  Since this weed grows very fast, if you use a summer cover crop, plant it at high density to insure good suppression.  Hay or straw mulch and synthetic barrier mulches effectively suppress this weed (Halverson and Guertin 2003).  Hand weeding should be done before seed capsules form to prevent dispersal of seeds during the weeding process (Lovett Doust et al. 1985).

Ecology

Origin and distribution:  Yellow woodsorrel is native to eastern North America and probably also eastern Asia (Lovett Doust et al. 1985).  It has been introduced into western North America, Europe, Africa and New Zealand.  It now occurs throughout most of the U.S.A. and southern Canada, except in the warmer parts of the Pacific coast and Intermountain West where it is absent or occurs only sporadically (USDA Plants).

Seed weight:  0.13 mg (Mohler unpublished data), 0.15 mg (Stevens 1932).

Dormancy and germination:  Freshly produced seeds of yellow woodsorrel are not dormant and will germinate immediately if sown on warm, moist soil (Mohler, unpublished data).  Seeds germinate at 48-85 °F (9-30 °C) (Anderson 1999), with optimum temperatures of 60-80 °F (16-27 °C) (Halverson and Guertin 2003).  Exposure of moist seeds to a high temperature of 97 °F (36 °C) will inhibit germination (Halverson and Guertin 2003).  Seeds require exposure to light after they have taken up water, with only a brief exposure to a low level of light being sufficient (Anderson 1999).  Thus, seeds that get incorporated into the soil will normally wait to germinate until they are exposed to a pulse of light during tillage.  

Seed longevity:  In undisturbed conditions, seeds persist in the soil for at least five years and probably much longer (Thompson et al. 1997).  Seed viability was 83% after one year (Anderson 1999).  Since the seeds germinate readily in recently disturbed soil, however, they are probably flushed out of the soil relatively quickly in regularly tilled and cultivated fields.

Season of emergence:  In temperate climates, yellow woodsorrel emerges from mid-spring through summer (Anderson 2008, Lovett Doust et al. 1985).  In climates with warmer winters such as California, this species can emerge throughout the year (Halverson and Guertin 2003).

Emergence depth:  This has not been reported, but, given the small seed weight, most seedlings probably arise primarily from the top 0.5” (1.3 cm) or less of soil.  Sprouts from rhizomes could emerge from deeper in soil, however, unless buried, the rhizomes typically lie just below the soil surface (Mohler, personal observation).

Photosynthetic pathway:  C3 (Halverson and Guertin 2003)

Sensitivity to frost:  Yellow woodsorrel tolerates light frost, but dies back to the ground following hard frost (Halverson and Guertin 2003).

Drought tolerance:  The species tolerates dry spells of several weeks (Halverson and Guertin 2003).  Leaflets fold along a center crease in response to stress (Halverson and Guertin 2003).

Mycorrhiza:  Yellow woodsorrel is mycorrhizal (Dhillion and Friese 1994, Lovett Doust et al. 1985, Harley and Harley 1987).

Response to fertility:  Yellow woodsorrel tolerates low fertility but is most prolific in highly fertile soils (Halverson and Guertin 2003).  Plants respond to fertilizer application by producing a flush of new leaves and flowers (Mohler, personal observation).

Soil physical requirements:  Yellow woodsorrel grows on a wide range of soils, but thrives in loamy soil (Lovett Doust et al. 1985).  It is an indicator of moist, fertile soils, but can tolerate drought-prone sites (Halverson and Guertin 2003).

Response to shade:  Yellow woodsorrel cannot grow in dense shade (Plants For The Future), but it tolerates the partial shade cast by many crops.

Sensitivity to disturbance:  Plants can be easily uprooted by hand-weeding soon after emergence (Lovett Doust et al. 1985), but can resprout from rhizomes after weeding or cultivation once they are established (Halverson and Guertin 2003).  The rhizomes lie just below the soil surface and are easily damaged, so resprouting is usually not a major problem.  Plants will assume a prostrate growth habit in response to mowing (Halverson and Guertin 2003).

Time from emergence to reproduction:  Spring emerging plants flower four to six weeks after emergence and set seeds two to four weeks later (Anderson 2008, Lovett Doust et al. 1985).  Plants emerging in midsummer can set seeds in as little as 5 weeks.  (Mohler, unpublished data).

Pollination:  Yellow woodsorrel is often self-pollinated but is also cross pollinated by insects (Halverson and Guertin 2003, Lovett Doust et al. 1985).

Reproduction:  Yellow woodsorrel reproduces either by seeds, and/or by perennating buds on rhizomes (Halverson and Guertin 2003).  Plants emerging from seeds in the spring and left undisturbed with minimal competition produced an average of 900 capsules each with an average of 23 seeds per capsule, thereby producing approximately 21,000 seeds per plant (Mohler, unpublished data).  Elsewhere, plants have been reported to produce from 570 (Stevens 1932) to 5,000 seeds per plant (Halverson and Guertin 2003).  Since newly produced seeds lack dormancy, the species can produce two complete generations per year in the northern U.S. (Mohler, unpublished data), and more in warmer climates (Halverson and Guertin 2003).  Vegetative reproduction by sprouts from rhizomes is rare in tilled fields but is common in less disturbed habitats (Halverson and Guertin 2003, Lovett Doust et al. 1985).

Dispersal:  The mature capsules rupture explosively, scattering seeds up to 13 ft (4 m) (Halverson and Guertin 2003, Lovett Doust et al. 1985).  Seeds pass alive through ruminant digestive tracts and are spread with manure (Mt. Pleasant and Schlather 1994).  Seeds may also be transported by rodents.  They probably also move with soil on shoes, tires, machinery, and by floating in waterways (Halverson and Guertin 2003).

Common natural enemies:  None of any consequence.

Palatability:  Leaves or young plants of yellow woodsorrel are sometimes added to salads or cooked dishes to add a sharp, sour taste (Lovett Doust et al. 1985).  The presence of the toxin, oxalic acid, which accumulates in the aerial parts of the plant gives the shoots their sour taste (Marshall 1987).  Leaves have similar Vitamin C content as that found in spinach and oranges (Zennie and Ogzewalla 1977).

References:

  • Anderson, R. L.  2008.  Seedling emergence of yellow woodsorrel in eastern South Dakota.  Western Society of Weed Science 2008 Research Progress Report, pp. 152-153.
  • Anderson, W. P.  1999.  Perennial Weeds: Characteristics and Identification of Selected Herbaceous Species.  Iowa State University Press: Ames, IA.
  • Dhillion, S. S., and C. F. Friese.  1994.  The occurrence of mycorrhizas in prairies: Application to ecological restoration.  Thirteenth North American Prairie Conference 13:103-114.
  • Halverson, W. L., and P. Guertin.  2003.  Factsheet for: Oxalis stricta L.  USGS Weeds in the West project.  29 pp.
  • Harley, J. L., and E. L. Harley.  1987.  A check-list of mycorrhiza in the British flora.  New Phytologist 105:1-102.
  • Lovett Doust, L., A. MacKinnon, and J. Lovett Doust.  1985.  Biology of Canadian weeds. 71. Oxalis stricta L., O. corniculata L., O. dillenii Jacq. ssp. dillenii and O. dillenii Jacq. ssp. filipes (Small) Eiten.  Canadian Journal of Plant Science 65:691-709.
  • Marshall, G.  1987.  A review of the biology and control of selected weed species in the genus Oxalis: O. stricta L., O. latifolia H.B.K. and O. pes-caprae L.  Crop Protection 6:355-364.
  • Mt. Pleasant, J., and K. J. Schlather.  1994.  Incidence of weed seed in cow (Bos sp.) manure and its importance as a weed source for cropland.  Weed Technology 8:304-310.
  • Plants For A Future - Species Database.  Copyright 1997-2003 Plants for a Future, Blagdon Cross, Ashwater, Beaworthy, Devon, EX21 5DF, UK.  Website: www.pfaf.org
  • Stevens, O. A.  1932.  The number and weight of seeds produced by weeds.  American Journal of Botany 19:784-794.
  • Thompson, K., J. P. Bakker, and R. Bekker.  1997.  The soil seed banks of North West Europe: methodology, density and longevity.  Cambridge University Press: Cambridge, U.K.
  • USDA Plants.  Natural Resources Conservation Service.  https://plants.usda.gov/java/
  • Zennie, T. M., and C. D. Ogzewalla.  1977.  Ascorbic acid and Vitamin A content of edible wild plants of Ohio and Kentucky.  Economic Botany 31:76-79.