Oxalis stricta L.

Identification of Yellow Woodsorrel

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 inch long by up to 0.2 inch 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 inches tall and unbranched or branching at the base. Stems are green to purplish and are covered in upward-facing, flattened hairs. Leaf shape and hairiness are similar to the seedling. Mature leaves are 0.5–1.1 inch wide, and individual leaflets are 0.25–0.5 inch wide and long. Leaf stalks are up to 2.5 inches long. The root system is fibrous, but the plant also produces shallow, spreading, white to pinkish rhizomes. Flowers grow in clusters of two to six from the leaf axils on stalks, which reach up to 1 inch long. The flowers are yellow, have five notched or rounded petals and five pale green sepals, and are 0.28–0.43 inch wide. Each flower is replaced by an upright, five-sided, pointed, cylindrical seedpod. The seedpods are hairy and 0.4–0.6 inch long. When ripe, the seedpods split and eject the seeds up to 6.5 feet from the plant. The seeds are flattened, red to brown, ridged transversely and 0.04–0.06 inch 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 of Yellow Woodsorrel

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. 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. 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 tilled 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 ensure good suppression. Hay or straw mulch and synthetic barrier mulches effectively suppress this weed. Hand weeding should be done before seed capsules form to prevent dispersal of seeds during the weeding process.

Ecology of Yellow Woodsorrel

Origin and distribution: Yellow woodsorrel is native to eastern North America and probably also eastern Asia. It has been introduced into western North America, Europe, Africa and New Zealand. It now occurs throughout most of the United States and southern Canada, except in the warmer parts of the Pacific coast and Intermountain West, where it is absent or occurs only sporadically.

Seed weight: Population mean seed weight ranges from 0.13–0.15 mg.

Dormancy and germination: Freshly produced seeds of yellow woodsorrel are not dormant and will germinate immediately if sown on warm, moist soil. Seeds germinate at 48–85°F, with optimum temperatures of 60–80°F. Exposure of moist seeds to a high temperature of 97°F will inhibit germination. Seeds require exposure to light after they have taken up water, with only a brief exposure to a low level of light being sufficient. 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. Seed viability was 83% after one year. 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. This species can emerge throughout the year in climates with warmer winters such as California. 

Emergence depth: This has not been reported, but given the small seed weight, most seedlings probably arise primarily from the top 0.5 inch 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.

Photosynthetic pathway: C3

Sensitivity to frost: Yellow woodsorrel tolerates light frost but dies back to the ground following hard frost.

Drought tolerance: The species tolerates dry spells of several weeks. Leaflets fold along a center crease in response to stress.

Response to fertility: Yellow woodsorrel tolerates low fertility but is most prolific in highly fertile soils. Plants respond to fertilizer application by producing a flush of new leaves and flowers.

Mycorrhiza: Yellow woodsorrel is mycorrhizal.

Soil physical requirements: Yellow woodsorrel grows on a wide range of soils but thrives in loamy soil. It is an indicator of moist, fertile soils but can tolerate drought-prone sites.

Response to shade: Yellow woodsorrel cannot grow in dense shade, but it tolerates the partial shade cast by many crops.

Sensitivity to disturbance: Plants can be easily uprooted by hand-weeding soon after emergence but can resprout from rhizomes after weeding or cultivation once they are established. 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.

Time from emergence to reproduction: Spring emerging plants flower four to six weeks after emergence and set seeds two to four weeks later. Plants emerging in midsummer can set seeds in as few as five weeks. 

Pollination: Yellow woodsorrel is often self pollinated but is also cross pollinated by insects.

Reproduction: Yellow woodsorrel reproduces either by seeds and/or by perennating buds on rhizomes. 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. Elsewhere, plants have been reported to produce 570–5,000 seeds per plant. Since newly produced seeds lack dormancy, the species can produce two complete generations per year in the northern United States and more in warmer climates. Vegetative reproduction by sprouts from rhizomes is rare in tilled fields but is common in less disturbed habitats.

Dispersal: The mature capsules rupture explosively, scattering seeds up to 13 feet. Seeds pass alive through ruminant digestive tracts and are spread with manure. Seeds may also be transported by rodents. They probably also move with soil on shoes, tires, machinery and by floating in waterways.

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. The presence of the toxin oxalic acid, which accumulates in the aerial parts of the plant, gives the shoots their sour taste. Leaves have similar vitamin C content as that found in spinach and oranges.

Summary Table of Yellow Woodsorrel Characteristics

Yellow Woodsorrel
Growth habitPerennial overwinter organEmergence period from perennial organsOptimum emergence depth (inches) from perennial organsTime/stage of lowest reservesPhotosynthesis typeFrost toleranceDrought toleranceMycorrhiza
short, branchedrhizomesmid-spring to summerC3moderatemoderateyes
Fertility ResponseImportance of seeds to weedinessSeed weight (mg)Dormancy of shed seedsFactors breaking dormancyOptimum temperarature range (F) for seed germinationSeedling emergence periodEmergence to flowering (weeks)
moderatehigh0.13–0.15noli60–80mid-spring to summer4–6

Table Key 

Perennial overwinter organ: Principal plant organ that survives winter and from which growth resumes in subsequent years.

Emergence period from perennial organs: Time of year when most emergence occurs from perennial overwintering organs in the typical regions of occurrence for each weed. Some emergence may occur outside of this range.

Optimum emergence depth from perennial organs: Soil depths (in inches below the soil surface) from which most shoots emerge from perennial organs. Lower rates of emergence usually will occur at depths above or below this range.

Time/stage of lowest reserves: Time of year and/or weed growth stage at which carbohydrate reserves are lowest. This usually corresponds to the time when the weed is most susceptible to weed management operations.

Frost tolerance: Relative tolerance of aboveground shoots to freezing temperatures (high, moderate, low).

Drought tolerance: Relative tolerance of aboveground plants to drought (high, moderate, low).

Importance of seeds to weediness: The relative importance of seeds to dispersal, genetic diversity and survival of the species as a weed in agricultural environments (high, moderate, low). Emergence to flowering: Length of time (weeks) after emergence from perennial organs to the beginning of flowering in the typical regions of occurrence. Note that this refers to established perennial plants, recognizing that some species may not flower in their initial year of establishment.

Further Reading

Halverson, W.L. and P. Guertin. 2003. Factsheet for: Oxalis stricta L. USGS Weeds in the West project. 29 pp.

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.