Cyperus rotundus L.

Identification

Other common names: nut grass, coco grass, nut sedge, coco sedge, coco, purple nutgrass

Family: Sedge family, Cyperaceae

Habit: Erect, grass-like perennial herb with triangular stems and three-ranked leaves that forms extensive networks by rhizomes and tubers.

Description: Seedlings and young vegetative shoots are upright, light green with a white midvein and somewhat stiff. The short stem is triangular with a solid center. Mature plants have dark green leaves and triangular stems, typically about 6 inches, but can reach up to 1–2 feet tall. Leaves do not have collar regions; they transition smoothly from blade to a sheathing base that completely overlaps the one below it, broadening the basal stem region. Waxy leaves are hairless, 0.13–0.25 inch wide, bluntly pointed at the tip, sturdy and thick at the base, and they have prominent paralleled veins. Leaves emerge in groups of three, alternating around the triangular base. The triangular flower stalk, growing from the plant base, is typically as tall as or taller than the leaves. The inflorescence consists of several spikes that originate from a common point in a spoke-like manner. The spikelets are closely clustered purple with a red or brown cast. Several long, thin, pointed, leaf-like bract structures are present immediately below the inflorescence; they are roughly horizontal and of equal to or greater length than the inflorescence. Dark seeds develop in three-sided, dark brown to black, 0.08–0.13 inch long, dry, elliptical fruit. Seeds are frequently non-viable; propagation occurs primarily via underground rhizomes, concentrated in the first foot of soil, that give rise to tubers. Tubers are white, turning dark brown or black as they age; they are covered by hard, rough, dark-red scales and ridges, and they give rise to new vegetative shoots, roots and rhizomes. 

Similar species: Yellow nutsedge (Cyperus esculentus L.) is a slightly taller plant with a yellowish cast throughout. Its leaf tips are long and tapered into narrow points; its inflorescences are yellow and more condensed and bottlebrush-like. Yellow nutsedge rhizomes produce tubers only at their ends, and tubers lose their scales as they mature.

Management

Purple nutsedge is considered a highly competitive and persistent weed in a wide range of crops throughout the world, and extreme measures are generally required to manage it. The most effective method for controlling purple nutsedge is to desiccate the tubers by thoroughly tilling the soil to the depth of the deepest tubers at the beginning of a period of hot, dry weather. For this approach to be effective, the tillage must break the tubers free of deep roots. Tubers in the upper few inches of hot, dry soil die in about eight days. Although a single tillage operation at the beginning of a hot, dry period of several weeks may be sufficient to kill all the tubers, multiple deep tillage operations will dry the soil and tubers faster, and help ensure successful control. 

Attempting to exhaust an intact tuber-rhizome system is essentially futile due to dormant tubers and huge underground reserves. Consequently, when attempting to exhaust tubers, begin by thoroughly tilling to break apart tuber chains so that tubers are induced to sprout. Subsequent cultivation to kill shoots should occur at less than three-week intervals since longer intervals will allow formation of new tubers. Even with prior deep tillage, exhausting tubers may require two growing seasons. Some tubers may remain dormant below the depth of tillage; the few tubers that sprout subsequent to the eradication effort should be dug out by hand. Although fallowing a field for nutsedge eradication requires taking the land out of production during the summer, winter grain planted in October and harvested in June will not interfere with the eradication campaign since purple nutsedge grows poorly in cool weather and the grain is highly competitive. A comparison of several integrated systems showed that tilling at three-week intervals from March to August was cheaper and was as effective for purple nutsedge control as was covering the ground with green or clear polyethylene film for the whole period or by preceding tillage with a turnip cover crop. All of these systems reduced tuber density, provided the subsequent fall pepper crop was either straw mulched or hand weeded. Inter-row cultivation reduces nutsedge competition with the crop but cannot by itself provide complete control. For example, two early season cultivations in cotton provided between 65% and 80% control, but the remaining infestation still significantly reduced yields.

The sharp tips of purple nutsedge rhizomes readily pierce black polyethylene agricultural film and paper mulch, and black film actually increases infestations by warming the soil and thereby encouraging sprouting and rapid growth. Heavier opaque materials can prevent shoot emergence, but some tubers will persist under the cover, and dense shoot clumps will form along the edges. Purple nutsedge emergence was severely limited through heavy paper mulch. Clear polyethylene mulches can be used to kill purple nutsedge by heating the soil to lethal temperatures (solarization). Some rhizome tips will, however, penetrate thin clear plastic films, particularly if the film is in contact with the soil. However, thicker film (e.g., 4 mil) with a 0.2–0.4 inch gap over the soil causes the leaves to emerge and become trapped under the mulch where they are scorched. In the desert regions of the Southwest, black plastic achieves lethal temperatures to a depth of 6 inches, but in the Southeast, infrared-type plastic that promotes capture of infrared radiation is most effective at achieving lethal temperatures. Although lethal temperatures only penetrate to about 4 inches in the Southeast, the increase in temperature and the increase in the difference between day and night temperatures at greater depths promote tuber sprouting, and the shoots that appear under the plastic are then scorched. Thus, translucent mulches provide some control even if temperatures lethal to the tubers are not achieved. Unlike most perennial weeds, purple nutsedge is significantly suppressed by dead mulches of organic material.

Swine are fond of the tubers, and 24–30 pigs can remove the tubers from one acre in a day. Chickens at a stocking rate of one bird per 125 square feet can eradicate a severe infestation in two years if they can be made to graze uniformly. At a stocking rate of 32 birds per acre, geese can keep a severely infested cotton field free of purple nutsedge competition, and a second year of goose grazing at a reduced stocking rate can eradicate the weed. 

Although purple nutsedge reduces production of all crops, some more competitive crops like green beans and transplanted cabbage are less affected than poor competitors like garlic and okra. Since purple nutsedge is sensitive to shade, crops that rapidly develop a leaf canopy are more suppressive of purple nutsedge. Sweet potatoes inhibit purple nutsedge by allelopathy, but its production is substantially reduced by the weed anyway. Dense grass cover inhibits the establishment of purple nutsedge. Even when tubers are planted, many fail to establish shoots in dense sod. However, the few weak plants that do establish send out horizontal rhizomes that explore the soil for openings where new, more vigorous shoots establish and form tubers. Consequently, disturbed areas in the sward are likely to become heavily infested if any purple nutsedge plants are nearby. Cover crops can slow the rate of tuber formation relative to land without a cover crop, but the population may increase anyway.

Ecology

Origin and distribution: Purple nutsedge is probably native to India, but it has been introduced to tropical and subtropical regions throughout the world. It is a serious pest in the Southeast from Virginia to central Texas, and in parts of Arizona and California; it occurs sporadically in the north central and northeastern United States. 

Seed and tuber weight: Seed lots contain many light, non-viable seeds. Heavier seed fractions, which contain at least some viable seeds, have seeds that weigh from 0.22–0.3 mg. 

Tuber weights can vary considerably, ranging from 160–440 mg in Alabama, 400–1,400 mg in Japan and 200–2,000 mg in Israel. Tubers forming near the soil surface tend to be substantially smaller than those deeper in the soil, though very deep tubers may be relatively small. 

Dormancy and germination: Purple nutsedge seed production is rare in the United States. Seeds are dormant when shed and slowly lose dormancy over several years of after-ripening. Damage to the seed coat (scarification), microbial action in the soil or exposure to high temperature (e.g., 140°F) increases germination. 

Most tubers have one to eight buds, but a few large tubers have up to 13. The bud farthest from the mother plant on a tuber or tuber fragment sprouts first and tends to suppress sprouting of the other buds. If that shoot is destroyed, the next lower bud will sprout. Similarly, the most recently formed tuber in a chain of tubers inhibits sprouting of tubers farther back on the chain, though these will sometimes sprout anyway. Breaking up chains experimentally or by tillage causes immediate sprouting of most tubers. The optimal temperature for sprouting is 77–95°F, and little sprouting occurs below 50–59°F. Short term chilling of tubers at 40°F for four days can accelerate tuber sprouting and subsequent vegetative and reproductive development. Extended chilling for two months promotes subsequent sprouting at suboptimal temperatures of 59–68°F, which promotes early emergence in temperate climates. Although many tubers will sprout at 68°F, shoots may fail to elongate or may elongate slowly. Fluctuating temperatures increase percentage sprouting, speed of sprouting and speed of shoot elongation, especially when temperatures are suboptimal. Temperatures above 109°F usually inhibit sprouting, and temperatures over 122°F kill the tuber within two days to less than an hour, depending on the temperature. Sprouting is inhibited at 10–20% soil moisture, but tubers sprout readily at 30–40% soil moisture. At greater than 50% soil moisture, sprouting is suppressed but the tubers survive. Inhibition of tuber sprouting in waterlogged soil is probably due to low oxygen concentration. Short day lengths prompt production of dormant tubers, whereas long day lengths favor formation of tubers that sprout immediately. Light speeds sprouting and increases the number of sprouts per tuber, but 100% of tubers will sprout in complete darkness. 

Tuber longevity: In an experiment in Costa Rica, tubers died off at a slowly increasing rate, and less than half were alive after 18 months. Depth of burial did not affect tuber survival. Tubers can survive 200 days of immersion.

Season of emergence: Shoots emerge continuously whenever the mean temperature is greater than about 59°F. In Georgia, emergence peaks in July.

Emergence depth: Seedlings emerge best from about 0.5 inch, with fewer emerging from near soil surface and none emerging from deeper than 1 inch. 

Studies differ regarding how well shoots can emerge from tubers placed at various depths, but they generally agree that tubers throughout the plow layer can produce emerged shoots. Some experiments found that shoots emerged best from tubers in the top 1–6 inches of soil, and also can emerge from 6 to 12 inches. Another pair of experiments found 50–70% emergence from tubers at 2 feet and 0–6% from 3 feet.

Photosynthetic pathway: C4

Sensitivity to frost: Tubers exposed to a temperature of 32°F survived for seven to 10 days; those exposed to 23°F or lower died within two hours. Another experiment, however, showed that tubers could survive eight hours of exposure to 25°F.

Drought tolerance: Tubers die when their moisture content drops below 15%. Tubers left at the surface of dry soil exposed to full sun were killed in four days. Under simulated field conditions, tubers at 2 inches and 4 inches in dry soil exposed to sunlight and protected from rain were killed after 12 and 16 days, respectively. In another experiment, tubers buried at 6 inches in dry soil for four weeks did not survive. Roots penetrate to 54 inches and are numerous and well branched deep in the soil; these help avoid water stress in an intact rhizome-tuber system and, consequently, undisturbed plants can survive six months of drought. 

Mycorrhiza: Mycorrhizal fungi infect purple nutsedge roots but decrease productivity. Presence of onion, a mycorrhizal crop, increases infection.

Response to fertility: Increasing N fertility increases the competitive ability of purple nutsedge in a wide range of crops. In radish, a poor competitor, nutsedge tuber production peaked at 98 pounds per acre of N, while shoot production peaked at 196 pounds per acre of N; radish yield declined linearly with increasing N application rates. For several other crops, application of low rates of N increased the weed’s growth and tuber production but still increased crop yield, whereas higher N application rates frequently decreased yield due to excessive competition from the weed. In experimental conditions, purple nutsedge grew best at pH 3.5–7, but it also thrives on soils with pH 8.7 or higher.

Soil physical requirements: Purple nutsedge thrives in soil ranging in texture from sand to heavy clay. Shoot growth is inhibited by a dense, poorly aerated layer in the soil; although roots penetrate such layers, rhizomes do not, and tubers will not form there. 

Response to shade: Shoot and tuber production decreases linearly with shade. At 60–80% shade, tubers are still produced, but they are small. Under high shade, plants allocate more energy to roots and rhizomes and less energy to tuber production. Screens producing 72–73% shade reduced tuber production by 10–70%. Severe shading of 99% by a crop or cover crop causes the top-growth to die back.

Sensitivity to disturbance: The proportion of tubers below the 8-inch plow layer varies greatly depending on location; in most temperate climate sites, at least some tubers lie below the normal depth of tillage, but few or none form deeper than 12 inches. Clipping shoots appears to promote sprouting of dormant buds on the tubers. Although mowing three times per week at 0.5 inch greatly reduced spread and tuber production, plants still produced 13–19 feet of rhizomes and 59–103 tubers per plant in about four months, and other experiments have similarly demonstrated the inability of clipping to control purple nutsedge.

Time from emergence to reproduction: Planting tubers may simulate what happens when tubers become independent of one another during tillage. Following planting in spring and summer, flowers and formation of dormant tubers usually occur within three to four weeks but may take as long as seven weeks. Flowering usually occurs with tuber formation or follows it by about one week. Formation of new shoots with tuber-like basal bulbs can occur within three weeks. Following planting in late fall and winter in a Mediterranean climate similar to California, the first tubers formed in about two months, and flowering was delayed until the following spring.

Pollination: Flowers are cross pollinated, mainly by wind, but most pollen is shriveled and non-viable. 

Reproduction: In the United States, purple nutsedge reproduces almost exclusively by tubers. Seed producing populations have been found in moist river valley fields in Alabama, but only if they were tilled in the spring. Generally, however, seed production is rare and, when seeds are produced, the proportion that is viable is low. A genetic study showed that all of the populations evaluated from the United States and Caribbean were likely a single clone; a lack of cross pollination in most locations may explain the rarity of seed production. 

Tuber production is rapid and continuous during spring and summer but slows during fall and winter. A single tuber can produce 99 tubers in 90 days. A tuber planted in late March produced a clone roughly 15 feet in diameter in one year, with shoots forming a nearly continuous sod in the occupied area. In a similar study, a single tuber produced a patch 238 square feet in area in 60 weeks. Stands of purple nutsedge grown from well-spaced tubers produced 1,000–3,000 tubers per square yard within 1.5–2 years.

Dispersal: Tubers are spread by erosion events and by moving water in streams. They are dispersed in nursery containers. They can be collected along with the crop during the harvest of potatoes and sweet potatoes and then dispersed to new locations when the crop is sold. Clones can spread more than 6 feet per year by rhizome expansion.

Common natural enemies: Moths in the genus Bactra bore into the basal bulb and damage the plant. Population increase of the moth is slower than that of the weed, so the moths do not control purple nutsedge unless populations are augmented by early season releases of larvae or adults. The nematode Tylenchus similiis attacks tubers, but the damage is insufficient to control the weed.

Palatability: Purple nutsedge has some medicinal uses but is not eaten as food by humans. The foliage quickly becomes fibrous and is poor forage, but it can serve as low quality forage in the absence of more desirable species. Swine are fond of the tubers.

Weed Characteristics Summary Table

Purple Nutsedge
Growth habitPerennial overwinter organEmergence period from perennial organsOptimum emergence depth (inches) from perennial organsTime/stage of lowest reservesPhotosynthesis TypeFrost toleranceDrought toleranceMycorrhiza
short sedgetuberssummer1–6C4lowhighyes
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)
moderatelow0.22–0.3yesscd, wst3–7

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

Bangarwa, S.K., J.K. Norsworthy, P. Iha and M. Malik. 2008. Purple nutsedge (Cyperus rotundus) management in an organic production system. Weed Science 56: 606–613.

Chase, C.A., T.R. Sinclair and S.J. Locascio. 1999. Effects of soil temperature and tuber depth on Cyperus spp. control. Weed Science 47: 467–472.

Hershenhorn, J., B. Zion, E. Smirnov, A. Weissblum, N. Shamir, E. Dor, G Achdari, H. Ziadna and A. Shilo. 2015. Cyperus rotundus control using a mechanical digger and solar radiation. Weed Research 55: 42–50.

Holm, L.G., D.L. Plucknett, J.V. Pancho and J.P. Herberger. 1977. The World's Worst Weeds: Distribution and Biology. The University Press of Hawaii: Honolulu.

Smith, E.V. and G.L. Fick. 1937. Nut grass eradication studies: I. Relation of the life history of nut grass, Cyperus rotundus L., to possible methods of control. Journal of the American Society of Agronomy 29: 1007–1013.

Smith, E.V. and E.L. Mayton. 1938. Nut grass eradication studies: II. The eradication of nut grass, Cyperus rotundus L., by certain tillage treatments. Journal of the American Society of Agronomy 30: 18–21.