Purple nutsedge

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Identification

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

Family:  sedge family, Cyperaceae

Habit:  Erect, perennial grass-like plant 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” (15 cm), but can reach up to 1-2 ft (30-61cm) 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” (0.3-0.6 cm) wide, bluntly pointed at the tip, sturdy and thick at the base, and prominent paralleled venation; they emerge in groups of three, alternating around the triangular base.  The triangular flower stalk, growing from the plant base, is typically as tall 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” (0.2-0.3 cm) long, 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 (Holm et al. 1977), 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 (Hershenhorn et al. 2015).  For this approach to be effective, the tillage must break the tubers free of deep roots (Andrews 1940).  Tubers in the upper few inches of hot, dry soil die in about eight days (Ranade and Burns 1925).  Although a single tillage operation at the beginning of a hot, dry period of several weeks may be sufficient to kill all the tubers (Andrews 1940, Hershenhorn et al. 2015), multiple deep tillage operations will dry the soil and tubers faster and help ensure successful control (Ranade and Burns 1925). 

Attempting to exhaust an intact tuber-rhizome system is essentially futile due to dormant tubers and huge underground reserves (Ranade and Burns 1925).  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 3-week intervals since longer intervals will allow formation of new tubers (Smith and Fick 1937, Smith and Mayton 1938).  Even with prior deep tillage, exhausting tubers may require two growing seasons (Smith and Mayton 1938, 1942).  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 (Smith and Mayton 1942).  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 crop is highly competitive (Smith and Mayton 1938).  A comparison of several integrated systems showed that tilling at 3-week intervals from March to August was cheaper and was as effective for purple nutsedge control as 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 (Bangarwa et al. 2008).  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 (Edenfield 2005).

The sharp tips of purple nutsedge rhizomes readily pierce black polyethylene agricultural film and paper mulch (Ranade and Burns 1925, Patterson 1998, Webster 2005a, Chase et al. 1998), and black film actually increases infestations by warming the soil and thereby encouraging sprouting and rapid growth (Webster 2005b).  Heavier opaque materials can prevent shoot emergence but some tubers will persist under the cover, and dense shoot clumps will form along the edges (Ranade and Burns 1925).  Purple nutsedge emergence was severely limited through heavy paper mulch (Cirujeda et al. 2012).  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 or 0.01 cm) with a 0.2 to 0.4” (0.4 to 1 cm) gap over the soil causes the leaves to emerge and become trapped under the mulch where they are scorched (Chase et al. 1998).  In the desert regions of the Southwest, black plastic achieves lethal temperatures to a depth of 6” (15 cm) (Wang et al. 2008), but in the Southeast, IR type plastic that promotes capture of infra-red radiation is most effective at achieving lethal temperatures (Chase et al. 1999).  Although lethal temperatures only penetrate about 4” (10 cm) of soil in the Southeast, the increase in temperature and the increase in the difference between day and night temperatures at greater depths promote tuber sprouting (Egley 1983, Miles et al. 2002), and the shoots that appear under the plastic are then scorched (Chase et al. 1999).  Thus, translucent mulches provide some control even if lethal temperatures are not achieved (Patterson 1998).  Unlike most perennial weeds, purple nutsedge is significantly suppressed by dead mulches of organic material (Hutchinson and McGiffen 2000, Bangarwa et al. 2008).

Swine are fond of the tubers, and 24 to 30 pigs can remove the tubers from one acre in a day (OSWALD).  Chickens at a stocking rate of 1 bird per 125 ft2 (12 m2) can eradicate a severe infestation in two years if they can be made to graze uniformly.  At a stocking rate of 32 birds per A (79 birds per ha), 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 (Mayton et al. 1945).

Although purple nutsedge reduces production of all crops (Keeley 1987), some more competitive crops like green beans and transplanted cabbage are less affected than poor competitors like garlic and okra (William and Warren 1975).  Since purple nutsedge is sensitive to shade, crops that rapidly develop a leaf canopy are more suppressive of purple nutsedge (Keeley 1987).  Sweet potato inhibits purple nutsedge by allelopathy, but its production is substantially reduced by the weed anyway (Peterson and Harrison 1995).  Dense grass cover inhibits the establishment of purple nutsedge.  Many tubers 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 can establish and form tubers.  Consequently, disturbed areas in the sward are likely to become heavily infested if any purple nutsedge plants are nearby (Ranade and Burns 1925).  Cover crops can slow the rate of tuber formation relative to land without a cover crop, but the population may increase anyway (Ranade and Burns 1925).

Ecology

Origin and distribution: Purple nutsedge is probably native to India, but it has been introduced to tropical and subtropical regions throughout the world (Holm et al. 1977).  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 U.S.A. (USDA Plants).

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 to 0.30 mg (Justice and Whitehead 1946). 

Tuber weight varies with the population and possibly the soil type: sandy loam in Alabama, 0.16 to 0.44 g (Smith and Fick 1937); sandy soil in Japan 0.4 to 0.8 g with a few heavier than 1.4 g, and a mean of 0.66 g (Ueki 1969); clay soil in Israel, 0.24 to 0.43 g (Horowitz 1972), 0.62 to 2.0 g (Hershenhorn et al. 2015).  Tubers forming near the soil surface tend to be substantially smaller than those deeper in the soil (Smith and Fick 1937, Horowitz 1972), though very deep tubers may be relatively small (Smith and Fick 1937).  Dor and Hershenhorn (2013) refer to tubers 0.1-0.5 g as small, 0.6-1.6 g as intermediate, and 1.7-2.0 g as large.

Dormancy and germination:  Purple nutsedge seed production is rare in the U.S.A.  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 or 60 °C) increases germination (Ranade and Burns 1925, Andrews 1946, Justice and Whitehead 1946). 

Most tubers have 1 to 8 buds, but a few large tubers have up to 13 buds (Ueki 1969).  The bud furthest 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 further back on the chain, though these will sometimes sprout anyway.  Breaking up chains, such as by tillage, causes immediate sprouting of most tubers (Smith and Fick 1937, Muzik and Cruzado 1953, Kawabaata and Nishimoto 2003).  The optimal temperature for sprouting is 77 to 95 °F (25 to 35 °C), and little sprouting occurs below 50 to 59 °F (10 to 15 °C) (Horowitz 1972, Ueki 1969, Shamsi et al. 1978).  Short term chilling of tubers at 40 °F (4 °C) for 4 days can accelerate tuber sprouting and subsequent vegetative and reproductive development (Dor and Hershenhorn 2013).  Extended chilling for 2 months promotes subsequent sprouting at sub-optimal temperatures of 59 to 68 °F (15 to 20 °C), which promotes early emergence in temperate climates (Shamsi et al. 1978).  Although many tubers will sprout at 68 °F (20 °C) shoots may fail to elongate or elongate slowly.  Fluctuating temperatures increase percentage sprouting, speed of sprouting, and speed of shoot elongation, especially when temperatures are sub-optimal (Miles et al. 1996, Nishimoto 2001, Kawabata and Nishimoto 2003, Wallace et al. 2013).  Temperatures above 109° F (43 °C) usually inhibit sprouting, and temperatures over 122 °F (50 °C) kill the tuber within two days to less than an hour, depending on the temperature (Ueki 1969, Smith and Fick 1937, Chase et al. 1999, Andrews 1940, Webster 2003).   Sprouting is inhibited at 10-20% soil moisture, but tubers sprout readily at 30-40% soil moisture (Andrews 1940).  At greater than 50% soil moisture, sprouting is suppressed, but the tubers survive (Holm et al.1977, Ueki 1969). Inhibition of tuber sprouting in waterlogged soil is probably due to low oxygen concentration (Ueki 1969, Nishimoto 2001).  Short day lengths prompt production of dormant tubers whereas long day lengths favor formation of tubers that sprout immediately (Holm et al. 1977).  Light speeds sprouting and increases the number of sprouts per tuber, but 100% of tubers will sprout in complete darkness (Ueki 1969, Muzik and Cruzado 1953, Nishimoto 2001).  

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 (Neeser et al. 1997).  Tubers can survive 200 days of immersion (Ueki 1969).

Season of emergence:  Shoots emerge continuously whenever the mean temperature is greater than 59 °F (15 °C) (Horowitz 1972).  In Georgia, emergence peaks in July (Hauser 1962).

Emergence depth:  Seedlings emerge best from about 0.5” (1.3 cm), with fewer emerging from near the soil surface and none emerging from deeper than 1” (2.5 cm) (Andrews 1946).

Studies differ regarding how well shoots can emerge from tubers placed at various depths, but 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 to 6” (2.5 to 15 cm) of soil, but can emerge from 6 to 12” (15 to 30 cm) (Andrews 1940, Chase et al. 1999, Hershenhorn et al. 2015).  Another pair of experiments found 50-70% emergence from tubers at 2 ft (61 cm) and 0-6% from 3 ft (91 cm) (Ranade and Burns 1925).

Photosynthetic pathway:  C4 (Elmore and Paul 1983)

Sensitivity to frost:  Tubers exposed to a temperature of 32 °F (0 °C) survived for 7 to 10 days, whereas those exposed to 23 °F (-5 °C) or lower died within two hours (Ueki 1969).  Another experiment, however, showed that tubers could survive 8 hours exposure to 25 °F (-4 °C) (Smith and Fick 1937).

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 4 days.  Under simulated field conditions, tubers at 2 and 4” (5 and 10 cm) in dry soil exposed to sunlight were killed after 12 and 16 days, respectively (Smith and Fick 1937).  In another experiment, tubers buried at 6” (15 cm) in dry soil for 4 weeks did not survive (Andrews 1940).  Roots penetrate to 54” (135 cm) and are numerous and well branched deep in the soil; these roots help avoid water stress in an intact rhizome-tuber system, and consequently, undisturbed plants can survive 6 months of drought (Andrews 1940). 

Mycorrhiza:  Mycorrhizal fungi infect purple nutsedge roots but decrease productivity.  Presence of onion, a mycorrhizal crop, increases infection (Muthukumar et al. 1997).

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 lb N/A (110 kg N/ha) while shoot production peaked at 196 lb N/A (220 kg N/ha) and radish yield declined linearly with increasing N application rates (Santos et al. 1998).  For several other crops, application of less than 54 lb N/A (60 kg/ha) increased growth and tuber production of the weed but still increased crop yield, whereas higher N application rates frequently decreased crop yield due to excessive competition (Okafor and De Datta 1976, Morales-Payan et al. 1998, 1999). In experimental conditions, purple nutsedge grew best at pH 3.5 to 7.0 (Al-Ali et al 1978), but it also thrives on soils with pH 8.7 or higher (Andrews 1940, Rao 1968).

Soil physical requirements:  Purple nutsedge thrives in soil ranging in texture from sand (Ueki 1969) to heavy clay (Andrews 1940).  Shoot growth is inhibited by a dense, poorly aerated layer in the soil; although roots can penetrate such layers, rhizomes and tubers will not form there (Andrews 1940).

Response to shade:  Shoot and tuber production decreases linearly with shade (Keeley 1987).  At 60-80% shade, tubers are still produced, but they are small (Jordan-Molero and Stoller 1978, Santos et al. 1997a).  Under high shade, plants allocate more energy to roots and rhizomes and less energy to tuber production (Santos et al. 1997a).  At 72-73% shade tuber production was reduced by 10 to 70% (Hauser 1962, Jordan-Molero and Stoller 1978).  Severe shading of 99% by a crop or cover crop causes the top-growth to die back (Holm et al. 1977).

Sensitivity to disturbance:  The proportion of tubers below the 8” (20 cm) plow layer varies greatly depending on location; in most temperate sites, at least some tubers lie below the normal depth of tillage but few or none form deeper than 12” (30 cm) (Smith and Fick 1937, Smith and Mayton 1942, Ueki 1969, Horowitz 1972).  Clipping shoots appears to promote sprouting of dormant buds on the tubers (Santos et al. 1997b).  Although mowing three times per week at 0.5” (1.3 cm) greatly reduced spread and tuber production, plants still produced 13 to 19 ft (4.1 to 5.9 m) of rhizomes and 59 to 103 tubers per plant in about 4 months (Summerlin et al. 2000), and other experiments have similarly demonstrated the inability of clipping to control purple nutsedge (Horowitz 1965, Santos et al. 1997b).

Time from emergence to reproduction:  Planting of 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 3 to 4 weeks but may take as long as 7 weeks.  Flowering usually occurs with tuber formation or follows it by about 1 week (Ranade and Burns 1925, Smith and Fick 1937, Muzik and Cruzado 1953, Horowitz 1972, Hauser 1962, Holm et al. 1977).  Formation of new shoots with tuber-like basal bulbs can occur within 3 weeks (Horowitz 1972, Holm et al. 1977).  Following planting in late fall and winter in a Mediterranean climate similar to California, the first tubers formed in about 2 months, and flowering was delayed until the following spring (Horowitz 1972).

Pollination:  Flowers are cross pollinated, mainly by wind, but most pollen is shriveled and non-viable (Holm et al. 1977). 

Reproduction:  Purple nutsedge reproduces almost exclusively by tubers in the U.S.A.  Seed producing populations have been found in moist river valley fields in Alabama, but only if they were tilled in the spring (Justice and Whitehead 1946).  Generally, however, seed production is rare (Smith and Fick 1937, Holm et al. 1977) and, when seeds are produced, the proportion that are viable is low (Muzik and Cruzado 1953, Justice and Whitehead 1946).  A genetic study showed that all of the populations evaluated from the U.S.A. and Caribbean were likely a single clone (Okoli et al. 1997); 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 (Horowitz 1972).  A single tuber can produce 99 tubers in 90 days (Rao 1968).  A tuber planted in late March produced a clone roughly 15 ft (4.5 m) in diameter in one year, with shoots forming a nearly continuous sod in the occupied area (Horowitz 1972).  In a similar study, a single tuber produced a patch 238 ft2 (22 m2) in area in 60 weeks (Webster 2005b).  Stands of purple nutsedge grown from well-spaced tubers produced 800 to 2,500 tubers per square yard (1,000 to 3,000 per m2) within 1.5 years (Hauser 1962, Horowitz 1972). 

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 (Holm et al. 1977).  Clones can spread more than 6 ft (1.8 m) per year by rhizome expansion (Horowitz 1972).

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 (Frick and Chandler 1978).  The nematode Tylenchus similiis attacks tubers but the damage is insufficient to control the weed (Holm et al. 1977).        

Palatability:  Purple nutsedge has some medicinal uses but it 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 (Holm 1977, Urbatsch 2006).  Swine are fond of the tubers (OSWALD).

References:

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• USDA Plants Database.  Natural Resources Conservation Service.  http://plants.usda.gov
• Urbatsch, L.  2006.  USDA, NRCS Plant Guide: Purple nutsedge, Cyperus rotundus L.  Natural Resources Conservation Service.  http://plants.usda.gov/plantguide/pdf/pg_cyro.pdf.
• Wallace, R. D., T. L. Grey, T. M. Webster, and W. K. Vencill.  2013.  Increased purple nutsedge (Cyperus rotundus) tuber sprouting with diurnally fluctuating temperature.  Weed Science 61:126-130.
• Wang, G., M. E. McGiffen, and E. J. Ogbuchiekwe.  2008.  Crop rotation effects on Cyperus rotundus and C. esculentus population dynamics in southern California vegetable production.  Weed Research 48:420-428.
• Webster, T. M.  2003.  High temperatures and durations of exposure reduce nutsedge (Cyperus spp.) tuber viability.  Weed Science 51:1010-1015.
• Webster, T. M.  2005a.  Mulch type affects growth and tuber production of yellow nutsedge (Cyperus esculentus) and purple nutsedge (Cyperus rotundus).  Weed Science 53:834-838.
• Webster, T. M.  2005b.  Patch expansion of purple nutsedge (Cyperus rotundus) and yellow nutsedge (Cyperus esculentus) with and without polyethylene mulch.  Weed Science 53:839-845.
• William, R. D., and G. F. Warren.  1975.  Competition between purple nutsedge and vegetables.  Weed Science 23:317-323.