Sandburs

Images above: Left: Field sandbur growth habit (Joseph DiTomaso, University of California, Davis). Right: Field sandbur inflorescence (Joseph DiTomaso, University of California, Davis).

Images above: Left: Longspine sandbur growth habit (Joseph DiTomaso, University of California, Davis). Right: Longspine sandbur inflorescence (Joseph DiTomaso, University of California, Davis).

Image above: Southern sandbur inflorescence (Joseph DiTomaso, University of California, Davis).

Identification

Other common names:

• Field sandbur: coastal sandbur, mat sandbur
• Longspine sandbur: bur-grass, sandbur-grass, bear-grass, hedgehog-grass, field sandbur
• Southern sandbur: hedgehoggrass

Family:  grass family, Poaceae

Habit:  Tufted, usually annual grasses with stems rooting at the lower nodes but curving upward to become erect.  Field sandbur sometimes persists through winter in the southern U.S.A. (CDFA).

Description:  Seedlings are rolled in the bud and have membranous, hairy ligules less than 0.06” (0.15 cm) long.  The bur from which the seedling emerged usually remains attached at the base of the plant.

• Field sandbur:  Hairs are present only on the blade base and sheath margin.  Seedlings are often purple.
• Longspine sandbur:  Seed leaf is purple toward the base.  Early leaves are rough, narrow, and often have long hairs at the base.  The collar is narrow, pale and distinct.  Sheaths are flat, smooth, often red, and have fine hairs on the edges. 
• Southern sandbur:  Seedlings are upright and hairless except on the sheath margin and at blade base.  Blades are green, flat, and abrasive.  Blades and sheaths may redden with age.

Mature plants tiller readily and root at stem nodes, forming clumps of prostrate to upright stems.  Sheathes are compressed and open.  Collars are angular.  Ligules are very short and membranous, with a fringe of hairs as long as or longer than the membrane.  Blades are flat, rough, and sometimes folded.  Roots are fibrous and shallow.

• Field sandbur:  Stems are 8-32” (20-80 cm) long.  Sheaths are hairless or sometimes hairy on the margin.  Ligules are 0.02-0.06” (0.05-0.15 cm) long.  Blades are 1.5-12” (4-30 cm) long by 0.1-0.25” (0.25-0.6 cm) wide, hairless or with a few long, straight hairs at the base.
• Longspine sandbur:  Stems are 4-30” (10-76 cm) long and light green but sometimes red at base.  Sheaths are keeled, hairless to sparsely hairy, with translucent edges.  Ligules are 0.02-0.07” (0.06-0.18 cm) long.  Blades are 2.5-7.5” (6.3-19 cm) long by 0.1-0.3” (0.25-0.76 cm) wide, light green, and hairless, but rough on upper surface.  
• Southern sandbur:  Stems are 9-36” (23-90 cm) long.  Sheaths are dark green to red, keeled, and generally hairless.  Ligules 0.02-0.08” (0.05-0.2 cm) long.  Blades are 2-12” (5-30 cm) long by 0.12-0.37” (0.3-0.9 cm) wide, and occasionally hairless but usually with very short, rough hairs on the upper surfaces and no hairs on the lower surfaces.

Inflorescences are narrow, unbranched terminal spikes of spiny burs.  Spikes of southern and longspine sandbur may be partially contained in the last leaf.  Burs are green when young, turning yellow or brown as they mature.  Spines of unripe burs are often purple. Each bur contains 1 to 4 spikelets, and each spikelet has 2 flowers.  Burs detach readily when mature and contain one or more brown, oval to egg-shaped seeds.

• Field sandbur:  Inflorescences are 1.25-5.5” (3-13 cm) long by 0.4-0.8” (1-2 cm) wide, with 6 to 12 tightly clustered burs.  Burs are oval to spherical, 0.2-0.4” (0.5-1 cm) long by 0.1-0.2” (0.25-0.5 cm) wide, hairless to moderately hairy, with 8 to 45 spines.  Spines are 0.08-.0.22” (0.2-0.6 cm) long.  Each bur contains 2 spikelets.  Seeds are 0.1” (0.25 cm) long by 0.04-0.08” (0.1-0.2 cm) wide.
• Longspine sandbur:  Inflorescence is 1.25-4” (3-10 cm) long by 0.4-0.8” (1-2 cm) wide, with 4 to 20 burs.  Burs are spherical and 0.2-0.4” (0.5-1 cm) long.  Spines are 0.13-0.25” (0.33-0.64 cm) long, numerous (45 to 75), and are arranged irregularly.  Each bur contains 1 to 4 spikelets.  Seeds are 0.08-0.16” (0.2-0.4 cm) long by 0.06-0.1” (0.15-0.25 cm) wide.
• Southern sandbur:  Inflorescences are 1-4” (2.5-10 cm) long by 0.4-0.7” (1-1.8 cm) wide with 5 to 50 burs.  Burs are 0.72-0.4” (0.5-1 cm) long, with a ring of flexible spines at the base.  The upper spines are 0.08-0.2” (0.2-0.5 cm) long.  Each bur contains 2 to 4 spikelets.  Seeds are 0.06-0.13” (0.16-0.32 cm) long by 0.06-0.09” (0.13-0.22 cm) wide.

Similar Species:  Distinctive collars paired with rough foliage and inflorescences composed of burs set these species apart from most other grasses.  Young seedlings, however, may be confused with foxtail species (Setaria spp.) or barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.].  Seedlings of foxtails have new leaves that uncurl as they emerge, while sandbur leaves unfold.  Barnyardgrass seedlings do not have a ligule, while sandbur seedling ligules are a short fringe of hairs.  Sandbur seedlings can often be distinguished from other young grasses by the presence of a bur at the base of the plant.

Management

The timing of sandbur emergence depends on the species and region, but in many places most seedlings emerge relatively early in the spring.  If this is the case, then delaying tillage for planting row crops will help suppress the weed.  Similarly, rotating with summer planted crops will help manage sandburs.  Shallowly incorporating the burs in the fall will increase seed mortality and promote earlier emergence from the remaining seeds.   If small mammal seed predators are abundant, however, leaving the burs on the soil surface through the winter may be a better approach for encouraging seed mortality. In this case, shallowly incorporate the burs early in the spring to promote germination, and then kill the young plants later in the season when preparing the seed bed for crop planting.  Incorporation of a high glucosinolate rapeseed green manure crop reduced longspine sandbur emergence and greatly suppressed growth in a greenhouse experiment, but this approach may be more effective on broadleaf species in a mixed weed community under field conditions (Boydston and Hang 1995).

Sandbur species compete poorly with well established crops (Parsons and Cuthbertson 2001).  Consequently, a strong stand of a winter grain will suppress sandbur.  In Australia, interseeding alfalfa into winter wheat further reduced longspine sandbur seed production if the alfalfa seeding rate was higher than 5.1 lb/A (5.7 kg/ha) (Twentyman 1974a).  These species will tend to emerge with spring grains, decrease crop yield and produce many burs, so spring grains should be avoided if sandbur is a problem.

Because these species can emerge from deep in the soil, rotary hoeing may be relatively ineffective at reducing density.  A tine weeder with stiff tines that break or bury the sandbur seedlings may be better.  Cultivate between crop rows before the plants begin to root at the nodes.  In irrigated regions, delaying irrigation immediately before and after a cultivation will reduce the amount of re-rooting of the dislodged seedlings (R. Boydston, personal communication).

A dense, well managed pasture resists invasion by sandbur species, but overgrazing allows establishment and seed production (Twentyman 1974a, CDFA).  Hand weed local infestations, for example, in disturbed ground around feeding and watering stations, before the sandbur spreads.  If sandbur is more widespread and the forage stand is good, use rotational grazing to intensively, but not excessively, utilize pasture in the spring – the perennial grasses should recover more rapidly than the young sandbur plants and competitively suppress them.  Then mow the pasture when the sandbur flowering stalks are still in the boot to reduce seed production.  Additional mowing may be needed when secondary inflorescences begin to form.  If sandbur is a severe problem in the pasture, consider replanting using a winter grain nurse crop.  If necessary, take the grain as forage before the sandbur flowering stalks are out of the boot.  Prevent contamination of hay with unpalatable burs by mowing the hay while the inflorescence is still in the boot. 

Ecology

Origin and distribution:  Field sandbur is native to southern North America (Parsons and Cuthbert 2001), and it occurs throughout the southern and southwestern states (USDA Plants).  It has been introduced into southern South America, the Middle East, southern Asia, Australia (Holm et al. 1979), Europe and South Africa (Parsons and Cuthbertson 2001).  Longspine sandbur is native to southern North America (Parsons and Cuthbertson 2001) but has spread into most of the U.S.A. and parts of southern Canada (USDA Plants).  It has been introduced into Australia (Parsons and Cuthbertson 2001).  Southern sandbur originated in tropical America and occurs from the southern United States to Argentina and Chile.  It has been introduced into tropical Africa, Madagascar, India, Israel, Hungary, southeast Asia, Australia and islands of the Pacific Ocean (Parsons and Cuthbertson 2001, Holm et al. 1977).  In the U.S.A. it occurs in the Atlantic and Gulf Coast states from Maryland to Texas and across the southwest from Oklahoma to California (USDA Plants). 

Seed weight:  Field sandbur, 1.7-3.4 mg, mean 2.8 mg (Mohler, unpublished data); longspine sandbur, 6.8 mg (Stevens, 1932); southern sandbur, 2.4-7.9 mg, mean 5.4 mg.  The burs of longspine sandbur contain 1 to 3 seeds.  The central, uppermost (primary) seed is larger than the lower (secondary) seeds (Twentyman 1974b)

Dormancy and germination:  Seeds are normally retained in the burs until they germinate, and the surrounding bur and chaff partially inhibit germination (Twentyman 1974b).  Three months of 32-39 °F (0-4 °C) temperatures were sufficient to permit germination under favorable conditions (Boydston 1989).  Secondary seeds of longspine sandbur tend to be more dormant than primary seeds (Twentyman 1974b).  For example, in one experiment with whole burs, about twice as many primary seeds germinated in 14 days (71%) than did secondary seeds (Boydston 1989).   Seeds germinate best with fluctuating daily temperatures where the high temperature is 77-95 °F (25-35 °C) and the low temperature is 50-77 °F (10-25 °C) (Twentyman 1974b, Boydston 1989). Temperatures over 104 °F (40 °C) inhibit germination and induce secondary dormancy, and prolonged exposure of moist seeds to such temperatures kills a high proportion of seeds (Twentyman 1974b, Boydston 1989).  Unlike many weed species, light tends to reduce germination (Twentyman 1974b, Boydston 1989).  Germination of southern sandbur is stimulated by nitrate and by minor damage to the seed coat (scarification) (CDFA).

Seed longevity:  Few seeds of longspine sandbur persist in the soil for more than a few years, and, in contrast with most weed species, the seed bank is depleted more rapidly when the burs are buried.  In an experiment in Australia, 75% of longspine sandbur seeds buried at 1 or 4” (2.5 or 10 cm) produced seedlings within 3 years.  The remaining seeds died off at an average rate of 67% per year.  Fewer seedlings emerged from burs left on the soil surface (62%) and the mortality rate for the remaining seeds was 52%.  Thus, burial of burs reduces the seed bank both by facilitating seedling emergence and by promoting seed mortality.  In a second experiment, only 2% of primary seeds buried at 1.6” (4 cm) in whole burs remained after 10 months in undisturbed grazing land, and no primary seeds remained after 10 months in tilled soil.  In contrast, 81% of secondary seeds were still present in undisturbed land and 54% in tilled soil (Twentyman 1974b).

Season of emergence:  Field sandbur emerges in early spring (USDA Plants).  The timing of emergence in longspine sandbur varies regionally.  In eastern Washington, over 98% of seedlings emerged between mid April and June 1, with a few seedlings continuing to emerge until October (Boydston 1990).  In Colorado, most seedlings emerged between May 25 and June 15, with a few more continuing to emerge until August (Anderson 1997). 

Emergence depth:  Fewer seedlings emerged from longspine burs left on the soil surface (62%) than from those planted at 1 or 4” (2.5 or 10 cm) (78 and 72%), while minimal emergence occurs from 12” (30 cm) (Twentyman 1974b).  In contrasting study with opposite results, 79% of pregerminated longspine seeds placed at 0.4” (1 cm) emerged, and emergence percentage declined smoothly to 2% at 4.3” (11 cm) (Boydston 1989).  Despite this apparent contradiction, both studies indicate that many seedlings of longspine sandbur can emerge from seeds buried several inches deep.  Southern sandbur can emerge from as deep as 3.5” (9 cm) in clay soil and 4” (10 cm) in loamy soil (Parsons and Cuthbertson 2001).  Seedlings emerged from burs buried at 4” (10 cm) about 1 month later than from burs on the soil surface. 

Photosynthetic pathway:  Southern and field sandbur are C4 species and the whole genus appears to possess this pathway (Smith and Brown 1973).

Sensitivity to frost:  These species generally die during the winter but may survive a mild winter and produce additional burs the following spring (Parsons and Cuthbertson 2001).

Drought tolerance:  All three species prosper in excessively drained habitats in moderately low rainfall regions.  However, they also emerge primarily in the spring when soil moisture is relatively high.  Authors disagree on whether field sandbur becomes semi-dormant during drought conditions and then greens-up and resumes seed production after rain (Canfield 1934, USDA Plants). 

Mycorrhiza:  Field sandbur, southern sandbur, and other species in the genus are mycorrhizal (Peterson et al. 1985, Fracchia et al. 2009, Khan et al. 2008).

Response to fertility:  No information located. 

Soil physical requirements:  All three sandbur species prefer sandy soils, but will grow on fine-textured soils as well (Thomson 1943, Twentyman 1974a, Parsons and Cuthbertson 2001, USDA Plants).

Response to shade:  These species are shade intolerant (Bullock et al. 2002).  For example, southern sandbur died out of a new coffee plantation in Nicaragua as the canopy closed (Aguilar et al. 2003) and was only found in open, sunny habitats in a vegetation survey in Brazil (Klink and Joly 1989).

Sensitivity to disturbance:  All three species root at the nodes.  Large plants form tussocks that are difficult to bury with plowing and hard to control with cultivation.  Repeated mowing or heavy grazing before the plants flower reduces but does not completely prevent seed production.  Plants are most sensitive to mowing when in the boot stage of development (CDFA).

Time from emergence to reproduction:  Heads of longspine sandbur emerge 7-13 weeks after seedling emergence, with the time to flowering decreasing as the time of emergence is delayed (Boydston 1990, Anderson 1997).  Warmer June temperatures appear to speed flowering of spring emerging plants and warmer September temperatures appear to speed flowering of late emerging plants (Boydston 1990).  In longspine sandbur, 20% of the seeds are viable by heading and over 40% by flowering (Anderson 1997).

Pollination:  Field sandbur can self-pollinate (McKinney and Fowler 1991).  Many flowers of longspine sandbur apparently self-pollinate prior to opening (Anderson 1997). 

Reproduction:  Burs contain 1 to 3 seeds (McKinney and Fowler 1991, Twentyman 1974b). Well-watered longspine sandbur plants grown without competition from seeds sown in May, June, July and August in eastern Washington produced an estimated 133,000, 49,000, 5,000, and 40 seeds respectively (Boydston, 1990).  Longspine plants emerging in late May in an irrigated Colorado corn field produced 1,120 burs, and the number of burs produced declined rapidly with later emergence (Anderson 1997). Four-week-old seedlings of longspine sandbur planted into a drying pasture of winter annual forages in southeastern Australia produced only 1-133 seeds per plant, and production decreased with the density of skeleton weed (Twentyman 1974a).

Dispersal:  The burs of all three species disperse by clinging to fur, clothing, and tires.  The burs float and disperse along streams and irrigation canals (Parsons and Cuthbertson 2001).  These species also disperse in contaminated hay (CDFA).

Common natural enemies:  Many species of mice eat substantial quantities of sandbur seeds (Howard and Evans 1961, Twentyman 1974a, Best and Skupski 1994, Monk and Jones 1996). 

Palatability:  Sandbur plants are palatable to livestock when young (Holm et al. 1977), but once the inflorescence emerges, the burs irritate the mouths and throats of grazers that ingest them.  (Parsons and Cuthbertson 2001). 

References:

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• USDA Plants Database, Natural Resources Conservation Service.  http://plants.usda.gov