Sandburs

Other common names:

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

Field sandbur, Cenchrus spinifex Cav. = C. pauciflorus Benth. = C. incertus M.A. Curtis)

Longspine sandbur, Cenchrus longispinus (Hack.) Fernald

Southern sandbur, Cenchrus echinatus L.

Identification of Sandburs

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 United States.

Description: Seedlings are rolled in the bud and have membranous, hairy ligules less than 0.06-inch 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: The 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 inches long. Sheaths are hairless or sometimes hairy on the margin. Ligules are 0.02–0.06 inch long. Blades are 1.5–12 inches long by 0.1–0.25 inch wide, hairless or with a few long, straight hairs at the base.
• Longspine sandbur: Stems are 4–30 inches long and light green but sometimes red at base. Sheaths are ridged, hairless to sparsely hairy, with translucent edges. Ligules are 0.02–0.07 inch long. Blades are 2.5–7.5 inches long by 0.1–0.3 inch wide, light green and hairless, but rough on upper surface. 
• Southern sandbur: Stems are 9–36 inches long. Sheaths are dark green to red, ridged and generally hairless. Ligules are 0.02–0.08 inch long. Blades are 2–12 inches long by 0.12–0.37 inch 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 one to four spikelets, and each spikelet has two flowers. Burs detach readily when mature and contain one or more brown, oval to egg-shaped seeds. 

• Field sandbur: Inflorescence is 1.25–5.5 inches long by 0.4–0.8 inch wide, with six to 12 tightly clustered burs. Burs are oval to spherical, 0.2–0.4 inch long by 0.1–0.2 inch wide, hairless to moderately hairy, with eight to 45 spines. Spines are 0.08–.0.22 inch long. Each bur contains two spikelets. Seeds are 0.1 inch long by 0.04–0.08 inch wide.
• Longspine sandbur: Inflorescence is 1.25–4 inches long by 0.4–0.8 inch wide, with four to 20 burs. Burs are spherical and 0.2–0.4 inch long. Spines are 0.13–0.25 inch long, numerous (45–75) and arranged irregularly. Each bur contains one to four spikelets. Seeds are 0.08–0.16 inch long by 0.06–0.1 inch wide.
• Southern sandbur: Inflorescence is 1–4 inches long by 0.4–0.7 inch wide, with five to 50 burs. Burs are 0.2–0.4 inch long, with a ring of flexible spines at the base. The upper spines are 0.08–0.2 inch long. Each bur contains two to four spikelets. Seeds are 0.06–0.13 inch long by 0.06–0.09 inch 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 of Sandburs

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 mammals that feed on seeds 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 seedbed 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.

Sandbur species compete poorly with well-established crops. Consequently, a strong stand of a winter grain will suppress sandbur. In Australia, interseeding alfalfa into winter wheat further reduced longspine sandbur seed reproduction if the alfalfa seeding rate was higher than 5.1 pounds per acre. 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 rerooting of the dislodged seedlings.

A dense, well managed pasture resists invasion by sandbur species, but overgrazing allows establishment and seed production. Hand weed local infestations, for example in disturbed ground around feeding and watering stations, before the sandbur spreads. If sandbur is more widespread, use rotational grazing to intensively but not excessively utilize pasture in the spring; if the forage stand is good, 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 of Sandburs

Origin and distribution: Field sandbur is native to southern North America and occurs throughout the southern and southwestern states. It has been introduced into southern South America, the Middle East, southern Asia, Australia, Europe and South Africa. Longspine sandbur is native to southern North America but has spread into most of the United States and parts of southern Canada. It has been introduced into Australia. 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. In the United States it occurs in the Atlantic and Gulf Coast states from Maryland to Texas and across the Southwest from Oklahoma to California. 

Seed weight: Field sandbur, 1.7–3.4 mg, mean 2.8 mg; longspine sandbur, 6.8 mg; southern sandbur, 2.4–7.9 mg, mean 5.4 mg. The burs of longspine sandbur contain one to three seeds. The central, uppermost (primary) seed is larger than the lower (secondary) seeds.

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

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 inch or 4 inches produced seedlings within three 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 inch 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.

Season of emergence: Field sandbur emerges in early spring. 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. In Colorado, most seedlings emerged between May 25 and June 15, with a few more continuing to emerge until August. 

Emergence depth: Fewer seedlings emerged from longspine burs left on the soil surface (62%) than from those planted at 1 inch or 4 inches (78 and 72%), while minimal emergence occurs from 12 inches. A contradictory study showed that 79% of pre-germinated longspine seeds placed at 0.4 inch emerged, and emergence percentage declined smoothly to about 2% at 4.3 inches. Despite the 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 inches in clay soil and 4 inches in loamy soil. Seedlings emerged from burs buried at 4 inches about one 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.

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

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. 

Mycorrhiza: Field sandbur, southern sandbur and other species in the genus are mycorrhizal.

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. 

Response to shade: These species are shade intolerant. For example, southern sandbur died out of a new coffee plantation in Nicaragua as the canopy closed and was only found in open, sunny habitats in a vegetation survey in Brazil.

Sensitivity to disturbance: All three species root at the nodes, and 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.

Time from emergence to reproduction: Heads of longspine sandbur emerge seven to 13 weeks after seedling emergence, with the time to flowering decreasing as the season progresses. Warmer June temperatures appear to speed flowering of spring emerging plants, and warmer September temperatures appear to speed flowering of late emerging plants. In longspine sandbur, 20% of the seeds are viable by heading and over 40% by flowering.

Pollination: Field sandbur can self-pollinate. Many flowers of longspine sandbur apparently self-pollinate prior to opening. 

Reproduction: Burs contain one to three seeds. 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. 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. 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.

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. These species also disperse in contaminated hay.

Common natural enemies: Many species of mice eat substantial quantities of sandbur seeds. 

Palatability: Sandbur plants are palatable to livestock when young, but once the inflorescence emerges, the burs irritate the mouths and throats of grazers that ingest them. 

Summary Table of Sandbur Characteristics

Sandbur, field Sandbur, longspine Sandbur, southern
Growth habit	Seed weight (mg)	Seed dormancy at shedding	Factors breaking dormancy	Optimum temperature for germination (F)	Seed mortality in untilled soil (%/year)	Seed mortality in tilled soil (%/year)	Typical emergence season	Optimum emergence depth (inches)
short, tufted	1.7–3.4 6.8 2.4–7.9	Variable	scd, cms, at, ni	77/50 to 95/77	19–67	46	spring	0.4–4
Photosynthesis type	Frost tolerance	Drought tolerance	Mycorrhiza	Response to nutrients	Emergence to flowering (weeks)	Flowering to viable seed (weeks)	Pollination	Typical & high seed production (seeds per plant)
C4	moderate	high	yes	-	7–13	0	self	1,000 & 100,000

Table Key

General: The designation “–” signifies that data is not available or the category is not applicable.

Growth habit: A two-word description; the first word indicates relative height (tall, medium, short, prostrate) and second word indicates degree of branching (erect, branching, vining).

Seed weight: Range of reported values in units of “mg per seed.”

Seed dormancy at shedding: “Yes” if most seeds are dormant when shed, “Variable” if dormancy is highly variable, “No” if most seeds are not dormant.

Factors breaking dormancy: The principle factors that are reported to break dormancy and facilitate germination. The order of listing does not imply order of importance. Abbreviations are:

scd = seed coat deterioration

cms = a period subjected to cold, moist soil conditions

wst = warm soil temperatures

li = light

at = alternating day-night temperatures

ni = nitrates

Optimum temperature range for germination: Temperature (Fahrenheit) range that provides for optimum germination of non-dormant seeds. Germination at lower percentages can occur outside of this range. The dash refers to temperature range, and the slash refers to alternating day/night temperature amplitudes.

Seed mortality in untilled soil: Range of mortality estimates (percentage of seed mortality in one year) for buried seeds in untilled soil. Values were chosen where possible for seeds placed at depths below the emergence depth for the species and left undisturbed until assessment. Mortality primarily represents seed deterioration in soil.

Seed mortality in tilled soil: Range of mortality estimates (percentage of seed mortality in one year) for seeds in tilled soil. Values were chosen for seeds placed within the tillage depth and subjected to at least annual tillage events. Seed losses are the result of dormancy-breaking cues induced by tillage, germination and deterioration of un-germinated seeds.

Typical emergence season: Time of year when most emergence occurs in the typical regions of occurrence for each weed. Some emergence may occur outside of this range.

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

Photosynthesis type: Codes “C₃” or “C₄” refer to the metabolic pathway for fixing carbon dioxide during photosynthesis. Generally, C₃ plants function better in cooler seasons or environments and C₄ plants function better in warmer seasons or environments.

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

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

Mycorrhiza: Presence of mycorrhizal fungi. “Yes” if present; “no” if documented not to be present, “unclear” if there are reports of both presence and absence; “variable” if the weed can function either with or without, depending on the soil environment.

Response to nutrients: Relative plant growth response to the nutrient content of soil, primarily N, P, K (high, moderate, low).

Emergence to flowering: Length of time (weeks) after emergence for plants to begin flowering given typical emergence in the region of occurrence. For species emerging in fall, “emergence to flowering” means time from resumption of growth in spring to first flowering.

Flowering to viable seed: Length of time (weeks) after flowering for seeds to become viable.

Pollination: “Self” refers to species that exclusively self-pollinate, “cross” refers to species that exclusively cross-pollinate, “self, can cross” refer to species that primarily self-pollinate, but also cross-pollinate at a low rate, and “both” refers to species that both self-pollinate and cross-pollinate at relatively similar rates.

Typical and high seed production potential: The first value is seed production (seeds per plant) under typical conditions with crop and weed competition. The second value, high seed production, refers to conditions of low density without crop competition. Numbers are rounded off to a magnitude that is representative of often highly variable reported values.

Further Reading

Anderson, R.L. 1997. Longspine sandbur (Cenchrus longispinus) ecology and interference in irrigated corn (Zea mays). Weed Technology 11: 667–671.

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.

Parsons, W.T. and E.G. Cuthbertson. 2001. Noxious Weeds of Australia, 2nd ed. CSIRO Publishing: Collingwood, Victoria, Australia.

Twentyman, J.D. 1974. Control of vegetative and reproductive growth in sandbur (Cenchrus longispinus). Australian Journal of Experimental Agriculture and Animal Husbandry 14: 764–770.