Xanthium strumarium canadense
Aster family (Asteraceae)
Description: This native or adventive plant is a summer annual about 2-4' tall and little branched, except for short side stems appearing from the leaf axils. The stems are round or slightly ribbed. They are often speckled with purple and have short white hairs scattered across the surface. The alternate leaves are up to 8" long and 6" across. They are cordate or ovate-cordate with bases that are well-rounded or indented and tips that are broad and blunt. Their margins are shallowly lobed or coarsely toothed, while the upper surface has a sandpapery texture. Each leaf has a long petiole that is often reddish or reddish green and about as long as the leaf blade. The petioles usually have short white hairs. A single spike-like raceme of compound flowers develops from the axil of each upper leaf. These racemes are shorter than the petioles of the leaves, often 1-4" in length. In addition, the central stem terminates in a spike-like raceme that is similar to the racemes of the leaf axils. Because Common Cocklebur is monoecious, each raceme produces several male compound flowers along its upper half, while several female compound flowers occur in the lower half.
The male compound flowers are about ¼" across, consisting of numerous staminate florets that have stamens with prominent white anthers. Each male compound flower occurs on a short pedicel and is slightly rounded at the top, while at the base there are 1-3 series of white floral bracts. After shedding their pollen, the male flowers quickly fade away. The female compound flowers are up to 1½" long and 1" across. Each female compound flower contains 2 pistillate florets, which are nearly enclosed by a prickly floral bract with a bur-like appearance. The female compound flowers are initially green, but turn brown as they mature and are slow to detach from the racemes. They are sessile or have short petioles. The surface of the floral bract is covered with curly white hairs, while the prickles are hooked at their tips. At the apex of each bur, there are a pair of spines that are longer and more stout than the prickles. At the base of each spine, there is a small opening for the divided style of a female flower. These styles are inconspicuous and wither away in a short period of time. The blooming period occurs during the late summer or early fall, although some plants may bloom a little earlier or later. Pollination is by wind and there is no floral scent. Each female flower within the bur-like bract produces a single oblong seed that more or less tapers to a point at each end. The seeds are often covered with dark membranes. One of the seeds in each bur has the capacity to germinate the following year, while the the germination of the second seed is delayed for at least 2 years. The root system consists of a taproot that is stout and rather woody. This plant reproduces by reseeding itself, and often forms colonies.
Cultivation: The preference is full or partial sun, moist to mesic soil, and loamy or sandy soil. Occasional flooding is tolerated if it is not too prolonged. Young seedlings of Common Cocklebur exude toxic chemicals that can inhibit germination of other species of plants, or kill off their seedlings. Individual plants become less toxic as they mature.
Range & Habitat: Common Cocklebur occurs in most counties of Illinois and is quite common (see Distribution Map). The two varieties that Mohlenbrock (2002) has described, var. canadense and var. glabratum, are both common in Illinois. The pop-up map refers to the distribution of var. canadense only. Common Cocklebur is native to both Eurasia and North America; it is hard to distinguish between native and adventive races of this plant. Habitats include cropland (especially corn fields), fallow fields, the floodplain zone of rivers and ponds, degraded meadows that are poorly drained, dried-up mudholes, stabilized areas of beaches and sand dunes, vacant lots, and waste areas. Disturbed, poorly drained areas are preferred.
Faunal Associations: The flowerheads don't attract many insects because they rely on wind-pollination. Insects that feed on Common Cocklebur include the leaf beetle Ophraella communa, Rhodobaenus tredecimpunctatus (Thirteen-Spotted Billbug), the plant bug Ilnacora stalii, Uroleucon ambrosiae (Brown Ambrosia Aphid), and the larvae of a fruit fly, Euaresta aequalis (Bur-Seed Fly). The larvae of the last insect feed on the flowers and developing seeds of this plant. Among vertebrate animals, the Purple Finch and Franklin Ground Squirrel reportedly eat the seeds. Deer occasionally chomp off the upper half of mature plants before the bur-like flowers form, while horses and cattle may eat mature plants with the bur-like flowers, which can result in obstruction of the gastrointestinal tract. Pigs eat young plants, which are toxic, and can be poisoned if they consume them in sufficient quantity.
Photographic Location: The floodplain zone of a large drainage ditch in Judge Webber Park in Urbana, Illinois. The lower photograph shows both the male and female flowerheads.
Comments: Common Cocklebur resembles Arctium minus (Common Burdock) somewhat, but it has separate male and female flowers that are brownish white and green, respectively, while the latter species has perfect flowers with bright pink corollas. Because Common Cocklebur is a highly variable plant, different varieties have been identified (or even regarded as separate species in the past). The other variety of Common Cocklebur in Illinois, var. glabratum, has bur-like bracts that are nearly glabrous and they tend to be more oval-shaped and less broad than the bracts of var. canadense. Otherwise, they are very similar to each other. Another species that is adventive from the southwest, Xanthium spinosum (Spiny Cocklebur), is rarely encountered in Illinois. It has more narrow lanceolate leaves, and there is a tripartite spine at the base of each leaf.