Description

Cocklebur (Xanthium strumarium L.) is a large, annual broad-leaved weed with an erect, branched stem and broadly triangular leaves (Figure 1). The stem is rough-hairy, spotted with purple and up to 1.5 m high. The leaves are alternate on the stem, with toothed or lobed margins and rough-hairy surfaces. The flowers are small, green and inconspicuous (Figure 2). The fruit is an oval, brown, woody bur covered with spines and ending in 2 stout, hooked beaks (Figure 3). Each bur normally contains 2 seeds. Seedlings have long, narrow cotyledons (Figure 4), and the bur often remains attached to the root, which can be an identifying feature. Mature plants of cocklebur are sometimes mistaken for burdock (Arctium minus (Hill) Bernh.), which also has large, coarse leaves and spiny burs. Burdock, however, is a biennial that produces a rosette of large, coarse leaves the first year, a flowering stalk with purple flower heads the second year, and round burs densely covered with hooked spines.

Cocklebur (Xanthium strumarium L.)
Figure 1. Cocklebur (Xanthium strumarium L.)
Male and female flowers.
Figure 2. Male and female flowers.
Fruit (bur) and seed.
Figure 3. Fruit (bur) and seed.
Three-week old seedling.
Figure 4. Three-week old seedling.

Distribution

Cocklebur is found in all provinces of Canada except Newfoundland and is widespread throughout the United States. In Ontario, it is most common in the southwestern counties, but is present throughout the southern part of the province. It is typically seen in soybean fields or along beaches near Lake Erie or Lake Ontario. Cocklebur is associated with conventional tillage, rather than no-till, and with a high frequency of soybeans in the rotation.

Economic impact

Cocklebur is considered to be one of the most competitive weeds in soybeans. The economic threshold for cocklebur emerging at the same time as the crop is approximately 3-5 plants/10 m of row, depending on herbicide cost and crop price. In addition to direct yield losses through competition, infestations of cocklebur decrease soybean seed quality through increased foreign matter and higher seed moisture content. The thick, woody stems slow combine speed and decrease harvesting efficiency. Cocklebur is less competitive in corn than in soybeans, but can still cause significant yield losses. Cocklebur can be a particular nuisance in livestock operations because the young seedlings (but not adult plants) are poisonous to animals, and the burs become entangled in their hair or wool. Cocklebur is a close relative of ragweed and both the pollen and contact with the leaves can cause allergic reactions in sensitive individuals.

Biology and economy

Cocklebur seedlings usually emerge in May or June, when average soil temperatures are above 15°C, although emergence can continue throughout the summer with adequate moisture. Plants can tolerate frequent flooding and saline soil conditions. Flowering is triggered by day length, and begins in mid-August in Ontario. The first burs are often ripe in early September, with new flowers and burs forming until a killing frost occurs.

Seeds from immature, green burs will still ripen if plants are cut and left in the field. Seed production is proportional to plant size and ranges from several hundred to several thousand seeds per plant. The spiny burs are dispersed to new locations by clinging to animal fur, human clothing, and other material. They are also readily dispersed by water, because the burs contain air spaces that allow them to remain buoyant and float for up to 30 days.

Only 1 of the 2 seeds within each bur normally germinates in the year after production, leaving the second seed to re-infest the field in subsequent years. Cocklebur seeds generally do not survive for more than 5 years. Seedling emergence from burs lying on the soil surface is low, because the spines prevent good seed to soil contact and water uptake. Emergence increases when burs are buried by tillage or even just pressed into the soil by wheel traffic. Seedlings can emerge from up to 15 cm depth in the soil. Cocklebur populations decrease in the absence of tillage and in crop rotations where soybeans are grown only once every 3 years or less.

Control

Cocklebur is susceptible to many soil applied and postemergent herbicides recommended for the control of broadleaf weeds. In soybeans, cocklebur can be controlled by:

  • Basagran Forte (bentazon)
  • Classic (chlorimuron-ethyl)
  • Pursuit (imazethapyr)
  • Reflex (fomesafen)
  • Blazer (acifluorfen)
  • Reliance (chlorimuron-ethyl/thifensulfuron-methyl; STS soybeans only)
  • Roundup (glyphosate; RR soybeans only) applied postemergent.

Preemergent applications of Conquest (imazethapyr+metribuzin), Broadstrike (flumetsulam), Sencor/Lexone (metribuzin), Pursuit, and Valor (imazethapyr/pendimethalin), also provide good control, although seedlings emerging from deep in the soil may escape these treatments. Some populations of cocklebur have acquired resistance to Group II herbicides such as Pursuit in 9 soybean producing states of the U.S., but no resistant populations have been reported in Ontario.

In corn, most pre- and postemergent broadleaf herbicides provide good to excellent control of cocklebur, including:

  • Banvel II (dicamba)
  • Marksman (dicamba/atrazine)
  • Fieldstar (flumetsulam/clopyralid)
  • 2,4-D, Peakplus (prosulfuron+dicamba)
  • Pardner (bromoxynil)
  • Basagran Forte
  • Laddok (bentazon/atrazine).

For information concerning herbicides, rates of application, crops in which they may be used safely, and specific cautions, consult the latest edition of OMAFRA Publication 75, Guide to Weed Control.