Unharvested section of a soybean field due to a biennial wormwood infestation.īiennial wormwood competes more aggressively for resources than soybean, and season-long competition from nine biennial wormwood plants/yard2 (10 plants/meter2) can reduce soybean yield by 44 percent (Nelson 2001 Nelson and Kegode 2006). In late July, when day length is declining, biennial wormwood plants bolt as they prepare to reproduce and appear above the canopy of crops such as soybean (Figures 4 and 9).įigure 9. Upon emergence, biennial wormwood seedlings grow slowly and occur as rosettes for much of the early part of the growing season (Mahoney and Kegode 2004). (redroot and Powell amaranth) in soybean at Fargo, N.D. (green and yellow), common lambsquarters and pigweed spp. Cumulative emergence percentages for biennial wormwood, foxtail spp.
Distinguishing Biennial Wormwood from Common Ragweedīiennial wormwood can be distinguished from common ragweed by a close examination of the leaves. Comparison of biennial wormwood (right) and common ragweed (left) plants growing next to each other in the field. Biennial wormwood often is confused for common ragweed (Figure 6).įigure 6. Biennial wormwood grows slowly after emergence, remaining as a rosette until midsummer, when plants bolt and growth becomes rapid. Characterization of emergence patterns in eastern North Dakota indicated that the weed began to emerge in late June or early July in corn, dry bean, soybean and sunflower (Kegode and Ciernia 2003). Seedling emergence can occur during the entire crop growing season under moist conditions and favorable environmental conditions. Seedling Emergence, Growth and Development Distribution of biennial wormwood in the United States in 2000.
The eastward spread of biennial wormwood into agricultural areas may have resulted from selection of the annual biotype however, this aspect of biennial wormwood has yet to be investigated.įigure 5. Perhaps annual and biennial types of biennial wormwood coexisted in the region of its origin, but primary use of the adjective “biennial” infers that the biennial trait was predominant. 1999).īiennial wormwood is found throughout the United States except the South (Figure 5). Similarly, biennial wormwood was not a common problem in continuous cereal and cereal-legume fields in Manitoba, Canada, in 1993 but was detected in 1994 (Ominski et al. However, in a 1997 survey of South Dakota, biennial wormwood was found in 92 percent of soybean fields (Snyder 1997), whereas in a 2000 survey, biennial wormwood was present in 3.2 percent of 663 North Dakota fields that were sampled (Zollinger et al.
Surveys conducted in the late 1970s did not identify biennial wormwood in any field that was sampled (Dexter et al. Occurrence and Distributionīiennial wormwood is native to North America, specifically the Rocky Mountains and Sierra Nevada and Cascade ranges of the northwestern United States and western Canada.īiennial wormwood historically was classified as a noncropland weed and, as such, did not receive much attention until cropland infestations began to rise noticeably. Photo showing spikelike flower head of a mature biennial wormwood plant in early September (left) and mid-October (right) at Fargo, N.D. Bolting biennial wormwood plant.įigure 4. Stevens’ estimate of seed production possibly was from a biennial plant, whereas the later estimate was from an annual plant.įigure 3. Mahoney and Kegode (2004) later estimated that a single biennial wormwood plant produced 400,000 seeds. Biennial wormwood is a prolific weed, producing approximately 1 million seeds per plant (Stevens 1932). Biennial wormwood flowers consist of heads in clusters arranged in a spikelike form. Plants typically grow 3 to 7 feet (1 to 2 meters) tall with a woody stem averaging 1 to 2 inches (3 to 5 centimeters) in diameter. The leaves are hairless and have toothed margins.
Biennial wormwood stems arise from a tap root, are hairless and often are tinged red. Biennial wormwood is a small-seeded plant that behaves like an annual species.