Factors affecting horseweed emergence are important for management of this weed species, particularly because of the presence of herbicide-resistant biotypes. Horseweed emergence was highly variable and not strongly correlated to soil temperature (r² = 0.21), air temperature (r² = 0.45) or rainfall (r² = 0.32). Horseweed emerged mainly during April and September in Tennessee when average daytime temperatures fluctuate between 10 and 15.5 C. However, some horseweed plants emerged during almost any month when temperatures ranged from 10 to 25 C and adequate moisture was available at the soil surface. Horseweed densities ranged from a low of 30 to 50 plants m⁻² to a high of > 1,500 plants⁻² at one location. These extremely high densities illustrate the ability of horseweed to be an effective ruderal plant that can produce stands that approach monoculture densities if not controlled. The amount of crop residue remaining after harvest from the previous field season was in the order of corn > cotton > soybean > fallow. Residue from a previous corn crop reduced horseweed emergence compared with soybean and cotton residues in a no-tillage situation. Decreased horseweed density due to crop residue presence indicates that a systems approach may help reduce horseweed populations.

Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L.; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.