Experiments were conducted in 2002 and 2003 to determine the influence of soil thermal amplitude on common cocklebur emergence. Additionally, common cocklebur achenes were collected in fall of 2003 and of 2004 to assess changes in light and temperature requirements for germination over a 12-mo period under field conditions. Common cocklebur germination in response to the light environment and to constant and fluctuating temperatures were evaluated under controlled conditions following achene retrieval from the field. There was a linear inverse relationship between shade intensity and soil thermal amplitude, which explained 77% of the variability in emergence in the field over 2 yr. Emergence decreased as soil thermal amplitude declined, with 95% shading resulting in a 72 to 88% reduction in emergence. Neither red nor far-red light had much effect on germination, and burial did not induce a light requirement. Germination of achenes retrieved from the soil surface or buried in soil generally was not affected by red or far-red light, and the achenes did not acquire a red-light requirement following burial. Daily exposure to natural greenhouse light at 24 to 30 C was essential for germination immediately following achene maturation, whereas no germination occurred in darkness. A thermal fluctuation of 15 C increased germination percentages over those at constant temperatures regardless of time after maturation or retrieval-depth of achenes. The mean fluctuating temperature over all sample dates that was generally optimal for germination was 25 C (17.5/32.5 C low/high temperatures; 15 C daily fluctuation) in April or May and July or August in both years. Germination was generally optimum at constant temperatures of 35 or 40 C. The higher mean temperature requirement for germination at constant temperatures than at fluctuating temperatures likely contributes to reduced emergence in spring and summer months, when earlier emerging weed species or crops have already become established, and the thermal fluctuation requirement for germination reduces the likelihood of emergence in an environment where light availability is diminished.
Nomenclature: Common cocklebur, Xanthium strumarium L. XANST