Common cocklebur is an adaptable species found in diverse environments. We grew 28 common cocklebur accessions from 14 states at Urbana, IL, to compare their growth, physiology, and morphology in a common environment to determine if regional control recommendations of specific biotypes might be practical. Accessions were evaluated for midseason and maximum heights and widths, flowering date, insect damage, and petiole color in 1998 and 1999. Weight of aboveground tissue, photosynthetic rate, stomatal conductance, chlorophyll content, and leaf and bur morphology descriptions were also taken in 1999. Midseason and maximum widths, midseason height, flowering date, and insect damage differed significantly based on the year of evaluation. The year by accession interaction was a significant source of variation for midseason and maximum heights and widths, flowering date, insect damage, and petiole color. Within each year, accessions differed significantly for all measured variables except maximum width in 1998 and stomatal conductance in 1999. Accessions from northern states usually flowered earlier, and flowering date was negatively correlated with latitude of origin (1998 r = −0.76; 1999 r = −0.60). Nevertheless, some of the earliest flowering accessions were from the southern states. Results from cluster analysis indicated that several accessions had phenotypes more similar to accessions from distant sites than to nearby accessions. We conclude that the accessions differ for traits that may affect weediness, but proximal accessions differ too frequently to recommend regional, biotype-specific control strategies.
Nomenclature: Common cocklebur, Xanthium strumarium L. #3 XANST.
Additional index words: Biotype, common garden, ecotype, intraspecies variability.
Abbreviations: P, probability level; Pn, net photosynthesis; *, significant at P ≤ 0.05; **, significant at P ≤ 0.01.
