Most seed of broom snakeweed [Gutierrezia sarothrae (Pursh) Britt. & Rusby] and threadleaf snakeweed [G. microcephala (DC.) Gray] were dormant when collected soon after maturity, requiring 4 to 6 months of storage at room conditions to break dormancy. An alternating temperature regime of 10 C (16 h) and 20 C (8 h) favored snakeweed germination during the afterripening period, but germination was greatest under alternating temperatures of 20 C (16 h) and 30 C (8 h) after 24 months of storage. After 6 months of storage, maximum germination (70% or more) of threadleaf snakeweed seed occurred at continuous temperatures of 15 to 25 C; whereas, broom snakeweed seed germinated best at 15 and 30 C. Germination of both species in darkness was half or less of germination with daily 8-h periods of light. Germination of seed lots containing a high proportion of dormant achenes was enhanced by preimbibition storage at 50 C; after dormancy was naturally broken, exposure to 50 C prior to germination decreased early seedling vigor and growth but not germination.
