Seed biology of rush skeletonweed in sagebrush steppe

Rush skeletonweed (Chondrilla juncea L.) is an invasive, herbaceous, long-lived perennial species of Eurasian or Mediterranean origin now occurring in many locations throughout the world. In the United States, it occupies over 2.5 million ha of rangeland in the pacific Northwest and California. Despite the ecological and economic significance of this species, little is known of the ecology and life history characteristics of North American populations. The purpose of this study was to examine seed germination characteristics of 2 populations of rush skeletonweed in Idaho. Seeds from rush skeletonweed plants in southwestern Idaho were collected during the 1994 and 1995 growing seasons. Mature seeds were harvested on 6 dates between early July and early October 1994, and on 5 dates between early July and late September 1995. Fresh seeds from each harvest period were measured to determine seed weight, total germination, rate of germination, and viability (tetrazolium staining [TZ]) of non-germinating seeds. An aliquot of seeds collected in 1994 was also stored for 1 year to examine the effects of seed storage on germination. In southwestern Idaho, rush skeletonweed produces seeds continuously from mid-July through October. Seeds were capable of immediate germination without scarification or wet prechilling. Total germination generally ranged from 60 to 100% throughout the entire seed production period. Germination was also rapid, reaching 50% of total germination in less than 12 days. In general, germination was higher at the lower incubation temperature regime (20/10 degrees C), perhaps reflecting origins of this species in Mediterranean winter rainfall regions. The TZ testing indicated that 30 to 60% of non-germinating seeds were viable, suggesting that seeds may persist in the soil seed bank. Up to 60% of seeds remained viable following 1 year of storage. Stored seeds generally exhibited higher germination rates (average = 90%) than fresh seeds (average = 67%), indicating possible dormancy and afterripening effects. Germination characteristics of this species are consistent with those of other invasive alien species, and favor rapid population growth leading to community dominance.