Yellow Starthistle Information
Reproduction
-Flower pollination
-Flower & seed
dev
-Seed dispersal
Germination
& dormancy
-Seed prod & types
-Seed development
-Germination
-Stress & germ
-Seasonal germ
-Seed longevity
Growth & establishment
-Seedling establish
-Roots
--growth
& dev
--water use
---competition
-Shoots
--growth &
dev
--light
--temperature
--allelopathy
Biology and Ecology
Germination and dormancy (continued)
Seasonal germination pattern
Yellow starthistle seed germination is closely correlated with winter and spring rainfall events (Maddox 1981, Benefield et al. 2001). Although germination occurs throughout the rainy season, emergence is highest after early season rainfall events. The extended timing of germination increases the difficulty of controlling yellow starthistle populations during the late winter and early spring, as subsequent germination often results in significant infestations.
The germination rate of pappus-bearing seed is higher in late fall and winter, whereas non-pappus-bearing seed germinate more in spring. As a possible explanation, Roché (1965) found that non-pappus-bearing seed have a higher temperature requirement for germination. This could account for the increased germination of pappus-bearing seed in January and February. By the end of each growing season, however, there was no statistical difference between total germination of the two seed types (Benefield et al. 2001).
Effective late-season control strategies such as mowing, tillage, prescribed burning, or postemergence herbicides should be conducted after seasonal rainfall events are completed, but before viable seeds are produced. In addition, the use of preemergence herbicides applied from late fall to early spring should provide residual control extending beyond the rainy season.
Seed longevity and seedbank depletion
Several investigators have attempted to determine the time required to deplete yellow starthistle seedbanks assuming there is no new seed recruitment. In a study conducted in Idaho, Callihan et al. (1989, 1993) reported no effects of burial depth on achene longevity, and showed that the average longevity of non-pappus-bearing and pappus-bearing achenes was six and ten years, respectively. Even after six years of burial, 9% of the pappus-bearing seed germinated. In a similar study in California, Joley et al. (1992) found significant differences in achene longevity at various soil depths. After burying seeds in bags for one year, they reported 88% viability in seed planted 5 cm deep, but only 4% viable seed remaining in bags 1 cm deep. Consequently, there is some confusion as to longevity of yellow starthistle achenes under typical field conditions. The discrepancy between these two studies may be due to ecotypic variability among populations from Idaho and California, various differences in soil or climatic conditions that could influence the rate of microbial degradation, or invertebrate predation of the achenes.
In the same study, Joley et al. (1992) dispersed yellow starthistle achenes on the soil surface. After one year, they reported 80% depletion in the seedbank, and by 3 years only 3.9% of the original seeds had not germinated and were still viable. They attributed this rapid decline in the seedbank to germination, achene mortality and predation. In addition, one year of prescribed summer burning in Sonoma County, California, reduced the seedbank of yellow starthistle by 74% and three consecutive years of burning, with no further seed recruitment, depleted the seedbank by 99.6% (DiTomaso et al. 1999a). This suggests that the longevity of viable seeds under normal field conditions in California may be shorter than previously believed.
In a recent study, Benefield et al. (2001), determined the contribution of achene degradation and germination to yellow starthistle seedbank dynamics in a California site. After sowing 1000 seeds, both emergence and degradation were monitored within an 18 month time period. In the first year, 44% of pappus-bearing and 39% of non-pappus-bearing achenes germinated in the period from January to June. At the end of the first season (October), 88% of the ungerminated pappus-bearing achenes and 81% of the non-pappus-bearing achenes were damaged or degraded by microbial or insect activity. In the second season, beginning with the first rains in November and extending until July, an additional 7% of pappus-bearing and 9% of non-pappus-bearing seeds germinated. Thus, of the seed that emerged over the two year period, the vast majority germinated in the first year. These results also suggest that yellow starthistle seeds are relatively short-lived under California soil and climatic conditions. Furthermore, under these conditions, microbial degradation and invertebrate predation of starthistle seeds probably contribute significantly to the rapid depletion of the soil seedbank.
These studies indicate that yellow starthistle management programs may require only two to three years of effective control to dramatically reduce the soil seedbank and infestation. In support of this, three consecutive years of prescribed burning reduced yellow starthistle seedbanks by 99.6% and vegetative cover by 91% (DiTomaso et al. 1999a). For long-term sustainable management to be achieved, land managers will be required to prevent seed recruitment from the remaining seedbank germinants or new introduction of dispersed seeds from off-site sources.
