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
Reproduction
Flowering and pollination
In California, yellow starthistle typically begins flowering in late May and continues through September and sometimes much later. There are very low levels of self-fertilization in yellow starthistle (Harrod and Taylor 1995, Maddox et al. 1996, Sun and Ritland 1998). Thus, a significant amount of cross-fertilization insures a high degree of genetic variability within populations.
Honeybees play an important role in the pollination of yellow starthistle, and have been reported to account for 50% of seed set (Maddox et al. 1996). Bumblebees are the second most important floral visitor to starthistle flowers, but several other insects also contribute to fertilization of the ovules (Harrod and Taylor 1995).
In a study conducted by Barthell et al. (2000) on Santa Cruz Island in California, investigators found that honeybees visited yellow starthistle 33 times more than native bees. By comparison, native bees visited a native gumplant species (Grindelia camporum) in the same family 46 times more than honeybees. Also, they found that when honeybees were excluded from visiting starthistle but native bees were not, the average seedhead weight of yellow starthistle significantly declined. This supports the hypothesis that honeybees are the most important insect to yellow starthistle seed production.
Phenology of flower and seed development
On average, seedheads require approximately 21 days to progress from pre-bloom to petal abscission (Benefield et al. 2001). Flowers remain in full bloom for just over two days before they begin to senesce. Senescence requires an additional 14 days, with the late senescence stage requiring the longest transition time (7-9 days).
The time period from flower initiation to the development of mature viable seed is only 8 days. No germinable seeds are produced until 2% of the spiny heads initiate flowering (Benefield et al. 2001). By 10% flowering, an estimated 100 germinable seeds are produced per 100 flowerheads. Achene production increases exponentially as percent flowering progresses. Thus, to prevent seed production, it is most practical to gauge timing of late season control practices around flower initiation, as this stage is easily recognized. Effective long-term control may be compromised if control practices are delayed too long after flower initiation, as it will allow production of viable seed. Therefore, to prevent new achene recruitment, late-season control options such as tillage, mowing, prescribed burning, and herbicides should be conducted before approximately 2% of the total spiny heads have initiated flowering.
Seed dispersal
The pappus-bearing seed are usually dispersed soon after the flowers senesce and drop their petals. However, non-pappus-bearing seeds can be retained in the seed head for a considerable period of time, extending into the winter (Callihan et al. 1993). These seed have no wind dispersal mechanism and the majority simply fall to the soil just below the parent plant. With pappus-bearing seed, the pappus is not an effective long distance wind dispersal mechanism. Roché (1991a, 1992) reported that 92% of yellow starthistle seed fall within 2 feet of the parent plant, with a maximum dispersal distance of 16 ft over bare ground with wind gusts of 25 miles/hr. By comparison, birds such as pheasants, quail, house finches, and goldfinches feed heavily on yellow starthistle seeds and are capable of long distance dispersal (Roché 1992). Human influences, including vehicles, contaminated crop seed or hay, road maintenance, and moving livestock, can also contribute to rapid and long distance spread of the seeds.
