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
Growth and establishment
Seedling establishment
In several infested areas, yellow starthistle seed production can be over 50 million seed per acre (DiTomaso et al. 1999a, Maddox 1981, Sheley and Larson 1994a). In exposed areas, high germination can result in extremely dense seedling populations. Seedlings are more likely to establish in soils with deep silt loam and loam with few coarse fragments (Larson and Sheley 1994). In many areas, a significant amount of self-thinning occurs and only a small fraction of seedlings reach reproductive maturity (Larson and Sheley 1994, Sheley and Larson 1994a). Thus, in heavily infested areas, starthistle populations produce far more seed than are necessary to re-infest the following year.
Roots
Growth and development
Following germination, yellow starthistle allocates resources initially to root growth, secondarily to leaf expansion, and finally to stem development and flower production (Sheley et al. 1983c, 1993, Roché et al. 1994). Root growth during the winter and early spring is rapid and can extend well beyond 3 feet in depth. Starthistle roots elongate at a faster rate and to greater depths than potentially competitive species, including weedy annual grasses and clovers (Sheley et al. 1993). During this same time period, rosettes expand slowly. In a study conducted in Washington by Roché et al. (1994), roots grew at a mean rate of 0.5 cm per day and as fast as 2.1 cm per day. They found that 140 days after planting, roots grew out the bottom of 1.2 m long (4 ft) tubes. Little above ground rosette growth occurred during winter months when roots were quickly growing. Other studies also demonstrated rapid yellow starthistle root growth during late winter and early spring (DiTomaso, unpublished data, Sheley et al. 1993, Sheley and Larson 1992, 1994b). Using minirhizotron tubes in the field, root depth increased exponentially with time and by 64 days roots reached depths of 2 ft. Within 80 days from planting (end of March), roots in most plots extended beyond the depth of the acrylic tubes (3.5 ft). Yellow starthistle roots extended beyond 6 ft after two months of growth when grown in tubes in the greenhouse (DiTomaso and Kyser, unpublished data).
Rapid germination and deep root growth in yellow starthistle extends the period of resource availability into late summer, long after seasonal rainfall has ended and shallow-rooted annual grasses have senesced. By extending the period of resource availability, competition is reduced at the reproductive stage. This can greatly benefit starthistle by ensuring ample seed production into the dry summer months (Sheley et al. 1993).
The potential density of yellow starthistle in a particular site can be closely associated with soil depth and, thus, late season water storage capacity. A direct direct relationship exists between the number of starthistle plants per unit area and the soil moisture depth (Roché et al. 1994).
Shading of young rosettes can have a dramatic affect on root growth (Roché et al. 1994). Reduced root growth correlates with increased shading (DiTomaso, unpublished data). Roots of yellow starthistle required 94, 138, and 163 days to reach a 60 cm depth in the unshaded, 80%, and 92% shaded plots, respectively. Roots of plants growing under 80% light reduction required an additional month and a half to reach a 2 ft soil depth, whereas an additional two months was required for plants grown under 92% light reduction.
Since yellow starthistle plants germinate over an extended time period beginning with the first fall rains and ending with the last spring rain event, the resulting canopy would likely consist of plants in several stages of development. In dense stands of starthistle, the population would be represented by both large canopied plants receiving full sunlight and an understory of smaller shaded plants. Thus, light suppression would likely be a significant factor regulating root growth. The roots of larger plants exposed to full sunlight quickly grow to great depths, while roots of shaded plants underneath the starthistle canopy would occupy shallower depths for longer periods of time. Under these conditions, starthistle would rapidly deplete soil moisture from all depths in the soil profile and severely compete with deep and shallow rooted annual species.
