Yellow Starthistle Information
Mech control
-hand
-tillage
-mowing
Cultural control
-grazing
-burning
-re-vegetation
Biocontrol
-insects
-biocontrol table
-plant pathogens
Chem control
- risks
--spray
--water
--toxicology
--herbicide
resist
--effects
-herbicides
--preemergence
--postemergence
--late season
--pre- & post-
--imazapic
--clopyralid
--picloram
Management
Cultural control (continued)
Prescribed burning
Fire has long been an important component in the development and continuance of most grassland systems. In addition to controlling some important noxious annual grasses, such as barbed goatgrass (Aegilops triuncialis), medusahead (Taeniatherum caput-medusae) and ripgut brome (Bromus diandrus), prescribed burning can all be used successfully to control yellow starthistle (DiTomaso et al. 1999a).
As with mowing, the success of this method depends on proper timing. Unfortunately, the best time for burning is usually in early to mid-summer (late June to early July), which may not be feasible in some areas. At this time starthistle is in the very early flowering stage (similar to ideal mowing timing), and has yet to produce viable seeds, whereas seeds of most desirable species have dispersed and grasses have dried to provide adequate fuel. Fire has little if any impact on seeds within the soil. Since grassland fires only transiently heat the soil surface to about 200oC, seeds on the soil surface are not typically damaged and may actually be stimulated to germinate, as is probably the case with many legumes.
After a single year of burning, the resident seedbank of yellow starthistle will be sufficiently high enough to allow re-infestation the following year. However, burning will reduce the thatch layer, expose the soil, and recycle nutrients trapped in the dried vegetation. In the first growing season after the burn, plant diversity will often increase, particularly native species, both perennial grasses and forbs. It is speculated that this increase in plant diversity is due to increased light penetration and/or increased soil temperature earlier in the season (DiTomaso et al. 1999a). After three consecutive years of burning, DiTomaso et al. (1999a) showed that starthistle seedbanks were reduced by over 99%, with vegetative control greater than 90% in spring and summer measurements.
Despite its effectiveness, there are some risks associated with prescribed burning as a method of controlling yellow starthistle (DiTomaso 1997). Air quality issues can be a significant problem when burns are conducted adjacent to urban areas. This potential problem can be avoided by conducting burns in more isolated regions not adjacent to urban areas. Public relations problems can be avoided by educating residents of the intended goals of the project prior to the burn. A major risk of prescribed burning is the potential of fire escapes. This is particularly true when burns are conducted during the summer months. This can be minimized by proper preparation and involvement of local fire departments and the California Department of Forestry (CDF). Fire can also have a significant impact on small animals and insects unable to escape the burn. For example, burning for control of yellow starthistle during the summer undoubtedly damages seedhead feeding biocontrol insects and their larvae. In some areas, burning can lead to rapid invasion by other undesirable species with wind dispersed seeds, particularly members of the sunflower family.
Continuous burning as a control strategy can also increase soil erosion and impact the plant composition within a site. Species that complete their life cycle before the burn will be selected for, while those with later flowering times will usually be selected against. Although this is a potential concern, data from Sonoma County, California, showed that only a few plants were negatively impacted by prescribed summer burning and most of these were introduced species. Some plants were unaffected by burning, but several species, particularly native legumes, were enhanced by the burn (DiTomaso et al. 1999a, Hastings and DiTomaso 1996).
The ability to use repeated burning depends on climatic and environmental conditions. In areas where resources are ample and total plant biomass is abundant, two or three consecutive years of burning may be practical. However, in other environments or years, fuel loads may not be sufficient to allow multiple year burns. Consequently, prescribed burning may be a more appropriate option as part of an integrated approach. A possible combination may be a first year clopyralid treatment, which is likely to suppress legumes and stimulate grasses, followed by a second year burn. In the second year fuel loads should be high and the previously suppressed legume populations are likely to be stimulated by the burn (DiTomaso et al. 2000d). Since prescribed burning is a late season tool, it can be considered as part of a flexible strategic management plan. For example, if the risk of burning in a particular year is too high, another late season technique (mowing, manual removal, or postemergence herbicide treatment) could be substituted.
In addition to summer burning, yellow starthistle seedlings have been controlled using winter or early spring flaming techniques (Rusmore 1995). This reduces the risk of escaped fires and avoids major air quality issues. However, this technique is somewhat non-selective and the successful control of yellow starthistle has proven inconsistent. When spring drought follows a flaming treatment, control of starthistle can be excellent (Rusmore 1995). In contrast, a wet spring can lead to complete failure and increased starthistle infestation, particularly since competing species may be dramatically suppressed.
