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
Mechanical control
Mechanical control options for yellow starthistle typically include hand pulling, hoeing, weed whipping, tillage and mowing.
Hand pulling, hoeing or weed whipping
Hand pulling and hoeing are the oldest forms of weed control used by humans. Although they are labor intensive and relatively ineffective for the control of perennial weeds (with exception of the weed wrench on some shrubs such as the brooms), they typically cause minimal environmental impact. When using manual removal techniques it is important to minimize soil disturbance around the removed plants. Disturbance can create an ideal site for re-establishment of new seedlings or rapid invasion of another undesirable species (DiTomaso 1997).
Manual removal of yellow starthistle is most effective with small patches or in maintenance programs where plants are sporadically located in the grassland system. This usually occurs with a new infestation or in the third year or later in a long-term management program. It can also be an important tool in steep or uneven terrain where other mechanical tools (e.g., mowing and tillage) are impossible to use. To ensure that plants do not recover it is important to detach all above ground stem material. Leaving even a 2 inches piece of the stem can result in recovery if leaves and buds are still attached to the base of the plant (Benefield et al. 1999). The best timing for manual removal is after plants have bolted but before they produce viable seed (early flowering). At this time, plants are easy to recognize and some or most of the lower leaves have senesced. Hand removal is particularly easy in areas with competitive vegetation. Under this condition, starthistle will develop a more erect slender stem with few basal leaves. These plants are relatively brittle and easy to remove. In addition, they are unlikely to have leaves attached at the base and, consequently, rarely recover, even when a portion of the stem is left intact (see mowing for more detail).
In the Bradley method (Fuller and Barbe 1995), a larger starthistle population can be controlled through physical removal by starting at the outward edge of the population and moving in. The technique requires repeated visits but ensures that no new seeds are produced, and soil disturbance is minimized. Using the Bradley method, it is possible to control relatively large starthistle-infested areas (<40 acres) with low-cost and low-impact.
Tillage
Tillage can utilize plows or discs that can control annual weeds by burying plant parts. This is more effective on annuals than perennials. In contrast, harrows, knives, and sweeps can be used to damage root systems or to separate shoots from roots in younger plants and can also be used to damage roots in larger plants (Thomsen et al. 1996b). This technique must be applied when the surface soil is dry, or fragmented plant segments will re-grow and possibly magnify the problem (DiTomaso 1997).
Early summer tillage will control yellow starthistle provided the roots are detached from the shoots. Repeated cultivation can be used in the same season when rainfall stimulates additional germination between tillage practices (Thomsen et al. 1996b). This will rapidly deplete the starthistle seedbank, but may also have the same effect on the seedbank of other desirable species. To be effective, tillage must be conducted before viable seeds are produced. This technique is occasionally used on roadsides, but is often used in agricultural lands and probably accounts for the uncommon occurrence of starthistle as a weed in these environments. In wildlands and rangelands, tillage is usually not an appropriate option for control of yellow starthistle. It can damage important desirable species, increase erosion, alter soil structure, and expose the soil for rapid re-infestation if subsequent rainfall occurs (DiTomaso and Gerlach 2000a).
Mowing
Mowing is a popular control technique along highways and in recreational areas and has less impact on the environment than tillage. Although mowing can be a cost-effective method for control of starthistle, it is not feasible in many locations due to rocks and steep terrain. Even when mowing is employed as a control technique, it is not always successful and can decrease the reproductive efforts of insect biocontrol agents, injure late growing native forb species (Rusmore 1995), and reduce fall and winter forage for wildlife and livestock (DiTomaso 1997, DiTomaso et al. 2000d). In addition, its success depends on proper timing and the growth form of the plant. Mowing plants before the seedheads reach the spiny stage can suppress competing vegetation, thus enhancing light penetration and increasing the starthistle problem. Even repeated mowing conducted too early will not control starthistle and may extend its life cycle. Mowing after plants have produced viable seed will not substantially reduce the seedbank and the following year's infestation.
Despite the limitations of mowing, Thomsen et al. (1994, 1997) and Benefield et al. (1999) demonstrated the successful use of mowing for yellow starthistle control. Thomsen et al. (1994, 1997) consistently demonstrated over 90% control of yellow starthistle using two timely repeated mowings per year over a three year period. Benefield et al. (1999) also showed that mowing conducted at the early flowering stage, before viable seed production, was most effective in controlling yellow starthistle. However, they also demonstrated that the success of mowing as a control strategy not only depended on mowing timing, but also on the plant's growth form and branching pattern. Plants with an erect, high-branching growth form were effectively controlled by a single mowing at the early flowering stage, while sprawling, low-branching plants were not controlled even with repeated mowings at the proper timing.
Mowing may be an alternative strategy for small landowners that do not wish to use herbicides. A few land managers have successfully controlled yellow starthistle using continuous mowing over multiple years. However, since mowing is a late season management tool it is best employed in the later years of a long-term management program or in a lightly infested area. This gives the landowner or manager the ability to assess the level of infestation and the flexibility of choosing the most appropriate and cost effective option, which can include mowing. If the infestation is extremely low, with only a few plants present, hand pulling may be a more desirable method than mowing.
