"To stop or slow the spread of this newly invasive plant in California,
we must quickly develop effective management tools
and an informed management approach."
With several weather related events, fire, extreme heat, etc., there has been an increase of undesired species growing on rangelands. Invasive species out compete with the more desired species such as grass, forbs - forage for grazing animals, both livestock and wildlife. Once of these species is Dittrichia graveolens (L.) Greuter, commonly known as Stinkwort, is an invasive species that is expending on lrnaglends in Sonoma and Marin Counties.
Stinkwort is not considered as a palatable species to animals and can be poisonous to livestock and cause contact allergic dermatitis in humans. Minimizing the impact of this noxious weed requires a good weed management plan. It will be important to begin a control program for this and other invasive species given our recent fire. Noxious weed can thrive in the burned areas, and the recent fire exposed ground surfaces, reduced shade and increased light, creating a flush of nutrients. All of these conditions favor this weed.
In California, this weed is found primarily along roadsides. However, the biology of this annual plant suggests that it could also invade open riparian areas and overgrazed rangelands. Stinkwort has an unusual life cycle among annual plants: unlike most summer or late-season winter annuals, stinkwort flowers and produces seeds from September to December. Such basic biological information is critical to developing timely and effective control strategies for this rapidly expanding weed.
Stinkwort seeds are likely spread by wind, on the fur and feathers of mammals and birds and on motor vehicles and equipment, thus moving along transportation corridors. Stinkwort has very high seed viability, with an average of about 90% of the seeds capable of germination at the time they disperse from the plant. While the primary expansion has moved radially from the original infestation in Santa Clara County, it has been present in Sonoma County since early 2000.
What makes stinkwort's life cycle rather unusual is that it matures much later in the season than most annuals, even other late-season winter annuals. For example, yellow starthistle begins to send up a flowering shoot (bolt, referred to as moderate growth in April, begins flowering in late June, and — like most late-winter or summer annuals — has completed its life cycle by September or October. In contrast, stinkwort begins to bolt in mid-May, grows most of its branches and leaves between June and September and flowers and produces seeds from September to December. What makes stinkwort's life cycle rather unusual is that it matures much later in the season than most annuals, even other late-season winter annuals.
Aside from the tarweeds, there are few other late-season winter annual species with a similar life cycle in the native California flora. Some other weedy species, such as Russian-thistle, horseweed (Conyza canadensis (L.) Cronquist) and yellow starthistle (Centaurea solstitialis L.), have similar life history strategies, with only Russian-thistle and horseweed flowering within the same time frame as stinkwort. In contrast to stinkwort, Russian-thistle is a summer annual that germinates in spring.
Stinkwort can grow roots as deep as yellowstarthistle andvirgatetarweed, this occurs several weeks after these other grassland annuals grow their roots. Thus, it may be that stinkwort will not be a significant invasive plant of rangelands, except in years when there is significant late-season rain or when competitive winter annual species are removed by overgrazing. Nevertheless,stinkwort has also been observed in open riparian systems, where water is not a limiting factor and a slow-growing shallow root system will not limit its competitive ability. It is possible that this non-native species may eventually become a problematic weed in these more-open wetland areas.
Current control methods.
The challenge in controlling stinkwort is applying the appropriate management at the proper time. Although traditional methods of control, including mechanical and chemical techniques, can be effective, determining the most appropriate timing has been difficult. If management actions are not taken before plants begin to produce seeds in the fall, there is a risk that they will help disperse seeds rather than control stinkwort populations. For example, mowing may move seeds on equipment for long distances when conducted too late in the season. Unfortunately, a poor understanding of the biology of this plant and of how to control it effectively have led to unsuccessful management of growing infestations as well as much wasted time and money. However, management tools that prevent seed production for 1 to 2 years have the potential to greatly reduce the soil seed bank and, thus, the population size.
Biological or cultural practices.
There are currently two biological or cultural practices that can be employed to limit the ability of stinkwort to invade an area. One is to minimize disturbances such as overgrazing and soil manipulation in natural andrangeland sites. Second, pastures should be managed for dense, competitive stands of desirable perennial or annual grasses that maximize ground cover in spring, when stinkwort seedlings are beginning to establish.
Mechanical control options can take advantage of the stinkwort root system, which is slow growing and initially relatively shallow. Plants may be controlled by hoeing or pulling. However, because stinkwort can cause dermatitis, it is important to wear appropriate protective clothing (long sleeves, long pants, gloves) to minimize exposure to the irritating oils. Once in flower, stinkwort plants should be bagged and removed from the site to prevent seeds from maturing and dispersing after the plants have been cut and left on the soil surface. Mowing can provide partial control when conducted late in the season. However, buds remaining on branches below the level of the mower may regrow. Mowing a second time can give improved control, especially when conducted after the soil has dried out in mid- to late summer. In contrast, mowing too early, as is done on highways to reduce the threat of grass fires, will favor stinkwort by removing competing annuals while this weed is still small and lower than the mowing blades.
Postemergence herbicides are applied to small germinated seedlings or young plants. Thus, in contrast to preemergence herbicides that are generally applied to larger areas before seeds germinate, postemergence applications can directly target known infestations visible to the applicator. However, the sticky oils on the foliage, especially on mature plants, make it difficult to control stinkwort with postemergence herbicides. To overcome this, it may be necessary to use ester formulations of postemergence phenoxy-type herbicides (2,4-D, MCPA, triclopyr, etc.). However, these compounds are more volatile compared to salt formulations (commonly used in summer), and some should not be applied when ambient temperatures will reach or exceed 80°F.
In University of California experiments conducted for the postemergence control of stinkwort, it was found that the salt formulation of triclopyr at 24-ounce acid equivalent (a.e.) per acre (2 quarts Garlon 3A per acre) gave the most effective level of control following a postemergence application. Triclopyr is selective and relatively safe on grasses, but it must be used cautiously around vineyards, as grapevines are extremely sensitive to triclopyr drift. It is also important to note that control with postemergence herbicides is most effective when plants are young, actively growing and not exposed to stresses such as drought. For stinkwort, this is generally just before or at the time of bolting.
Glyphosate (Roundup Pro) at 1 quart product per acre also gave fairly good control, and anecdotal information from other land managers indicates that a rate of 2 quarts product per acre gives control similar totriclopyr at 2 quarts product per acre. Unfortunately, other herbicides, including aminopyralid (Milestone) and aminocyclopyrachlor (one of the active ingredients in herbicide Perspective), did not provide effective late-season post-emergence control of stinkwort. As previously discussed, plants also partially recovered from late-season mowing.
Pre- and early postemergence herbicides.
Because stinkwort germinates throughout the rainy season, the most effective control options are likely to be broadleaf selective herbicides with both pre- and early postemergence activity, which can control both new germinants and young emerged seedlings. A fairly new group of foliar- and soil-active growth regulator herbicides have proven very effective in winter and spring applications for control of yellow starthistle and other members of the sunflower family (Asteraceae). These herbicides have the ability to control both emerged young plants through foliar activity, as well as germinating seedlings through soil activity. These chemicals include clopyralid (Transline), aminopyralid (Milestone) and aminocyclopyrachlor, and they are generally safe on grasses. In preliminary research, UC found that winter applications of aminocyclopyrachlor and spring applications of Milestone VM+ (aminopyralid plus triclopyr) showed the greatest potential for controlling stinkwort. Early-season application of glyphosate, however, controlled competing vegetation and so allowed late-germinating stinkwort to thrive. Thus, glyphosate is best used later in the season as a post-emergence application.
It will be critical to assess the impact that stinkwort is making on your rangelands. A rancher should inventory and map the impacted areas of the ranch; then develop a management strategy to control this invasive plant. Having invaded sites mapped allows ranchers to assess future expansion and can help prioritize management activities. This work also lays a foundation for future implementation of specific management methods. To stop or slow the spread of this newly invasive plant in California, we must quickly develop effective management tools and an informed management approach.
For more information on control strategies: Stinkwort is rapidly expanding its range in California by Rachel Brownsey, Guy B. Kyser, Joseph DiTomaso,
- Author: Stephanie Larson
UCCE Sonoma County along with other UC colleagues, recently received a grant to identify the impact of grazing on the frequency and severity of wildfire in California. The project will ask three specific questions:
- Does grazing reduce the likelihood of fires at the landscape scale?
- Does fire severity differ between grazed and ungrazed lands?
- What are the synergies and tradeoffs of grazing management as a tool to directly reduce wildfire risk?
Our results will be used to:
- Suggest to individual land owners the potential for grazing to reduce their risk of wildfire
- Influence policy makers to reduce barriers to grazing in California.
With the reduced frequency of wildfires, woody plant cover increased and shrublands and woodlands expanded (Miller et al., 1994). Burning was reintroduced around 1945, with the primary purpose to convert bushlands to grassland and to maintain the rangelands brush-free. An average of 67,000 acres of brush was burned annually under State permit from 1945 to 1951; and from 1951 through 1952, 133,000 acres burn annually (Biswell, 1954). With environmental conservation movement in the 1960s and 70s, controlled burning again was not favored and fire suppression was the referred management tool. This allowed ecosystems to accumulate more fuels that are prone to burning on a regular interval. Management practices can greatly affect a landscape's fuel amount and distribution. Fuel load, or biomass, is one of the most influential and easily manipulated fuel variation affective fire intensity (Strand et al., 2014). Livestock grazing is one management technique that has been shown to decrease fine fuel loading and subsequent wildfire severity (Davies et al., 2010).
Fire fuel treatments are designed to alter fuel conditions so that wildfire is easier to control and less destructive (Reinhardt et al., 2008). Cattle grazing primarily alters fuel conditions by reducing the amount of herbaceous fine fuels, whereas goat and sheep grazing can potentially also reduce the shrub component. Other fuel treatments that can be used to accomplish these same objectives include herbicides, mechanical treatments such as mowing, prescribed/controlled fires, or a combination of these treatments (Nader et al., 2007). Many studies have reviewed and describe factors affecting fuel treatment costs but studies specifically on rangelands are limited. Least cost fuel treatments will vary with conditions and objectives, but grazing alternatives appear to be cost-competitive especially if the objective is reduce fire fuel loads where mowing or a prescribed burn are potential alternatives (Strand et al., 2014).
Over the years, management has played a significant role in shaping California's rangelands. On a yearly basis, grazing can reduce the amount and alter the continuity of fine fuels, potentially changing wildfire fire spread and intensity (Stand et al., 2018). With changing climate conditions, it is now more critical than ever that grazing, as a fire fuel reduction tool, be scientifically quantified in order to demonstrate its use to landowners, managers and policy makers. California provides a unique opportunity to analyze how grazing effects wildfire trends due to the presences of long-term fire and climate datasets as well as diverse conditions under which grazing takes place. California is likely to see an increasing number of extreme fire danger days, almost doubling from current numbers over the next 50 years (Yoon et al, 2015).
The research project will use data from the past 30 years of wildfires across the whole state of California, along with data on climate, vegetation type, land ownership and biophysical variables to determine if grazed areas burned less frequently and/or with less severity than non-grazed areas. In addition, we will seek to identify trade-offs and synergies between grazing wildfire management. In areas where there is a high probability of ignition and the area is grazed, is there an optimum residual dry matter (RDM) to be managed for to reduce risk? Are there barriers to reaching the optimum RDM levels? Grazing by livestock is likely the most cost effective and practical treatment to apply across large landscapes scales to manage herbaceous fuels (Davies et al., 2015). Grazing can alter fuel characteristics of an ecosystem; however, little is known about the influence of grazing on fire, in particular ignition and initial spread and how it varies by grazing management differences (Davies, et al., 2017).
This project promotes the scientific significance andstrength of the UC network through collaboration of advisors and specialist to benefit California beef cattle producers and the rangelands they graze. It will address the priority of managing rangelands for multiple ecosystem services especially documenting the “why should we” and when to graze working landscapes to reduce fire severity in California. The research project will lead to more informed lands landowners, managers, policy makers and public on the importance of managing rangelands, through grazing. Research results will address the following issues:
Reducing Wildfire Risk. Increasingly severe wildfires are impacting an array of communities, including many lower income areas such as Lake and Mariposa Counties.
Wildfires increase air pollutants such as PM, CO and O3, amplifying problems that are already more severe in less affluent, inland areas. Wildfires put firefighter's lives at risk, reduce state funds available for other needs, jeopardize infrastructure, and increase insurance and utility costs. Improved wildland management for fire has already become a critical issue, with important implications for Low-Income Communities.
Sustaining Water Supply. Concerns were raised about the water supply that serves many counties during recent years up and down the state. East Bay Municipal Utility District rerouted staff from projects on district owned land in the Mokelumne watershed to burned areas above their reservoirs to help revegetate the watershed, and decrease erosion into Pardee Reservoir, drink water supply for the East Bay. Fires in Sonoma County about Lake Sonoma could have impacted the water source for both Sonoma and Marin County residence. In addition, the severity of wild fires in Santa Barbara and Ventura caused mud slides which lead to reductions in water quality and reservoir capacity. Improved wildland management will become increasingly important for maintaining California's water supply.
Maintaining Tourism Economy. Many of the state's rural areas, especially in the Sonoma, Santa Barbara, and Ventura depend heavily on tourism for local economies. The occurrence of fires impacted the number of tourist visiting, along with the commodities grown in those counties, i.e. grapes, vegetables, etc. The 2018 Ferguson Fire closed Yosemite National Park for an unprecedented two weeks, a large impact for a heavily touristy area around Yosemite and central California Foothill communities. Improved wildland management is becoming an important issue for maintaining tourism economies across rural California.
Increasing Grazing Lands. Many state lands are not currently managed to control fine fuels. Using livestock to manage them will open opportunities for more grazing lands, potentially allowing for greater flexibility of managing herds already in the state and providing opportunities for newer ranchers. In addition, controlling brush and reseeding an area has been documented to provide up to 2,000 lbs./acre in additional forage (Biswell, 1954), providing not only access to more forage for livestock, but also decreasing brush as a fuel. Both the coast range foothills and the Sierra Foothills can benefit from reduced brush encroachment.
Citations upon Request