• Rainbow over Half Dome

    Yosemite

    National Park California

Treatment of Invasive Plants

Control and Treatment

Treatment methods are tailored to their effectiveness in the context of worker safety and site-specific resources and conditions. First, non-native invasive plants are prioritized for treatment. For the highest-priority species, plant and site-specific management objectives are then developed involving a combination of manual, mechanical and chemical control methods. Yosemite has used biological methods in the past, but none have been released in recent years.

All treatment methods have strengths and weaknesses, hence an array of tools and techniques are needed to control invasives in a large and complex park like Yosemite. Some treatments are appropriate in some situations while not in others. Special measures must be followed in protection zones such as designated Wilderness, special-status species habitat, wetlands, riparian zones, archeological and traditional use areas.

Invasive plant control in the park focuses on the handful of high-priority species that pose the greatest threat to the park resources. At present, 90% of our control resources are targeting the following five species in order of importance: yellow star-thistle, Himalayan and cut-leaved blackberry, bull thistle, and velvet grass. Four other high-priority species occur in the park but control efforts are minimal because the species have been eradicated or nearly so: Italian thistle, perennial pepperweed, and spotted and diffuse knapweed. Cheat grass is another highly problematic plant, but it is already so widespread in Yosemite, that it cannot be effectively controlled parkwide. Local control projects do occur in areas of special concern, and in wilderness areas where cheat grass is not yet widely established.
 
Four images of invasive plants
Targeted invasive species in Yosemite National Park from left to right: yellow star-thistle, blackberry, velvet grass, and bull thistle.
 
Prior to 2009, weed control in Yosemite was performed using only manual or mechanical methods. For fifteen years park managers tried to control aggressive invasive species such as Himalayan blackberry and yellow star-thistle with hundreds of volunteers. While many local successes were made, the spread of star-thistle and blackberry far outpaced the rate of control. The inefficient and ineffective use of resources and the need for repeat treatments has led managers to implement herbicides as one tool of a full range of methods employed by Yosemite managers, as explained below.


Manual Control

Manual control techniques use only non-mechanized hand tools. They are typically labor intensive, expensive and best suited to treat small patches or very sparse infestations spread across a large area. Examples include hand-pulling, lopping, and cutting the plants below the root crown using shovel type tools. Manual control methods can be appropriate treatment for perennials such as shrubs and trees that will not stump sprout after cutting or in areas where certain tools may not be appropriate, such as herbicide use near rare plants. Before plants produce seeds, flowering heads and any existing seeds are bagged and disposed of properly. One successful use of manual control has been the treatment of spotted and diffuse knapweed and mullein. These plants have been consistently hand-pulled and have not spread into the wilderness.

One great advantage of manual control methods is that they can be readily employed by volunteers. A major disadvantage of some manual control methods is the extensive ground disturbance and trampling of associated natives. These disturbed areas provide freshly exposed soil, the ideal habitat conditions for invasive plants to rapidly re-establish. Subsequent follow-up treatments can perpetuate the disturbance-invasive plant cycle. Furthermore, many areas in Yosemite have archeological resources below ground that can be disturbed by some manual methods.

 
Worker removing blackberry plants by hand.

Handpulling Himalayan blackberry may be locally successful. However, soil disturbance provides suitable conditions for other Non-native invasive species to colonize the area.


Mechanical Control

In situations where invasive plants have completely overwhelmed a site, mechanical methods may be appropriate. Hand-held motorized equipment, such as brush cutters quickly remove the above-ground portions of invasive plants. For example, using brush-cutters on flat terrain when 5% of the infestation is in bloom has proven effective in controlling yellow star-thistle in Yosemite. However, mechanical methods are often labor intensive, noisy, require more training and may involve significant collateral damage to non-target vegetation. Additionally, some of our staff have sustained injuries when working with brush-cutters in steep and rough terrain. Many mechanical control methods can only be employed during early season because of the late season fire hazards. Mechanical mowing is also not a preferred method in designated Wilderness.

 
Employee mowing thistle

The dry and crumbly soil condition makes mechanical treatment on steep slopes dangerous for workers and is no longer implemented. Spraying herbicides are one of the few safe alternatives for staff in such steep areas. Herbicide application occurs earlier in the season when it is cooler, soils are not dry and crumbly, and the grass is not dry and slick. 

Herbicides

The use of herbicides within Yosemite National Park is scrutinized by the public as well as park managers. Like all other methods of IPM, herbicide application methods also have unique shortcomings that must be carefully weighed against its benefits to protecting park resources. For instance, more substantial training, documentation, and equipment are needed, and the delivery of herbicides frequently requires a large amount of clean water which may not be readily available in remote sites. Furthermore, in California, the use of herbicide is more controversial than other IPM methods, which also requires unique considerations. However, judicious use of herbicide can significantly increase the efficiency and effectiveness of invasive plant control, and the likelihood of decisively controlling infestations. This is significant because manual and mechanical treatments alone have not been able to contain some invasives. One example came after many years of repeated unsuccessful manual treatments of tree of heaven in El Portal. A single cut-stump herbicide treatment with a minor follow-up treatment the subsequent year controlled the problem. Targeted herbicide applications such as these, kill invasive plants without any, or with very limited impacts to surrounding native plants. In Yosemite Valley, managers treated a single 8.65 acre patch of Himalayan blackberry in 2009. The following year, only 0.34 acres of blackberry remained at this site; a reduction of 96% in a single year! No other method can realize this level of cost-effectiveness. Such efficient methods also serve the Wilderness character, and repeated lengthy presence of staff conducting invasive manual treatments and the enduring presence of exotics do not. Naturally, when using herbicides, the park conforms to standards above and beyond those required by the law to ensure the highest protection for applicators, visitors, wildlife, aquatic biota, water quality, and native plant species.

There are many methods to apply herbicide; the following methods are commonly used in Yosemite National Park:

  • Foliar Spray: Leaves are sprayed with a mixture of herbicide, water, non-ionic surfactant and an indicator dye from a backpack or other sprayer. Precise mixes are formulated according to label requirements, equipment calibration rates, species type, life cycle, and other factors. The dye is used to identify what has been sprayed which reduces overspray and repeat treatments. The park uses three delivery systems:
  1. Backpack sprayer

  2. Truck mounted spray with 700 feet of hose extension and wand

  3. Hand sprayer (rarely used)

  • Cut Stump and Frill: Herbicides are applied to the freshly-cut stump of a woody species (tree, shrub, or vine). With the frill method, multiple cuts are made into the cambium layer, followed by immediate herbicide application. This method has the advantage of being very selective with high efficacy for some species. However, it is labor intensive, and potential worker exposure to herbicide concentrate is greater.
  • Wiper: Herbicides are applied to the leaves of plants with a wick, sponge, paintbrush, or similar tool. This method is also highly selective and labor intensive, but not as effective as foliar spray.

Additional Techniques

Other techniques include changing land management practices to promote native species diversity. For example, prescribed fire can be used to maintain the overall natural diversity and functioning of an ecosystem, and potentially to exploit vulnerabilities in the life cycles of invasive plants. Two or more methods can also be combined to produce more synergistic results such as burning before herbicide treatment to remove excess biomass or thatch.

  • Controlled Burning: Most plant communities in Yosemite National Park evolved in the presence of periodic fire. After being suppressed for decades, fire is now recognized as an important tool for maintaining the health of these communities. In some cases, fire is useful as a tool for managing invasive species. Fire is part of the ecological drivers in many ecosystems in the park, and it can be used to treat large areas effectively. However, fire is non-selective and logistically difficult to implement. Furthermore, implementing fire is potentially hazardous and risky, and it has the potential to compound invasive plant problems. Firefighting equipment can be contaminated with invasive plant seeds, which is of particular concern when off-park crews and equipment are employed. During the post fire period of disturbance, the establishment and spread of invasive annual grasses such as cheat grass is promoted. Such species have the capability to completely displace native plant communities.
  • Restoration: Invasive plants are less likely to become established where diverse native plant communities are already established. Restoration activities may incorporate reseeding as appropriate to aid in recovery and prevent invasive plant re-infestation. Active restoration is particularly useful in heavily disturbed areas, such as roadsides that lack a native seed bank.
  • Biological Control: Yosemite National Park introduced a chrysomelid beetle (Chrysolina quadridemina) in Yosemite Valley to control St. John's wort (Hypericum perforatum) in the 1980s. In 1994 and 1995, the peacock fly (Chaetorellia australis), the hairy weevil (Eustonopus villosus), and the false peacock fly (Chaetorellia succinea) were introduced in El Portal to help control yellow star-thistle. Biocontrol can be an effective method of controlling some species. Currently, no invasive species in the park require the further release of biological control agents to meet management objectives.

 

Herbicides

The use of herbicides within Yosemite National Park is scrutinized by the public as well as park managers. Like all other methods of IPM, herbicide application methods also have unique shortcomings that must be carefully weighed against its benefits to protecting park resources. For instance, more substantial training, documentation, and equipment are needed, and the delivery of herbicides frequently requires a large amount of clean water which may not be readily available in remote sites. Furthermore, in California, the use of herbicide is more controversial than other IPM methods, which also requires unique considerations. However, judicious use of herbicide can significantly increase the efficiency and effectiveness of invasive plant control, and the likelihood of decisively controlling infestations. This is significant because manual and mechanical treatments alone have not been able to contain some invasives. One example came after many years of repeated unsuccessful manual treatments of tree of heaven in El Portal. A single cut-stump herbicide treatment with a minor follow-up treatment the subsequent year controlled the problem. Targeted herbicide applications such as these, kill invasive plants without any, or with very limited impacts to surrounding native plants. In Yosemite Valley, managers treated a single 8.65 acre patch of Himalayan blackberry in 2009. The following year, only 0.34 acres of blackberry remained at this site; a reduction of 96% in a single year! No other method can realize this level of cost-effectiveness. Such efficient methods also serve the Wilderness character, and repeated lengthy presence of staff conducting invasive manual treatments and the enduring presence of exotics do not. Naturally, when using herbicides, the park conforms to standards above and beyond those required by the law to ensure the highest protection for applicators, visitors, wildlife, aquatic biota, water quality, and native plant species.

There are many methods to apply herbicide; the following methods are commonly used in Yosemite National Park:

  • Foliar Spray: Leaves are sprayed with a mixture of herbicide, water, non-ionic surfactant and an indicator dye from a backpack or other sprayer. Precise mixes are formulated according to label requirements, equipment calibration rates, species type, life cycle, and other factors. The dye is used to identify what has been sprayed which reduces overspray and repeat treatments. The park uses three delivery systems:
  1. Backpack sprayer

  2. Truck mounted spray with 700 feet of hose extension and wand

  3. Hand sprayer (rarely used)

  • Cut Stump and Frill: Herbicides are applied to the freshly-cut stump of a woody species (tree, shrub, or vine). With the frill method, multiple cuts are made into the cambium layer, followed by immediate herbicide application. This method has the advantage of being very selective with high efficacy for some species. However, it is labor intensive, and potential worker exposure to herbicide concentrate is greater.
  • Wiper: Herbicides are applied to the leaves of plants with a wick, sponge, paintbrush, or similar tool. This method is also highly selective and labor intensive, but not as effective as foliar spray.

Additional Techniques

Other techniques include changing land management practices to promote native species diversity. For example, prescribed fire can be used to maintain the overall natural diversity and functioning of an ecosystem, and potentially to exploit vulnerabilities in the life cycles of invasive plants. Two or more methods can also be combined to produce more synergistic results such as burning before herbicide treatment to remove excess biomass or thatch.

  • Controlled Burning: Most plant communities in Yosemite National Park evolved in the presence of periodic fire. After being suppressed for decades, fire is now recognized as an important tool for maintaining the health of these communities. In some cases, fire is useful as a tool for managing invasive species. Fire is part of the ecological drivers in many ecosystems in the park, and it can be used to treat large areas effectively. However, fire is non-selective and logistically difficult to implement. Furthermore, implementing fire is potentially hazardous and risky, and it has the potential to compound invasive plant problems. Firefighting equipment can be contaminated with invasive plant seeds, which is of particular concern when off-park crews and equipment are employed. During the post fire period of disturbance, the establishment and spread of invasive annual grasses such as cheat grass is promoted. Such species have the capability to completely displace native plant communities.
  • Restoration: Invasive plants are less likely to become established where diverse native plant communities are already established. Restoration activities may incorporate reseeding as appropriate to aid in recovery and prevent invasive plant re-infestation. Active restoration is particularly useful in heavily disturbed areas, such as roadsides that lack a native seed bank.
  • Biological Control: Yosemite National Park introduced a chrysomelid beetle (Chrysolina quadridemina) in Yosemite Valley to control St. John's wort (Hypericum perforatum) in the 1980s. In 1994 and 1995, the peacock fly (Chaetorellia australis), the hairy weevil (Eustonopus villosus), and the false peacock fly (Chaetorellia succinea) were introduced in El Portal to help control yellow star-thistle. Biocontrol can be an effective method of controlling some species. Currently, no invasive species in the park require the further release of biological control agents to meet management objectives.

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