What is Non-native Invasive Plant Removal?
The Napa River riparian corridor, like most California landscapes, is host to many non-native and invasive plants. Often introduced as ornamentals, non-natives and invasive plants readily establish after escaping from landscaped settings and spread rapidly. Other non-native, invasive plants were presumably introduced through movement of livestock or in conjunction with introduction of more desirable plants. Invasive plant species are problematic when they outcompete native plant species, which then, in turn, increases the area’s vulnerability to further invasion.
Some invasives, such as many of the common grass species found in California today, are so widespread and well-established that many botanists consider them here to stay (and for which eradication or control efforts are likely futile except on a very localized basis).
What are common non-native invasive plant species in the Napa River watershed?
- Giant reed (Arundo donax),
- Ivy – English (Hedera helix), and Cape (Delairea odorata),
- Tamarisk or saltcedar (Tamarix spp.),
- Pampas grass (Cortaderia spp.),
- Himalayan blackberry (Rubus discolor)
- Large periwinkle (Vinca major),
- Purple star thistle (Centaurea calcitrapa) and yellow star thistle (C. solstitialis),
- French broom (Genista monspessulana) and Scotch broom (Cytisus scoparius),
- Giant waterfern (Salvinia molesta)
- Eucalyptus species
- Acacia species
- Kikuyu grass (Pennisetum spp), and
- Tree of Heaven (Ailanthus altissima)
Why are non-native invasive species a problem?
- Invasives threaten natural habitat value by displacing native plant species and associated native animal species.
- Invasive plant species often are prolific reproducers that have higher tolerance for certain environmental conditions, such as frequent wildfires, drought or shade, which can give them a competitive edge over native species.
- Non-native plant species co-evolved with natural enemies (such as herbivores and seed predators) which are absent in the new habitat, allowing the species to expand rapidly.
- Some non-native invasive species, such as saltcedar (Tamarix spp.), can increase soil salinity under certain conditions and create environmental conditions that are inhospitable to growth and germination of native riparian plant species.
- Invasive riparian plants can affect terrestrial wildlife and fish species associated with the river corridor by altering fluvial geomorphic processes, stream temperature, nutrient cycling and food web dynamics.
How can we stop the spread of invasive species?
- The goal of sustainable weed management is the development and maintenance of healthy, desired native plant communities that have the ability to resist continued weed infestation. It incorporates a combination of preventative weed establishment strategies and weed management techniques that shape the composition and structure of the plant community. Common practices include prevention and early detection strategies that are effective in hindering spread and establishment into weed-free areas. Small or newly established patches are responsive to eradication programs. Large infestations require an integrated approach in management towards reestablishing healthy plant communities.
- Methods of invasive plant eradication or control are commonly categorized as physical, thermal, biological, managerial, or chemical methods.
- Physical methods include manual and mechanical removal techniques. Physical methods are most effective when combined with another control method such as herbicide application.
- Thermal methods include broadcast burning or spot treatment with a flame thrower. Prescribed burning for pest plant control must be done in accordance with local fire regulations and the consequences of fire for native plants, soil chemistry, and air quality must be considered.
- Biological control involves the introduction of insects or pathogens which are highly selective for a particular weed species. Biological control has some risk of non-target impacts associated with it, though very few have been reported for biological control of weed pests. Currently, few biological methods have been approved for use by the U.S. Department of Agriculture.
- Managerial weed control methods include prescribed grazing, covering with plastic or weed cloth, and the encouragement of competitive displacement by native plants.
- Chemical control includes both broadcast and spot application of herbicides or less toxic alternatives. Remember to use pesticides wisely: always read the entire pesticide label carefully, follow all mixing and application instructions and wear all recommended personal protective gear and clothing. Contact the California Department of Agriculture or the Napa County Agricultural Commissioner for any additional pesticide use requirements, restrictions, or recommendations.
- The most desirable, and often most effective approach to controlling or eradicating noxious plants is that of an integrated weed management (IWM) plan. This involves the optimum use of several control strategies to control pest plants, such as the manual or mechanical removal of the woody plant material combined with the application of herbicide to the remaining stump. Integrated weed management is generally accepted as the most effective, economical, and environmentally sound long-term method of controlling or eradicating pest plants. If various control techniques are used, however, they must be compatible with one another to achieve the best results.