What is an "invasive plant"?
An invasive plant is a weed of natural habitats. It is a non-native plant that was introduced into California through human action. (Whereas native plants are the result of natural processes of plant migration and evolution.) An invasive plant is a species that, after being introduced, is capable of spreading into natural areas without further help from humans. Not all non-native plants are invasive; fortunately only a small fraction of the thousands of plant species that have been introduced into California have escaped cultivation, and only a small fraction of those have spread into wildlands. Invasive plants vary widely in their ability to occupy native habitats. Some are true invaders and will readily move into any suitable ecosystem. Others - probably the majority - are more colonizing; they are capable of growing in areas that have been disturbed by human or natural causes, but do not succeed in areas currently inhabited by natives.
Where do invasive plants come from?
They come from all over the world; giant reed (Arundo donax) from Europe, pampasgrass (Cortaderia selloana) from South America, bermudagrass (Cynodon dactylon) from South Africa, and tree of heaven (Ailanthus altissima) from China. Invasive plants have been both intentionally and unintentionally introduced into California. Unintentional introductions have typically arrived accidentally, for example; as contaminants in crop seed; in hay or forage; on machinery, equipment or vehicles; or in or on livestock. Examples of these weedy species are yellow star thistle (Centaurea solstitialis) and many other thistles, ripgut brome (Bromus diandrus), and common plantain (Plantago major).
About half of the invasive plant species in California were intentional introductions for what was felt, at least at the time of introduction, to be beneficial purposes. Some species were planted extensively along streams for erosion control; these include pampasgrass and saltcedar (Tamarix ramosissima). Hottentot fig (Carprobrotus edulis) is an iceplant species that has been used for stabilizing seaside sand dunes. Most species, however, are escaped horticultural species, such as; Spanish broom (Spartium junceum), cape ivy (Delairea odorata) (aka German ivy, Senecio mikanoides), Mexican fan palm (Washingtonia robusta), and fountain grass (Pennisetum setaceum).
What are the impacts of invasive plants on natural areas?
Invasive plants can rapidly degrade the quality of wildlands by altering natural processes and reducing biodiversity. The most common impact is a change in the biological structure of the site. Changes in the plant communities (groups of species) will generally drive changes in the populations of animal inhabitants. Because native insects seldom feed on non-native pest plants, insect numbers usually diminish. Native bird and reptile populations decline in turn, because insects were their main source of food. Invasive plants may be inedible or undesirable to native herbivores, both small and large. And birds may not be able to build nests in the non-native plants.
Invasive plants can also change the physical environment. In riparian zones (streams and rivers), dense stands of invasive species such as giant reed and saltcedar can clog channels and increase the severity of flooding. Invasive plant species may not root as well as natives, resulting in stream embankments washing away easier. Large infestations, such as the 1.3 million acres of saltcedar in river systems of the Southwest US, make this much more than a minor or local problem.
Invasive plants can also cause wildfires of increased frequency, intensity, and size. For example, non-native annual grasses such as red brome (Bromus madritensis) have become increasingly dominant in the Mojave Desert, creating carpets of dry fuel that carry fire rapidly across the landscape. The exotic grasses return in higher densities after a fire, increasing susceptibility to future fires. In 1999, Joshua Tree National Park suffered the largest fire on record, burning 14,000 acres of junipers, pinyon pines, and Joshua trees. These slow-growing native perennials do not recover well from the more frequent fires. Although a Joshua tree can live for hundreds of years, once burned, it will take a hundred years for another to take its place.
Invasive plants can also alter nutrient relationships within an ecosystem, which may impact other life in the habitat. Arundo has invaded streamsides throughout southern California. Because giant reed doesn't shade the water as well as native willows and cottonwoods, stream temperatures rise. This stimulates algae growth and increases stream acidity. Reproduction decreases in the native fish and amphibian populations, and a rich streamside ecosystem is lost to a monoculture of giant reed. Other exotic plants take up salts from deep in the soil and deposit them on the soil surface, inhibiting the growth of natives that are less salt tolerant.
What areas of the state are most affected?
There is nowhere in California that has escaped completely from this problem. Highly populated regions of the state, especially the coastal zones, have had the most problems. This isn't surprising; the greater human presence and conversion of natural habitat into houses, shopping centers, and the like reduces the amount of room for the natives. The natural habitat most at risk in southern California is coastal sage scrub; only about 5% of this habitat remains intact after 3 centuries of European American settlement. It also increases the urban/wildland interface, the place where domestic landscapes meet natural landscapes. This is an especially important factor for the invasion of escaped ornamental plants into wildlands.
Riparian zones are sensitive sites often plagued with exotic weed problems. These areas are culturally important to Californians who enjoy the amenity value offered by rivers and streams, not to mention the water provided for residential, agricultural, and commercial use. Unfortunately, invasive plants like giant reed and saltcedar have huge appetites for water, sucking up as much as three times the amount used by native plants!
Rangelands, both private and publicly owned, have also been highly affected. Yellow starthistle occupies over 15 million acres in California, choking out native plants, displacing native animals that depend on those plants, and reducing the quality and availability of forage. Because of its needlelike spines, both wildlife and livestock avoid grazing in heavily infested areas. In addition, horses that feed on large quantities of yellow starthistle (in the early spring before the spines come out) develop an incurable and fatal brain disease.
Who is involved in controlling invasive plants?
The landowner or land manager is the individual or entity most affected by invasive plants. And, because federal, state, or local government agencies publicly own much of California, we all share this problem. At the federal level, agencies that have active programs against invasive plants are the Bureau of Land Management (BLM), the U.S. Fish and Wildlife Service, the National Parks Service, and the U.S. Forest Service. In California, the State Parks, Department of Food and Agriculture, Department of Forestry, and Department of Fish and Game all fight invasive plant species. Locally, the agencies involved are County Parks, City Parks, Water and Flood Control Districts, and the Agricultural Commissioner. Private lands owned by conservation organizations (such as The Nature Conservancy) are also infested with invasive plants. In many cases, a large share of the money spent on land conservation and restoration is utilized to control these exotic pest plants. The Nature Conservancy, in particular, has been at the forefront of a national effort focused on this issue. The California Native Plant Society (CNPS), although not a landowner, also works to address the problem of non-native species interfering with natural vegetation in California. Many CNPS members volunteer their time on weekends and holidays controlling invasive plants on public and private land. The problem of invasive plants is becoming a more important issue for other environmental groups such as the Sierra Club.
How are these pest plants controlled?
Invasive plants are controlled with the same basic strategies and tactics as weeds in landscapes and agricultural lands. However, these are pests of natural areas, which requires heightened sensitivity to environmental concerns.
The first step in control is quarantine and eradication. Government agencies in charge of quarantine are the U.S. Department of Agriculture's Animal Plant Health Inspection Service (USDA-APHIS) and the California Department of Food and Agriculture (CDFA). The County Agricultural Commissioner (CAC) is responsible for enforcing quarantine and eradication regulations against weeds listed by the state of California. Under quarantine laws and regulations, CDFA and CAC can require private landowners to remove listed weeds from their property. They can also restrict movement of plants, especially nursery stock, out of the area.
Existing infestations are controlled using physical, chemical, and biological methods. Physical control is divided into various approaches, such as mechanical, burning, or grazing methods. Mechanical control can be with a disk or harrow to completely disrupt the ground, but this method is often avoided due to the destruction of too many native plants or animals. Mowing or chopping is more common, either by hand with swing blades or brush cutters, or with larger tractor-mounted equipment. Hand removal of plants is done in limited areas or infestations. Controlled burns or flaming techniques can be used to clear off an area, but this is generally not effective unless followed by chemical treatment of re-growth. Sheep or goat grazing is practiced in some areas, but it is not widespread because of costs associated with shepherding.
Herbicides are widely used throughout California for control of invasive plants, even by environmental groups that generally oppose pesticides. Although many herbicides exist that can kill these pest plants, only a small number are legal to use in wildland situations. Special application techniques have also been developed, such as cut stump (spraying the stump of a plant immediately after cutting) or frilling (making angled cuts into the stems and spraying herbicide into the cuts). Cut stump and frill treatments use relatively high concentrations of herbicides applied to localized areas on the plant to avoid damage to non-target plants.
Biological control ("biocontrol") perhaps offers the best approach to long-term management of invasive plants, but it is neither simple, nor cheap, nor free of controversy. The basic premise behind biocontrol is that invasive plants have been removed from their native range where other organisms (herbivores, insects, and/or parasites) kept their population in balance. In order to restore balance where a non-native plant is causing a problem, a biocontrol agent (usually an insect herbivore) is introduced from that plant's origin. However, the biocontrol agent cannot be released in the U.S. if it feeds on desirable and/or native vegetation. This specificity requirement can be difficult to achieve. Over time, levels of control (i.e., reduction in the pest plant population) of up to 95% are possible, but 60% to 70% is more common. Thus, biological control will not completely eliminate or eradicate a weed; some level of weed infestation has to persist for the process to work over the long run. Large up-front costs are incurred in the process of finding and testing likely biocontrol agents, but this is balanced by much lower maintenance costs. However, biological control projects, notably with saltcedar, have been challenged by the U.S. Fish and Wildlife Service and some environmental organizations in recent years. They assert that biological control poses undue threats to non-target organisms or endangered species. This is a minority opinion among scientists involved in biocontrol, but one that requires consideration. The history of biocontrol is very good; examples of introduced biocontrol agents feeding on non-target plants are very rare.