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Exotic and Invasive Species

Invasive weeds are not native to California. They have been introduced from another country. Invasive weeds are colonizing species that usually rely on some form of soil disturbance for initial establishment. Because they are not native they have few natural enemies here to restrict their growth and spread. Over time these weeds will replace native and cultivated plants. Most invasive weeds aggressively spread through roots as well as from seeds.

No matter who you are or where you live, invasive weeds affect your way of life. Widespread infestations reduce productivity and value of the land. While weeds have long been recognized as a problem in agriculture, they pose an ever increasing threat to the integrity of all wildland ecosystems.

What happens if invasive weeds are not controlled?

Water levels drop; these plants consume enormous quantities of water.
Native plants are displaced; they can't compete with aggressive invasive species.

  • Wildlife habitat is lost.
  • Diversity of terrestrial and aquatic life is reduced.
  • Land values decrease.
  • Scenic values are impaired.
  • Fire hazards are increased.
  • Quality of pasture land is reduced.

How can I help control the spread of invasive weeds?

Land owners and managers can help in supporting, implementing, or taking part in a weed management program. Weed management includes the integration of control methods to prevent new weed introductions, and eradication of existing invasive weeds.

Join forces in combating invasive weeds. Control can be achieved by several methods, including: hand pulling, cutting, spraying with a suitable herbicide, or a combination of these methods. For specific treatment methods contact the Resource Officer at one of the district offices or the Agricultural Commissioner Office in the county you live.

Invasive Weeds are spread by:

  • Disturbing native vegetation. This often prepares a seed bed for invader weeds.
  • Introducing foreign plants, seeds or pods to the area. They can spread aggressively into native habitat.
  • Rangeland that is overgrazed. Sustain desirable native vegetation of the area.
  • Construction and recreation vehicles can unknowingly transport invasive weeds, seeds or pods.
  • Transporting gravel, fill dirt, and feed stuffs such as, hay, grain, etc. contaminated with weed seeds.
  • Be alert to the problem of domestic animals transporting weed seeds in their fur, feathers and droppings.

Two of the most common invasive weeds in the Los Padres National Forest that destroy riparian (streamside) habitat are Arundo and Tamarisk

Tamarisk

Also called salt cedar, tamarisk is one of the most aggressive non-native plants found in the forest. It was originally planted in California as a windbreak, to provide shade and help control erosion. The tamarisk has quickly spread into natural wetlands, where it tends to form dense thickets along streams and springs, displacing native trees such as cottonwood, willow and mesquite. In areas of the Los Padres National Forest where watercourses are small or intermittent and tamarisk has taken hold it has severely limited the available water; it can even dry up the water source. Each mature plant can consume about 300 gallons of water every day! If all tamarisks were removed from southern California, we would save enough water to serve a city the size of San Diego for an entire year!

The plant has a very high salt content that affects the soil and native plants around it. Worst of all, a single tamarisk plant can produce 500,000 seeds each year and each seed can germinate within 24 hours! Also, during the spring it can grow as much as one foot per month. Tamarisk can usually out-compete native plants for water. It can eliminate excess salt from the tips of its leaves. When the leaves are shed, this salt increases the salinity of the soil, further reducing the ability of native plants to compete. Because of its ability to spread, its hardiness, its high water consumption, and its tendency to increase the salinity of the soil around it, the tamarisk has often completely displaced native plants in wetland areas.

From a wildlife point of view, the tamarisk has little value and is usually considered detrimental to native animals. The leaves, twigs and seeds are extremely low in nutrients, and, as a result, very few insects or wildlife will use them. Because of the tamarisk's ability to eliminate competition and form single-species thickets, wildlife populations have dropped dramatically.

Tamarisk is difficult to eradicate. It re-sprouts readily after cutting or burning. Research and many programs are now in place to reduce or eradicate tamarisk, and laws are being enacted to eliminate its sale and importation. It is found that, when tamarisk is eradicated and native species allowed to grow, water returned to wetland areas and wildlife again thrived.

Arundo

Arundo, also called giant reed or giant cane, is another common invasive plant which is crowding out native plants along streams in Los Padres National Forest. It can grow four inches a day, and up to 30 feet tall. Arundo originally brought from Asia in the 1820's for roofing material, livestock fodder, erosion control and as an ornamental plant.

This destructive bamboo-like species may be even more abundant than tamarisk because it thrives in sun and shade in riparian areas (along streams). When sections of the stem or root break off, they start new plants down stream. The plant grows fast and survives fire. It out-competes other streamside plants, which in turn displace the shade trees, such as alders, willows and cottonwoods which provide a healthy ecosystem with habitat for fish and wildlife. It's taking up valuable space on our public lands!

Native plants that provided food and shelter for fish and wildlife are severely diminished by the fast growing and multiplying Arundo. The number of song birds, fish species and amphibians that depend on the limited streams in the Los Padres are loosing their habitat due to this invasive species.

A couple of other problems are caused by Arundo. It wastes water. It consumes water that could be used by native plants which do provide habitat for fish and wildlife. It also causes flooding problems. During times of high stream flow, the roots are easily undercut and cause clogged culverts and stream channels downstream.

The following section, the invasive weeds will be described and noted where they are found on the forest. Please keep your eye out for new infestations on our national forest! 

Invasive Weeds by Ranger District

Last updated March 27th, 2025

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