Invasive Plants

“Invasive species means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health” (Federal definition from Executive Order 13112 in 1999).

Invasive plant species have been both accidentally and purposefully introduced into California at least since the gold rush era. Over the decades many of these plants have become established pests. They often outcompete and greatly diminish native plant cover over time. Loss of habitat due to non-native plant invasion is the second greatest threat to rare and endangered plants. Some of these invaders can also change ecosystem processes such as wildfire frequency and intensity, hydrology, or soil chemistry.

Many of these invasive introduced plants, especially those causing damage to agriculture interests have been listed as “noxious weeds” by the California Department of Food and Agriculture (CDFA). The California Invasive Plant Council (Cal-IPC) maintains an invasive plant inventory that categorizes non-native invasive plants that threaten the state’s wildlands. The categories are based on an assessment of the ecological impacts of each plant.

Many invasive plants are exceptional competitors and are able to outcompete most native plants species. They are often introduced by soil-disturbing activities. Invasive plants often begin growth while the native perennial plants are still dormant. Once on site, they grow rapidly and effectively utilize available soil water and nutrients. Another advantageous trait of these invaders is that they are prolific seed producers, with seed banks that remain viable for several years. New infestations are facilitated by large and small soil disturbances.