County Lists Invasive Plants of Greatest Concern for Threat of Wildfire
By E.A. Barrera

Perennial grasses can be found all along the roadways where the fires burned. Photo by Jamey Ritter

University of California Invasive Plant specialist Carl Bell said that most of the grasses common in Southern California are non-native species from Europe and other countries.

Annual Grasses Include: Wild Oats, Bromes, Rye

"These grasses germinate in the winter and complete their life cycle before summer," said Bell. "Their dried tissues can provide fuel throughout the summer and fall fire season. Mature grass seeds that fall to the ground in early summer will escape damage from high fire temperatures. In the desert, red Brome and Mediterranean grasses helped fuel large fires that have repeatedly burned some areas and converted them from native perennial desert scrub to non-native annual grassland.

Perennial Grasses: Giant Reed, Pampas, Fountain

Bell notes that Giant Reed is the most common invasive plant in riparian areas of Southern California. This is due to the fact that the reed grows in dense masses to 20-30 feet tall, creating large amounts of biomass that become dry and flammable in the fall.

New growth Eucalyptus thrives in the foreground while a native shrub in the background recovers from being burned three years ago.

"Fires in these areas are expansive and intense, and flames spread easily from the surface into the canopies of the tallest native trees," notes Bell. "Soon after a fire, Giant Reed re-sprouts quickly from the large rhizome system. By comparison, natives recover very slowly. Gaps left on the river banks provide invasion points for other weedy plants, such as castorbean and poison hemlock."

Pampas grass is a large clumping grass, about 6 to 8 feet tall. It is a weed of coastal sage scrub and the upland areas of riparian zones.

"Although it remains green year round, it can readily burn because its old leaf litter and dead flower plumes provide ample flammable fuel. The large clumps can become massive torches and hinder fire fighter access to burns," said Bell.

Fountain grass is a smaller clumping grass that can establish in wide range of sites in southern California.

Giant reeds are covering charred branches along stretches of Mussey Grade. Photos by Jamey Ritter

"After the San Diego wildfires of 2003, it was often the first plant to re-sprout along streets and highways. Like pampas grass, it stays green year round, but the leaf litter and old shoots easily burn," said Bell.

Herbaceous Broadleaf Plants:

• Mustard grasses:

• Filarees;

• Fennel;

• Thistles;

• Saharan mustard

Like the non-native grasses, Bell notes that Southern California has a number of Mediterranean forbs (herbaceous broadleaf plants) introduced by European immigrants. He says these winter annuals grow readily during the rainy season, then die or go dormant by the beginning of the summer fire season.

"These species do not usually produce as much fine fuels as do the grasses, but their dried seed stalks burn and the seed within the soil are not damaged or killed by fire," said Bell. "Since they are early germinators like the annual grasses, they can out compete the natives. In the desert, Saharan mustard produces large amounts of fuel that, when combined with an under story of annual grasses, can be an important contributor to the wildfire problem."

Woody Trees and Shrubs:

• Saltcedar;

• Acacias;

• Eucalyptus

Bell says many woody non-invasive plants burn and recover quickly after a wildfire. Like the Giant Reed, these trees and shrubs are often invasive in riparian habitats where they increase the frequency and intensity of fires.

"Saltcedar, in particular, is responsible for intense summer fires along desert rivers. In some cases, fires associated with

salt cedar have eliminated stands of native mesquite, cottonwood and willow along the Colorado River," said Bell.