Invasive Plants of California's Wildland

Highway iceplant (Carpobrotus edulis) is a ground-hugging succulent perennial that roots at the nodes, has a creeping habit, and often forms deep mats covering large areas. Shallow, fibrous roots are produced at every node that is in contact with the soil. Highway iceplant has been widely planted for soil stabilization and landscaping, and is well known by most Californians for its succulent three-sided leaves and its propensity to form deep mats and monospecific stands. In California flowering occurs throughout the year, peaking in late spring and early summer; flowers do not appear to require specific pollinators.

Highway iceplant is easily confused with its close relative, the more diminutive and less aggressive Carpobrotus chilensis (sea fig), and the two species hybridize readily throughout their ranges in California. The large, two and a half- to six-inch-diameter, soli-tary flowers of highway iceplant are yellow or light pink, whereas the smaller, one and a half- to two and a half-inch-diameter sea fig flowers are deep magenta. Hybrid flowers are pink and intermediate in size. C. edulis hybrids also appear to be invasive pest plants in wildlands (Albert et al. in press).

Highway iceplant is found in coastal habitats from north of Eureka, California, south at least as far as Rosarita in Baja California. It has been planted (along with Carpobrotus hybrids) and is still abundant along highways, on military bases, and in other public and private landscapes. It spreads beyond landscape plantings and has invaded foredune, dune scrub, coastal bluff scrub, coastal prairie, and most recently maritime chaparral communities. Establishing readily after disturbance, its seedlings are often seen along roads and on trails and gopher mounds, as well as in areas of open sand and recently burned areas. It is intolerant of frost, and is not found far inland or at elevations greater than approximately 500 feet (150 m).

Its ability to establish and grow in native plant communities differs from one community to another (D’Antonio 1993). In coastal prairie it requires rodent disturbance to provide suitable open soil and is usually outcompeted by grasses at the seedling stage. Once established, however, highway iceplant can spread rapidly by vegetative means. In foredune and dune scrub areas, establishment is limited by herbivory (probably mostly by rabbits) but not by competition, although growth is slow in the dry, low-nutrient conditions. In the less harsh conditions of backdune scrub areas, seedling mortality is high as a result of herbivory, but a moderate rate of growth allows for fairly rapid vegetative spread.

Highway iceplant tolerates a range of soil moisture and nutrient conditions and can establish and grow in the presence of competitors and herbivores. These qualities and others have meant that in many natural areas it has formed nearly impenetrable mats that dominate resources, including space. It has invaded foredune, dune scrub, coastal bluff scrub, coastal prairie, and maritime chaparral communities, and competes directly with several threatened or endangered plant species for nutrients, water, light, and space (State Resources Agency 1990). It can suppress the growth of both native seedlings (D’Antonio 1993) and mature native shrubs (D’Antonio and Mahall 1991). In addition, it can lower soil pH in loamy sand (D’Antonio 1990a) and change the root system morphology of at least two native shrub species (D’Antonio and Mahall 1991).

An indirect effect of highway iceplant on the communities it invades can be the build-up of organic matter in normally sandy beach and dune soils, especially in areas where dieback and regrowth have occurred or in areas where iceplant has been treated with herbicide. This can result in invasion by non-native plants that normally would not be able to establish in sandy soils. Another indirect effect is the stabilization of dune sands, resulting in a change in the natural processes that sustain dune community formation over time.

Highway iceplant can reproduce both vegetatively and by seed. Flowering occurs almost year round, beginning in February in southern California and continuing through fall in northern California, with flowers present for at least a few months in any given population. Seed production is high, with hundreds of seeds produced in each fruit. Fruits mature on the plant and are eaten by mammals such as deer, rabbits, and rodents. Germination is enhanced by passing through animal digestive systems. Seeds in scat were found to have a higher germination rate than seeds from fruits that were not eaten (D’Antonio 1990a, Vila and D’Antonio 1998). Because of the ability to produce roots and shoots at every node, any shoot segment can become a propagule. This allows for survival of individual branch segments when they are isolated from the rest of the plant by being severed or buried by sand. For this reason it is important to remove all material from the site when attempting to eradicate this species.

Active growth appears to occur year round, with individual shoot segments growing more than three feet (1 m) per year (D’Antonio 1990b). All segments can produce roots at the nodes when in contact with soil, allowing for the formation of broad, thick mats. The impact on native competitors changes with the availability of water throughout the year, with the greatest impact occurring in times of drought (D’Antonio and Mahall 1991).

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Manual methods: Highway iceplant is easily removed by hand pulling, making it a good target for community or school group restoration projects. Because the plant can grow roots and shoots from any node, all live shoot segments must be removed from contact with the soil to prevent resprouting. If removal is not possible, mulching with the removed plant material is adequate to prevent most resprouting, but requires at least one follow-up visit to remove resprouts.

Mechanical methods: Mechanical removal by bobcat or tractor is efficient for areas in which there are no sensitive resources, although in order to prevent significant soil removal, the use of a brush rake attached to the scoop is recommended (Pickart, pers. comm.). Mechanical removal is effective at any time of year.

Insects and fungi: There are currently no biological controls for Carpobrotus edulis. The iceplant scale insects, Pulvinariella mesembryanthemi and P. delottoi, have a small impact on some individuals (Washburn and Frankie 1985), but would likely not be useful as a control tool. In addition, occasional parasitism by dodder (Cuscuta sp.) can be seen, but its impact appears to be minimal.

Grazing: Because of the salty and astringent quality of the leaves and the fibrous to woody quality of stems, grazing is unlikely to be an effective control for highway iceplant.