Carlsbad is preparing to launch an aggressive effort, including the widespread application of a chemical herbicide, to eradicate the invasive plant Ward’s weed from about 200 acres where it has taken root in several of the city’s habitat preserves.
A native of the Mediterranean region, the species’ first North American appearance was in Carlsbad’s Rancho La Costa Preserve in 2008. Since then, the plant has spread to the undeveloped habitat management areas east of El Camino Real between Palomar Airport Road and Alga Road.
Ward’s weed has been found in three other locations in San Diego County and nowhere else so far in the United States, according to a city staff report. About 90% percent of the known infestation is in Carlsbad, and it’s unknown how it arrived.
The plant “spreads extraordinarily fast and can grow in dense mats that choke out native plants,” the report states. “Once the infestation reaches a certain point, eradication will be infeasible, and the plant could spread throughout California and the United States.”
Another danger is that the plant quickly dries out and becomes highly flammable after its annual spring growth spurt, creating a fire hazard that lasts until the next rainy season.
Staffers have asked the City Council to join San Diego County and two nonprofits in spending up to $400,000 on a two-year eradication program. If approved Tuesday, the city would contribute up to $200,000; the Nature Collective, which was formerly the San Elijo Lagoon Conservancy, has agreed to contribute $100,000; San Diego County would spend $82,400; and the Center for Natural Lands Management would kick in $11,600.
“Invasive, non-native plants are one of the greatest threats to the city’s preserve system,” the city report states. By crowding out native plants, the invaders also push out native wildlife including important pollinators, mammals and birds.
The plant has seed pods that hang on throughout the dry season and burst open with the first rains, distributing up to 30,000 seeds per square meter. The seeds stick to clothing, muddy boots and the feet of animals to become widely dispersed.
Small infestations can be controlled by hand-pulling the weeds. Mowing the plant is ill-advised because it tends to spread the seed.
Organic herbicides are not effective because they kill only the above-ground portion of the plant, and it resprouts from the roots. Plants as small as 1 inch tall can produce seeds, and multiple herbicide treatments can trample any nearby desirable native plants.
The city plans to use the herbicide Gallery, which includes the active ingredient isoxaben, probably in November or December before the heaviest rains. Gallery is considered a “pre-emergent” herbicide that suppresses seed germination and does not harm most woody shrubs, grasses or bulb species.
The City Council in 2017 unanimously approved a policy that opposed the use of chemical pesticides and herbicides in favor of organic alternatives when possible. However, the policy allowed exceptions for infestations such as Ward’s weed.
“There is a stipulation within our nontoxic policy that if there is a threat to native plants and public preserves ... or a fire threat, there is an allowance” for chemical herbicides, said Mary Anne Viney, a Carlsbad resident and member of the local nonprofit Preserve Calavera.
She was among residents who pushed the council to pass the pesticide policy, which required the city to back away from widely used herbicides such as Roundup in favor of organic solutions whenever possible.
The Ward’s weed battle is “concerning” because herbicides can have a toxic effect, she said, but “it’s one of those things, darned if you do and darned if you don’t.”
People who care about native species have been aware for some time that Ward’s weed is a threat, Viney said, and “we need some tool to help us.”
Most of the infested areas are on privately held property, including parts of the Bressi Ranch, Bressi Spectrum and Rancho Carrillo preserves.
Diehl writes for the San Diego Union-Tribune.