Marin Bans Tropical Milkweed Sales

MARIN COUNTY, CA — Marin County has banned the sales of tropical milkweed, a non-native plant that’s harmful to monarch butterflies, but those with the plant currently growing in their gardens don't need to remove them, officials said.

The move comes the heels of the California Department of Food and Agriculture designating tropical milkweed as a B-rated weed, county officials said, noting that such weeds are rated as pests that do not meet state cleanliness standards.

Marin County Agricultural Commissioner Stefan Parnay directed all retailers to remove and destroy all tropical milkweed in stock in an Aug. 24 memo.

“We want to thank the nursery staffs for their understanding and assistance in helping us protect the monarch butterflies,” Parnay said in a statement.

“Their customers can play an important role by buying native milkweeds and native nectar plants that will improve the health of pollinators. Planting native plants is the way to go.”

In July, the International Union for Conservation of Nature designated monarch butterflies as endangered.

The recorded monarch population in the Bay Area, which migrates from central Mexico, has declined by more than 80 percent since the 1990s.

The decrease is tied to climate change, pesticides, disease, and habitat loss. Monarchs depend on milkweed to complete their life cycle; the plants are the only ones on which the butterflies will lay eggs and are the sole source of food for their caterpillars.

Unlike native milkweed species, tropical milkweed does not die back in winter, which can confuse monarchs into breeding when they should be migrating. Tropical milkweed is also a host plant to a protozoan parasite of monarchs called Ophryocystis elektroscirrha, or OE.

Spores are carried on adult monarchs as they migrate, and the dormant OE spores are deposited on the tropical milkweed leaves. As the monarch caterpillars eat the plant, they ingest the OE spores and become infected.

Such infections have been linked to lower migration success, reduction in body mass, lifespan, mating success, and flight ability.

The Master Gardener help desk is a garden problem-solving and information resource for all Marin residents.

Marin County Director of the UC Cooperative Extension David Lewis recommends homeowners contact the UC Marin Master Gardener Help Desk for more information by calling (415) 473-4910 or emailing HelpDesk@marinmg.org.