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Agriculture officials in Marin County have banned the sale of a harmful plant in an effort to protect an endangered insect.

An Aug. 24 memo from the county’s agriculture commission to nurseries in the county followed a state ruling that designated tropical milkweed as a “B-rated weed,” which are considered pests that do not meet state cleanliness standards, according to a news release from the commission.

Tropical milkweed — which goes by several names including Mexican butterfly weed, blood flower and cotton bush — can harm monarch butterflies, which are endangered, by confusing them during migration season. Rather than dying back in the winter like native milkweed, tropical milkweed remains bright, which can confuse monarchs.

The tropical variety also produces a parasitic spore that can be harmful to monarch caterpillars.

Marin County Agriculture Commissioner Stefan Parnay directed all plant retailers to remove and destroy the tropical milkweed they currently have in stock.

County residents who have planted tropical milkweed in their gardens and yards do not need to remove it, according to the new guidance, but are advised to prune the flowers back in the fall to a height of six inches.