Monarch butterflies are fussy eaters that dine only on milkweed, so to boost their dwindling numbers, gardeners have been busy sowing milkweed plants in their yards. Now there’s news that some milkweeds are more hindrance than help, according to Ron Vanderhoff, general manager of Roger’s Gardens nursery in Corona del Mar and a board member of the Orange County chapter of the California Native Plant Society.
The most common milkweed in gardens is a non-native known as tropical milkweed (Asclepias currassivica), which blooms profusely with rich orange flowers the monarchs love, but also harbors microscopic protozoans that survive on the plants during winter and sicken the caterpillars who eat them in the spring.
The Xerces Society, an invertebrate conservation group, recommends that home gardeners plant native milkweed species instead, such as narrow-leaf milkweed (Asclepias fascicularis), “a relatively showy plant reaching about 3 feet in height, with pale pink to cream-colored flowers.”
Unlike the tropical variety, the native plants die down to the ground in the winter, so overwintering critters aren’t an issue, Vanderhoff says. If you must keep your tropical milkweed (which have more showy flowers), Vanderhoff says gardeners should cut it to the ground in December, and once more in February, to ensure no nasty protozoans survive.