Dwindling monarch butterfly populations have prompted some gardeners to pepper their landscapes with milkweeds, the various plants in the Asclepiadaceae family on which monarchs lay their eggs. Chubby, zebra-striped monarch caterpillars gorge themselves on the plants’ milky alkaloid sap, which makes them poisonous to birds.
The question for many isn’t whether to grow milkweed, but how — and which kind.
Milkweed-asclepias-curassvica In Connie Day’s Santa Monica garden, a tiger-colored monarch spars with another butterfly, chasing it from a patch of milkweeds.
“The challenge is keeping the food here,” Day says, noting that a few monarchs can defoliate a plant in a couple of weeks. Day favors South American blood flower (Asclepias curassavica), right, because it’s leafy all year, unlike native milkweeds, which are dormant in winter. The South American blood flower catches the eye with brash little flowers in smoldering colors, including ember red and orange-singed yellow.
But some scientists recommend the natives. These experts suspect that the supply of exotic milkweeds is prompting some monarchs to loiter in California, and those that don’t migrate seem to be more vulnerable to parasites, according to entomologist Brent Karner. And, true to their name, the nonnative plants are a bit weedy.
The loveliest of the California natives is showy milkweed (Asclepias speciosa). Its carnation-scented blossoms are two-tiered: Five-fingered pink coronas are backed by petals that blush a rosy mauve. Velvety leaves glow in soft light. Alas, monarchs seem to prefer speciosa’s homelier cousin, the narrow-leafed milkweed (Asclepias fascicularis).
So what to do with a spindly, often-bare plant? Scatter it among low-growing native shrubs, says Mike Evans, president and co-owner of Tree of Life Nursery in San Juan Capistrano.
Monarch-caterpillar “Place it behind some native grasses or irises,” says Los Angeles County Arboretum and Botanical Garden Curator Jill Morganelli. “That way you can still see the flowers but you miss a lot of the ‘skeletonization’ that happens because the caterpillars don’t eat all the way up to the flowers.”
Look for a sunny place — away from pets and traffic — where the butterflies can spot the plant from above. After seeing and smelling a plant, they come to it and test it, biologist Bob Allen says.
“The female takes her front legs, which have chemical receptors, and scratches the leaf, tasting it with her feet to confirm it’s milkweed,” he says.
The aforementioned South American blood flower is widely available, as is butterfly weed (Asclepias tuberosa); but for the California species you’ll have to visit a native plant nursery or troll ButterflyEncounters.com.
Start from seed to avoid systemic pesticides that some growers apply. To encourage leafy growth, cut milkweeds back after they bloom. Adult monarchs need nectar, so entice them with plantings of flat, upward-facing flowers.
There’s no guarantee you’ll be graced with monarchs, but Southern Californians have a good shot. Plus, milkweed flowers feed a variety of butterflies.