Photo copyright © 2020 Mariposa Native Plants
A perennial herb with large, almost hand-sized leaves and large, round clusters of purple, pink, or even white flowers. It is one of the native California milkweeds in the Asclepias genus that support the monarch butterfly caterpillar. There is a monarch butterfly on a showy milkweed umbel in the photo above. Showy Milkweed tends to grow around watercourses, and one often finds it around roadside culverts and stream overcrossings. It spreads through underground rhizomes, and it forms large patches. In the past, it was called "River Milkweed". A.speciosa is probably the most attractive milkweed native to California and to the western U.S. as a whole. Milkweeds are deep-rooted, tend to emerge later in the spring as compared to other California native plants, thrive in the hot summer when the other plants have turned brown, and go dormant in the early winter.
In California, A.speciosa is found mainly in the north, from the San Francisco Bay Area north in the Coast Ranges to Oregon, down through the western Sierra foothills, in Yosemite Valley, and in the Owens River Valley. It ranges in elevation from sea level to 8,000 ft. (2,500m). Sun: full/part sun. Temperature: cold tolerant to -5°F (-20°C). Soil: should be well-drained, but A.speciosa tolerates some wetness in the ground, as long as the plant is not inundated. Soil acidity: pH from 6 to 8.5.
Fairly easy to care for once established. Minimal moisture requirements for the Showy Milkweed are fairly low, but it does a lot better when there is some moisture available. A specimen that is planted early in the year from a 1-gallon container should be given supplemental water (1 gallon, 4 liters) every week for the first summer. Once the fall and winter rains arrive, stop supplemental watering. In the second summer, A.speciosa should need no supplemental water, but, again, the plant thrives with a little additional H2O. One last point: Do give some thought to where you are going to locate this plant in the garden or landscape. It does spread into dense patches. That is good for the monarch butterflies, but you do have to worry about the milkweeds overwhelming some other prized plant. The spread of a milkweed patch can be halted merely by digging up the peripheral shoots as they emerge.
Foothills (to 2500 feet), lower mountains (2500-3500 feet), middle mountains (3500-6000 feet), and high mountains (above 6000 feet).
Deer resistant. Gophers will eat the roots of young milkweeds. It would be prudent to provide Showy Milkweed plantings with a gopher screen.
All of the native California milkweeds have specific insects associated with them. One of the most obvious is the Oleander Aphid, Aphis nerii. This is a small, yellow aphid that sucks fluids from the plant. The Oleander Aphid is annoying, but it seldom harms the plants in any significant way. You can blast the aphids off of the plant with a stream of water, holding the plant stem with your hand so that it doesn't snap under the force of the jet of water. You can brush the aphids gently off with a twig, too. The main thing, however, is that one should not use insecticides on Showy Milkweeds; this would be harmful to the butterflies and their caterpillars. Typically, the Oleander Aphid only infests milkweed plants, oleanders, and other plants in the broader Dogbane, or Apocynaceae, family of plants. Milkweeds contain a white, sticky sap that is toxic, and this thwarts other pests that have not developed special adaptations for ingesting the plant fluids.
After the monarch butterflies lay eggs on a native milkweed plant, it takes a few weeks for the tiny caterpillars to emerge. They begin to feed on the plant's leaves and stems, really begin to grow large, and then eventually devour almost the entire leaf and small stem structures on the milkweed. This is good, and the milkweeds are actually adapted to this carnage. The caterpillars will go into a pupal stage, turn into butterflies, and leave the plants. The milkweeds actually recover quite quickly, pushing out fresh new growth, and it's something that is awesome and worrisome to watch happen, but also something that we don't really have to fret about.
The native California milkweeds have grown in popularity among native plant gardeners in recent years. The reason behind the sudden interest is that the monarch butterfly is in decline in the western U.S., and the milkweeds are the only plants upon which the butterflies lay their eggs. Re-establishing large milkweed patches in the state is the best way to provide monarch caterpillar habitat and fend off the extinction of the western monarch. There is a special relationship between the milkweed and the monarch. The caterpillars feed on the plant's leaves and stems, absorb the toxin from the plant's phloem layers, and become toxic themselves. The monarch butterfly, when it emerges from the pupal stage, retains this toxicity. Both the caterpillar and the butterfly are brightly colored, signaling to potential predators (birds, rodents, reptiles, frogs) that this insect is not something that it should attempt to eat.
(Above) Showy Milkweed, grown from seed, about 10 weeks old, in one-gallon nursery pot. Photo copyright © 2018 Mariposa Native Plants
#1 container, about 1 gallon. Seeds: two ecotypes are avaliable, Mariposa and Sacramento Valley.