With the recent declines in numbers of monarch butterflies leading to the popular insect becoming a candidate for listing as an endangered species, more and more gardeners are thinking about growing milkweed. Milkweed, after all, if the only kind of plant monarch caterpillars can eat, and so growing milkweed in your garden means you're providing monarchs with a nursery and larder for their young.

But there's a problem: there are about 140 known species of milkweed, some of them potentially invasive in California wildlands. In fact, not all milkweeds are of equal benefit to monarch butterflies. There's even some thought that one popular tropical milkweed may be harming North American monarchs by changing their migration habits.

Fortunately, there are fifteen species of California native milkweed that gardeners can choose from to give monarchs a helping hand. Not all of them are readily available in nurseries, but with a little searching you should be able to find at least one species appropriate for your part of the state.

In a guide to milkweeds recommended for California butterfly gardens, the invertebrate conservation group the Xerces Society recommends two widespread and widely available species of milkweed that not only do well across much of California but which are also reasonably easy to procure from nurseries, at least as seeds. Pictured above is one of the two: the narrow-leaved milkweed, Asclepias fascicularis. A relatively showy plant reaching about three feet in height with pink and white blooms, narrow-leaved milkweed is native to all of California save the hottest parts of the Low Desert and the coldest parts of the High Sierra.

With a blooming period lasting from May through October, narrow-leaved milkweed provides plenty of nectar for adult monarchs as well as many other species of beneficial insects. As with other species of milkweed, pinching off the blooms once they're spent, a process called "deadheading," can greatly prolong the blooming season. That way, the plant keeps putting out more flowers in an effort to produce seeds. (Let a few flowers go to seed toward the end of the summer, and you'll have plenty of seeds for next year's season.)

Gardeners in the vast swath of California characterized by big trees may do well to try growing the other species in the Xerces Society's top two Milkweeds: the showy milkweed, a.k.a. Asclepias speciosa. The species' common name is apt: under good growing conditions these plants can reach five feet in height, with a bloom potentially running from May through September if you take care to deadhead the faded blooms.

If you're lucky enough to have naturally moist garden soil here in droughty California, showy milkweed can be almost invasive, sending out aggressive creeping roots that can easily crowd out other garden plants.

The Xerces Society is working to make seeds of both narrow-leaved and showy milkweed more readily available through retail nurseries: more information and a short list of nurseries is available in their California milkweed guide.

Central and Southern California gardeners inclined to pout over the likely unsuitability of showy milkweed for their warmer, drier gardens can take heart: with a little bit of creative online searching, you can find a perfectly attractive milkweed best suited to more chaparrally gardens. California milkweed, or Asclepias californica, is a droughty, fuzzy-leaved milkweed that inhabits dry slopes from the Bay Area and Yosemite foothills to the South Coast.

About three feet tall at best with pink and purple flowers between April and July, California milkweed fits right into wild gardens in the California chaparral country. That's if you can find the seed. Scour local small native plant nurseries, or find gardeners (or owners of wild land) with existing California milkweed plants and ask them if you can have some seeds.

Mojave milkweed | Photo: ©2010 James M. Andre, used by permission

Searching for plants and seed may well bring you luck with a few other showy California native milkweeds, including the frankly stunning heartleaf milkweed (Asclepias cordifolia) whose photo we put at the top of this article. To be honest, the search for hard-to-find seeds can be half the fun of growing appropriate natives.

But some native milkweeds, especially those from California's deserts, are just unlikely to end up in commercial seed stocks any time soon: they're just too rare, and too hard to grow outside their native habitat.

And thus the best way to use those rare milkweed species to promote the health of our monarch butterfly population is to protect the wild habitat in which they grow. That's especially important in the desert, where widely scattered populations of milkweed may act as crucial stepping stones in the butterflies' migration.

Mojave milkweed (Asclepias nyctaginifolia), shown in the photo just above, provides an example of how a rare species can be devastated by inappropriate development of its habitat. A few years ago there were 25 known occurrences of Mojave milkweed in the state of California. But 16 of those occurrences were on the footprint of the Ivanpah Solar Electric Generating System, so there are a lot fewer Mojave milkweeds in California now.

Sometimes, it turns out, the best way to garden is to leave the plants where they are in the first place.