FOR: Narrow-leaf and showy milkweed (seasonal availability); also many native nectar plants, both annual and perennial. Open Mar–Nov.

FOR: The Marin chapter of the California Native Plant Society offers an extensive array of native bulbs, annuals, perennials, shrubs and even sometimes trees at their April and October plant sales, held on the grounds of Falkirk Cultural Center in San Rafael. Home Ground supplies about a third of the plants each sale through their propagation activities. Check www.cnpsmarin.org for dates. Flowering plants for pollinators are the emphasis in spring; shrubs are the emphasis at the fall sale. Also available: Home Ground’s line of locally packaged native wildflower seeds.

FOR: Neem seed meal to treat orange aphids, bulk organic fertilizers and soil additives, egg meal (to make a deer deterrent), lots of plants for pollinators, good advice.

The genus name, Asclepias, derives from the Greek god and healer, Asclepios. The Roman god of healing and medicine was named Aesculapius, and there was good reason for naming this genus of plants after this god. The plants contain organic compounds that have been used to treat various ailments and conditions of the heart and nervous systems, as well as the stomach and intestines. All parts of the plant contain cardiac glycosides and cardenolides, specifically one called oleandrin, which are concentrated in the milky latex sap.

From ancient times, these compounds have been used as heart tonics, as diuretics, and emetics. Theses toxic compounds are fatal when ingested in large quantities, so they were also used as poisons on arrowhead tips, to kill mice and rats, and to poison people. In modern days, they are used to treat congestive heart failure and cardiac arrhythmia because they can increase the output force of the heart and decrease the rate of contractions with actions based on a cellular level.

Oleanders are in a family (Apocynaceae) that is closely related to milkweeds, and there are famous stories of death by “white oleander.” Some other plants that contain cardiac glycosides are: foxgloves (digitalis spp.), hellebores, lily of thev, and some species of kalanchoe. The Cane Toad (Bufo marinus) also produces this deadly toxin. Humans have used parts of these plants for medicinal purposes and have found many other beneficial uses as well.

Asclepias tuberosa, native to the eastern US, is also known as pleurisy root; the root was used by early colonists to treat inflammations of the pleura (a thin membrane covering the lungs). The milky sap was also used as an emetic and a diuretic.

Asclepias curassavica, Mexican or Tropical milkweed, and sometimes also called bloodflower, is known by the people in its native range as soladitos; it was used to treat scorpion stings and to cauterize wounds.

Asclepias syriaca (an eastern species) and Asclepias speciosa (the western counterpart) were both used by the native peoples as a food and fiber plant. Both these species are large in stature, with big leaves emerging early in spring, large clusters of fragrant flowers in early summer, and large fuzzy seed pods forming later in summer. The tender young leaves, as well as the newly formed seedpods, were cooked and eaten as a vegetable.

Later in the season, when the leaves had dried and fallen, the now-bare stalks were harvested for the fibers, from which a very fine twine was made.

Early colonists also collected the floss, or silk, (the tiny “parachutes” attached to each seed), spinning it to make candlewicks. They called the plants “silkweeds”!

During World War II, the US government set up milkweed farms in order to harvest quantities of the silken floss as a substitute for kapok fibers. The “silk” was used as a filling in flying suits and life jackets.

In more recent times, milkweeds have been the subject of eradication campaigns on roadside ditches and field edges in the Midwest and in many grasslands and fields through the West where cattle are pastured because the plant is poisonous to most herbivores.

Most vertebrates and the smart herbivores, such as deer, know to avoid this plant; they never browse it. But I know from the experience of growing thousands of these plants over the years, that the small seedlings are eaten down to a nub by rats, which usually kills the little plants. Lately, I’ve also noticed that rats will eat the seeds, pulling stalks close to the ground to get to the seedpods, and then leaving nothing but the silk and the empty seedpod. Roman snails (the large introduced garden snail) also eat the plants! How do these creatures manage the toxins? Are they self-medicating and deriving a benefit from the toxic compounds in the plants?

The toxins protect a number of invertebrate species besides the Monarchs that associate closely with the milkweeds, and they all exhibit some combination of the warning colorations of red, orange or yellow, with black and white. There are large and small red and black seed bugs (family: Lygaeidae) that feed on the milkweed seeds, and a bright red milkweed longhorn beetle (Family: Tetraopes) that feeds on the foliage. There’s also a milkweed tussock moth (Family: Lymantriidae) whose colorful spiny larvae feed on the plants, and three species of tiger moths (Family: Arctiidae) that use asclepias as a larval host plant.

Besides the creatures that feed on milkweeds, there are numerous other arthropods that visit the flowers regularly. The most spectacular are our giant spider wasps (Family: Pompilidae) with blue-black bodies and bright orange wings that they constantly flick while hunting for prey. Spiders, especially crab spiders, position themselves amongst the flower clusters to wait for prey, and predatory arthropods such as mites, beetles, ants, lacewings and some “true” bugs all hunt on milkweed plants.

Many of these creatures also predate on the Monarchs; eggs and early instars are eaten by invertebrate predators, at which stage the milkweed toxins don’t offer much protection. The benefit of the toxin is more obvious in the later instars, when visual vertebrate predators like birds are a bigger problem. By these later stages, the larvae have eaten enough milkweed to build up the levels of toxins in their bodies, which is then passed on to the adult in the pupal stage. Even then, the individual doesn’t necessarily benefit, as the distastefulness of their bodies has to be learned by the predator, but it does benefit the species, and as well as all sorts of Mullerian mimics who assume the same warning colorations as the Monarchs.

Milkweed for the Monarch Butterflies

Asclepias fascicularis
Milkweeds provide important resources for many beneficial creatures, including Monarch butterflies. The fine leaves of this native milkweed gives it’s a soft, wispy look. The plant isn’t long-lived, but reseeds itself readily. Summer blooms, 3’ tall, low water, full sun, dormant in winter.

Asclepias speciosa
Monarch butterflies depend on milkweed to provide food for their caterpillars once they hatch. Star-shaped flowers in large clusters are produced from late spring to summer. Slow to get started but can get 3-5 ft. tall once established, low water, cut to the ground in winter.

Milkweeds are literally the life blood of the Monarch Butterfly. Both species, Narrow-leaf Milkweed (Asclepias fascicularis), and Showy Milkweed (Asclepias speciosa) are relatively easy to start from seeds. The trick is to pre-soak the seed in warm water for about 3 or 4 hours before sowing them. The seeds are a good size, but flat, so the best technique is to gently scratch them into the soil mix just a little, then cover them with vermiculite and tamp that down for solid seed-to-soil contact. 

These species germinate more readily in a controlled indoor situation. I use a gro-light set for 12 hours a day initially, and a bottom heat mat which keeps the soil temperature at about 70 degrees. Germination starts to occur within 4 or 5 days, increasing over the next several weeks to a month or more. I often start the process in late December, which allows enough time to grow large plants with vigorous root systems, ready for fall planting. The Narrow-leaf Milkweed is faster and easier than the Showy, and is also the better plant both for the Monarchs and for a garden situation.

Within a month or so the true leaves have developed, and when the seedlings are about 3 inches tall with a nice sturdy stem and several sets of leaves, they can be transplanted. Depending on available space we transplant to 4” square pots, or use long thin pots – sometimes called “liners” – for the first transplant. We use taller, narrow pots for the more mature seedlings, too. This style of container allows a really good root system to develop.

For seedlings started indoor under controlled conditions, the hardening-off process is important. Expose the seedlings gradually at first to morning sun only, keeping them inside at night, but without bottom heat. Then add more hours in the sun, and otherwise in a protected area (such as an outdoor covered porch). Finally, you can put them outdoors in full sun, and they can be exposed to cooler nighttime temperatures.

One thing I’ve learned through hard experience is that rats will eat Milkweed seedlings down to a nub, and that seedling rarely recovers. Snails and slugs will also eat Milkweed seedlings! It seems really strange that these creatures are able to handle the potent chemicals contained within the plant – but they obviously can – so be sure to protect your new seedlings. I make box-type cages that cover a whole standard nursery flat from ¼” galvanized wire mesh, and I also cover the bottom of the flat with the wire to keep the rodents out.

Another creature to be aware of when growing Milkweeds is the orange aphid, (Aphis nerii) which is a non-native species introduced with Oleandars (Nerium oleander).  Aphids reproduce by a system called parthenogenesis, which is basically self-reproduction by females giving live birth to other pregnant females. This is why the colonies build up so fast, and because the aphid is feeding on a toxic plant, they are also somewhat toxic, and the usual native predators cannot control really large populations.

I monitor the Milkweed seedlings closely and at the first sign of aphids, I either crush them or wash them off the plant with a strong stream of water. Next in the arsenal is a soap spray, and after that a Neem-oil spray to control the populations of this tiny beast. It is important, because large populations of aphids can completely enervate a plant, destroying the good leafy growth early in the season. Once this happens the plant is not in very good shape to host Monarch larvae, which, here in Marin, usually don’t show up until later in summer, long after the aphids have colonized the plants.

The very best Milkweed of all for the larvae of the Monarch butterfly is a non-native “Tropical” or “Mexican” Milkweed (Asclepias curassavica). This particular species of Milkweed (there are 14 species native to California alone, and dozens more native to the Americas, as well as other temperate climates around the world) has the highest level of Cardiac Glycosides of the species that are easily cultivated.