Milkweed and Monarchs

Milkweeds in the Wild

Asclepias species usually grow in grassland habitats; here in Marin much of it has been eradicated from areas where cattle have been grazed. I've only found populations of Narrow-leaf Milkweed (A. fascicularis) growing naturally in two places in Marin; at Sky Oaks, in the meadow close to the boundary of the Meadow Club, and in scattered clusters on Mt. Burdell, fairly close to the largest of the  serpentine outcrops. Milkweeds are poisonous plants, so most browsing and grazing animals avoid it; which also means that it is a deer-proof plant.

Monarch larva on Indian Milkweed
Monarch larva on Indian Milkweed, photo by C T

When I’m camping during the summer, mostly in Sonoma, Mendocino and Lake Counties, I do find stands of other species of Milkweeds; A. speciosa, A. eriocarpa, and A. cordifolia, as well as Narrow-leaf Milkweed. Indian Milkweed (A. eriocarpa) grows in large drifts in hot, dry conditions in an open exposed meadow at about 1,800 feet elevation; but Purple Milkweed (A. cordifolia) wants a bit of shade, so I find it growing at the base of shrubs or on north facing slopes.

Showy Milkweed
Showy Milkweed

Showy Milkweed (Asclepias speciosa) is a beautiful plant in bloom, with large fragrant clusters of pink-tinged white flowers; I see it growing in ditches, sometimes even along roadsides, and at higher elevations. It will grow in a garden though it is often difficult, and can take years, to get the plant established. I’ve collected seed from the other two wild species, but no luck, so far, in growing viable plants from those seeds!

Narrow-leaf Milkweed
Narrow-leaf Milkweed, photo by CT

Since Milkweed is scarce in Marin, habitat gardeners can really help Monarch populations by including Milkweeds in their gardens. I grow three different species in my Novato garden; Showy Milkweed, Narrow-leaf Milkweed, and Mexican Milkweed (also called Tropical Milkweed, or Bloodflower).

Monarch nectaring on Mexican Milkweed
Monarch nectaring on Mexican Milkweed, photo by Bob Stewart

Milkweeds in the Garden

Milkweeds are herbaceous perennials, dying back to thick fleshy rootstocks during the colder months. Showy Milkweed has roots that wander, and will eventually spread out to where they want to grow. This species is very appreciative of water during the summer months, and will respond with vigorous, spreading growth and lots of beautiful flowers. This plant did beautifully in a garden I built which included a small pond with a seep. Showy Milkweed spread toward the moisture of the seep, and created a drift that was spectacular and powerfully fragrant when in bloom!

Narrow-leaf Milkweed is a better choice for a drought-tolerant garden, where it needs full sun, good drainage, and perhaps a little extra water during the hottest months. However, I soon discovered an idiosyncrasy; this plant wants to choose where it will grow; offering the habitat gardener a beautiful lesson in giving up total control over the garden. 

Asclepias fascicularis seems to establish best where it seeds itself in; over the years I’ve planted out healthy, vigorous specimens, only to watch them fade out and die after just one season. By that time the seeds have been blown about the garden, and new seedlings appear in various places where they persist and thrive year after year! Narrow-leaf Milkweed has no problem with frosty nights, keeping its leaves long after Showy Milkweed is nothing but bare stems. Leaves are eventually shed, and then I cut it right back to its base; as much to invigorate new growth on the plants as to try to break the cycle of the dreaded orange aphid which colonizes the milkweeds.

Asclepias curassavica is not native to California; it is a tropical plant, and as such is frost tender. In very cold areas it is grown as a summer annual. Seeds germinate readily; especially if the seed is pre-soaked and the seed starting flat is placed over bottom-heat. In many ways this is the best of all Milkweeds for the garden; it is fast growing, with lots of orangey-red flowers that attract pollinators. There are also hybrids; ‘Silky Red’ with deep red flowers, and ‘Silky Gold’ with clear yellow flowers. This Milkweed also grows beautifully in large containers; and when grown in a pot it is also easier to move it to a protected site for the colder months. 

Milkweed Toxins

Milkweeds attract all manner of insects, many of which are able to utilize the toxic quality of the plant to their advantage.  The toxin is a ‘cardenolide’ which at best causes vomiting in a vertebrate predator, and at worst can cause death. The Monarch larva sequesters this toxin within its body as it feeds on the plant. The toxin is passed on through the pupal stage to the adult; the bright colors of both the larva and the Monarch butterfly are ‘warning colorations’ that alert potential predators to their toxicity.

The level of cardenolides varies with each species, and, of all the Milkweeds, A. curassavica has one of the highest levels of toxicity; which makes it the best host plant for the larvae of the Monarch butterfly. The adult female is able to take a chemical reading of a plant before she lays her eggs on it; the higher the level of toxicity, the more protection her offspring have!

Milkweeds and Aphids

In my garden, and at the nursery, most larvae show up on the Mexican Milkweed. But so does the Orange Oleander Aphid (Aphis nerii); this aphid is native to the Mediterranean region, and closely associated with Oleanders. Along with Oleanders, this insect has been introduced to many other parts of the world, and is now quite cosmopolitan. Large populations can build up quickly, and can really debilitate a Milkweed plant if left unchecked.

Oleander aphids
Oleander aphids

I have found that the best way to control the aphids is to find them early before the big colonies develop. Luckily, here in Marin, most of the Monarchs arrive in late summer, so there is plenty of time to deal with the aphids, without worrying about damaging butterfly eggs or larvae, or beneficial predators, and still have vigorous host plants ready for the Monarchs when they arrive.

Aphids are ‘true bugs’ that feed on plant fluids with sucking mouthparts. When there are just a few bugs on the plant I simply rub them out. When they reappear in greater numbers, I spray them off with a stream of water. If the colonies continue to grow, and I’m sure the various predatory species aren’t present yet, I will use an insecticidal soap spray. And then, if that still hasn’t brought the aphid colonies under control, I start to prune off the most heavily colonized stems of the plant. I put these into the yard waste can so that they are hauled away from my garden. The plant will continue to put out new growth, and the gardener has to continue to monitor the aphid colonies.

The usual predators of aphids; Lady Bird Beetles, both adults and larvae, Lacewing larvae, and Syrphid fly larvae, can’t do a really good job of keeping this aphid in check because of the Milkweed toxins. The Oleander Aphid is also toxic (it is orange as a warning) and I’ve read that beetle and lacewing larvae that have fed on these aphids are sometimes malformed as adults. The toxins are also in the aphid’s exudate (commonly called honeydew) so even the ants aren’t that interested in this aphid. Tiny wasps (Braconidae) do parasitize the aphids, turning them into ‘aphid mummies’, and that does help somewhat to keep populations at manageable levels.

aphids and LB larvae
Aphids and Lady Bird Beetle larvae

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