Monarchs depend on milkweed

This often overlooked plant is a prime food source for Monarch caterpillars. By planting a few milkweeds in your garden, you can help the struggling Monarch population.|

Most of us are familiar with monarch butterflies (Danaus plexippus plexippus), but how many of our gardens have milkweed, the essential host food plants for caterpillars? Not only are these plants striking, educational and entertaining elements of a garden; they can help sustain rapidly declining monarch populations as they migrate north in spring and south in the fall.

There are two populations of monarch butterflies - western and eastern - that fly along separate seasonal migratory routes to and from overwintering sites in forests north of Mexico City. A portion of the western monarch populations overwinter at sites in coastal Central and Southern California. Populations of western monarchs have been rapidly declining. In the 1990s as many as 700 million monarchs flew along migratory routes.

Now, numbers are down dramatically, especially in western coastal populations. They have fallen from 4.5 million in the 1980’s to only 28,429 as counted in January 2019. A number of factors have affected them. As monarchs rely on milkweed for reproduction, the loss of it on main migration routes means fewer monarchs will travel north.

The loss of milkweeds due to urbanization and agricultural intensification, pesticide use, and erratic weather affect viability of populations. In corn and soy growing areas, the use of Roundup TM Ready Crops, has caused fields to be free of any plants but the crop plants.

Formerly milkweeds and other plants were present in and around fields and provided both larval host plants and adult nectar plants. We can easily plant or protect existing milkweeds in our gardens and farms. Milkweeds are deer resistant.

A number of native milkweed species grow in our area. On valley floors, the narrow leaf milkweed (Asclepias fascicularis) is often found growing in ditches, moist areas, and near fences, creeks and rivers, and in and around orchards and vineyards. Of all the milkweeds, it is not a showy plant, but it the longest blooming and one of the easiest to grow.

The green, narrow leaves and small pink and white flowers don’t jump out in the landscape, and, when mistaken for a weed, it is often mown or weed whacked. This milkweed also grows well in our gardens. But with regular water and fertile soil, it can be invasive.

The best place to grow it is in areas out of main flower beds that receive regular irrigation.

In a garden setting, it will bloom most of the summer, and is visited by a large number of bees, beneficial insects and butterflies.

Narrowleaf milkweed should be available at native plant nurseries in our area.

Another native milkweed found in drier valley floors, low hills and in drier vineyards, orchards, livestock pastures and along fencerows is the woolypod milkweed, Asclepias eriocarpa.

This is a very beautiful plant, with almost white oval leaves, and beautiful round umbels of white flowers. The whole plant may only be 1 foot tall. It is not invasive, but very slowly forms colonies over time.

Even though it is beautiful, it is often unrecognized as a plant of value, and consequently is disked or mowed in vineyards, or weedwacked around homes.

Resprouting from the roots, it will readily regrow - even after controlled burns or fires. If this occurs too late season, seed pods may not mature. Milkweed is toxic. Livestock, including horses, cows and sheep don’t eat it and even in heavily stocked and overgrazed pastures, plants will be untouched.

Though very garden worthy, woolypod milkweed is hard to find at nurseries. This is a summer blooming milkweed.

The earliest native milkweed to emerge and bloom is the heartleaf milkweed, Asclepias cordifolia. It grows on hillsides in both moist and very dry soils along some parts of the San Francisco Bay Area and through Oregon and Nevada. Leaves are large, bluish, waxy and pointed with distinctive white veins. Flowers clusters are a very showy burgundy and white. It does not seem to form colonies as readily as many of the other species. Blooming in spring while grass is still green, this distinctive milkweed is often overlooked. It goes dormant in summer after seed pods have ripened.

The most widely grown native milkweed and easiest to find in nurseries is the Showy milkweed, Asclepias speciosa. Very large, pink, showy flower clusters are held aloft on top of striking stalks of gray with large oval leaves. Showy milkweed has very strong upright growth, is very drought resistant or can tolerate fairly regular irrigation. It will spread, but doesn’t overwhelm. Any unwanted shoots are easily pulled. The showy milkweed grows in many soil types and over much of the western U.S. Butterflies nectar on its blooms. The selection ‘Davis’ has showy, fragrant white flowers, and the leaves are a striking white.

Another milkweed that grows well in our area is common milkweed, Asclepias syriaca. Native to much of the Midwest and eastern seaboard, this milkweed’s pinky-mauve flowers are divinely and exotically fragrant. It is worth planting for the scent alone. The flowers are incredibly attractive to honeybees and appear to drive them to a happy bee frenzy. This is the largest milkweed, and closely resembles showy milkweed, except the leaves are more green than gray, the flowers are mauve, and the plant is larger.

Tropical milkweed, Asclepias curassavica is a non-native milkweed that does not die back in frost-free climates. Year-round growth allows buildup of a protozoan parasite (Ophrycystis elektroscirrha) that kills monarchs. If you have this plant, make sure to cut in back late summer to avoid parasite build-up, or better yet, replace it with one of the native species.

Milkweeds grow easily from seed, but patience is necessary as seed takes about one month to germinate.

Many milkweed plants are plagued with orange oleander aphids in mid or late summer. This is a non-native aphid that is able to tolerate the toxins found in the plant. As the aphids secrete honeydew, mold can form, turning older plants into a sticky mess. If this happens, simply cut plants down. A way to prevent this is to have several milkweed plants. The aphids seem to target older plants and are not a problem on younger foliage.

Cut half of the plants down to the ground in summer as aphids first appear and multiply. Plants will regrow with fresh, young foliage that will be prime condition for south-bound migrating butterflies in late summer.

Cut down old spent plants as they decline. Watch for caterpillars. Monarch butterfly migration begins in August and continues into early October. Milkweed bugs can also be a problem, sucking plant juices of flowers and seedpods, and causing them to wither.

A pest of young caterpillars is the non-native European paper wasp, serving as its main food source. In the cool of the morning or evening, vacuum nests from under house eaves, on bushes or other areas.

A good place for monarch related resources is the Xerces Society for Invertebrate Conservation (

Kate Frey’s column appears every other week in Sonoma Home. Contact Kate at:,, Twitter @katebfrey, Instagram @americangardenschool

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