Monarchs depend on milkweed
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.