Monarchs and Milkweeds, What Gardeners Need to Know

//Monarchs and Milkweeds, What Gardeners Need to Know

Monarchs and Milkweeds, What Gardeners Need to Know

monarch butterfly on asclepias curassavica tropical milkweed

Recently, gardeners and the public in general are becoming aware of the plight of one of our most well-known butterflies, the Monarch.

Once a common sight throughout most of North America, Monarch numbers are on a serious decline and the iconic butterfly is even being considered for listing as an Endangered Species.

Milkweed plants, in all their various forms, are the exclusive foodplant of Monarch butterflies, so gardeners can play an important role in monarch ecology, and hopefully their recovery. Almost always, more milkweed plants means more Monarch butterflies.

In Southern California, gardeners have milkweed choices. For decades the most common milkweed species in our gardens has been a Central American species sometimes called Tropical Milkweed, or more correctly Asclepias currassivica. It is a magnet for monarchs and even a single plant in a garden will soon display a few colorful caterpillars dining on its leaves.

In Southern California another milkweed also grows well, and it has been here for thousands of years. It is our native milkweed, usually called Narrow-leaf Milkweed or more accurately Asclepias fascicularis. It grows in many of our local canyons, hillsides and mountain foothills; and fortunately, now occasionally in our gardens.

Danaus plexippus on Asclepias fascicularis, UCI Arboretum Roger's Gardens
Tropical Milkweed (Asclepiasis currassivica) is a popular non-native species and a prolific bloomer.

Caption: Tropical Milkweed (Asclepiasis currassivica) is a popular non-native species and a prolific bloomer.

There is growing concern within the science community that non-native milkweeds may be causing changes in monarch migration habits and increasing the prevalence of a debilitating disease among the adult butterflies. Fortunately, our monarch butterflies in California and other western states are non-migratory, so the issue of effecting their migration habits does not seem to apply to our region. The monarchs in California are not those that winter in Mexico or that you read about when you were in school. Ours don’t travel from the Northern parts of North America to Southern areas every winter and then back again the following spring.

Nonetheless, there still may be an issue. The invertebrate conservation group The Xerces Society recommends native milkweed species. Narrow-leaved milkweed, or Asclepias fascicularis, is a relatively showy plant reaching about three feet in height with pale pink to cream colored flowers. With a blooming period lasting from May through October, narrow-leaved milkweed provides plenty of nectar for adult monarchs, but more importantly, it provides the necessary leaves that Monarch butterflies depend upon. As with all 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 fruit and reproduce.

Narrow-leaf Milkweed (Asclepias fascicularis) is a native species that grows on our local hillsides and is an excellent foodplant for Monarch butterflies.

Caption: Narrow-leaf Milkweed (Asclepias fascicularis) is a native species that grows on our local hillsides and is an excellent foodplant for Monarch butterflies.

Native Narrow-leaved Milkweed is perfectly suitable to most of our gardens, as long as you understand its growth cycle and know what to expect. Unlike Tropical Milkweed, which retains foliage all winter and even a few flowers, our native milkweed is completely winter dormant. About November the foliage will quickly yellow and begin to dry. Shortly after this, the entire plant is generally cut back to near the soil line to let it rest until the following spring, when it will burst out with a fresh season of growth.

It is this winter ‘die-down’ that is exactly what tropical milkweed doesn’t do – and that may be the problem. A certain bacterial disease can persist on the winter foliage of tropical milkweeds. Then, when monarchs feed on these tropical milkweeds the bacterium can transfer from the plant to the larvae and eventually to the adult butterfly, weakening and even killing them.

Monarch caterpillars and cocoons (pupae) are nearly as beautiful as the adults.

Caption: Monarch caterpillars and cocoons (pupae) are nearly as beautiful as the adults.

Caption: Indian milkweed (Asclepias eriocarpa) is another Southern California native species, but is seldom grown in gardens.

If growing the tropical milkweed in a garden it is important to manage them in a manner that protects the Monarch butterflies. Each winter, beginning about Christmas, cut the entire plant to about four – six inches from the ground. This will not harm the milkweed at all. In fact, doing so refreshes the plant and makes it look even better the following year. Some gardeners even cut it a second time a few weeks later, just to be safe.

This winter cut-back removes all the milkweed foliage and therefore the overwintering disease, preventing it from establishing on the plant and infecting the butterflies.

Whether native or tropical, make your milkweed choice carefully and manage your plants intelligently. Your efforts will help bring back the Monarch’s and restore their populations to historic levels.

  • A close relative of the Monarch is the Queen Butterfly

Caption: A close relative of the Monarch is the Queen Butterfly, which also feeds on Milkweeds. Here it is nectaring on Narrow-leaf Milkweed in Irvine.

By |2018-07-12T13:13:23+00:00July 12th, 2018|Gardening|0 Comments

About the Author:

Ron Vanderhoff is a lifelong Southern California gardener and the General Manager and Vice President of Roger’s Gardens. He is a local director of the California Native Plant Society and serves on a number of state and local advisory committees involving horticultural education and natural resource protection. He was a principal contributor to The Butterflies of Orange County and The Wildflowers of Orange County and the Santa Ana Mountains, as well as a special contributor to The Sunset Western Garden Book.

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