UPDATE: Roger’s Gardens no longer sells or recommends tropical milkweed. The evidence is overwhelming that these plants are actually harming our Monarch butterflies, not helping them. Please join us and only cultivate native milkweed species, and share this information with friends and fellow gardeners.

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. Since the 1980’s, California’s monarch population has declined an estimated 97%.

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. But now, as difficult as it is to believe, we are learning that this may not necessarily be true.

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 evidence 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 far less migratory than their Eastern brethren, so the issue of effecting their migration habits does not apply as much 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. But they do move around regionally, just not like the big migrations of some areas.

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, and none of the harmful bacteria. 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 disappears completely in the winter – and that is perfect for monarchs!

It is this winter ‘die-down’ that is exactly what tropical milkweed doesn’t do – and that is 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 transfers 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.

Roger’s Gardens no longer sells or recommends tropical milkweed. We suggest all tropical milkweed be removed from your garden, but we know that some gardeners will resist this request, especially when they see larvae abundantly feeding on it.

If growing 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, below all leaves. This will not harm the milkweed at all. In fact, doing so refreshes the plant and makes it look even better the following year. About a month later, cut it down again. Some conscientious butterfly gardeners even cut it a third 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 healthy 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.

For more information about western Monarchs, milkweeds and butterfly health see: https://xerces.org/save-western-monarchs/