(08/02/22) BATON ROUGE, La. — Monarch butterfly numbers have declined enough that one group of concerned scientists has placed the beloved insects on a watch list of endangered species.

Many Louisiana gardeners want to help the migratory monarchs by planting milkweeds, the host plant for the butterfly’s larvae, but knowing which species of milkweed to plant can require research, said Anna Timmerman, an associate extension agent for horticulture for the New Orleans area who researches native plants.

“A lot of the more prairie species, or what I would call upland species, have taproots, and anywhere there is high water table or excess rainfall, they really struggle,” Timmerman said.

In mid-July, the International Union for Conservation of Nature (IUCN) added the migratory monarch butterfly (Danaus plexippus) to its “red list,” labeling the species endangered. The group estimates the population of migratory monarchs has shrunk by between 22% and 72% over the past decade, depending on the type of measurement used.

These butterflies migrate from Mexico and California in the winter to breeding grounds throughout the United States and Canada, and a lack of milkweed plants has contributed to their decline, according to the IUCN. Monarch larvae feed exclusively on milkweed plants, according to the AgCenter Bug Biz Pest Management and Insect Identification Series fact sheet on monarch butterflies.

Milkweeds are not among the most popular ornamental plants, said Heather Kirk-Ballard, state consumer horticulturist with the LSU AgCenter, but their ecological benefits outweigh their less showy appearance.

“Most people might not look at it as one of the most ornamentally attractive plants, but as consumers trends have shifted, they want more native, more environmentally friendly plants,” she said. “I do see people want more native milkweeds. They’re desired for their ecosystem services. They're desired because they're good for the environment — they help provide ecological stability.”

One of the most popular milkweed plants for home ornamental use has been the nonnative Mexican or tropical milkweed (Asclepias curassavica), which can become problematic when it stays green through the winter.

Because a tropical milkweed will retain its leaves longer, it exposes monarch larvae to a harmful protozoan parasite, Ophryocystis elektroscirrha, or OE, that often travels with monarchs on their migration, according to the Monarch Joint Venture, a partnership of dozens of nongovernmental nonprofit organizations and federal and state agencies, such as the Natural Resources Conservation Service and the U.S. Forest Service.

Native milkweed leaves die back after blooming, and the parasites die with them. So, many butterfly researchers recommend gardeners plant native milkweeds, Kirk-Ballard said.

While many websites detail lists of milkweed plants native to the United States, not all will thrive in Louisiana, and some milkweeds that grow well in some regions of the state do not perform consistently in other areas, Timmerman said. Swamp milkweed (Asclepias incarnata) and aquatic milkweed (Asclepias perennis) will grow across Louisiana, but in coastal areas they grow more consistently than others, Timmerman said. While its name suggests otherwise, aquatic milkweed does well out of the swamp.

“It doesn’t need to be submerged or in wet locations,” Timmerman said. “It does great in patio pots and raised beds, and it can tolerate the humidity and the rainfall all summer long.”

White swamp milkweed (Asclepias perennis), which is a separate species from swamp milkweed, has become more widely available in garden centers and does well across Louisiana.

Common milkweed (Asclepias syriaca) does well in north Louisiana, Timmerman said, and butterfly weed (Asclepias tuberosa) thrives in central, west and north Louisiana and on the north shore of Lake Pontchartrain.

Milkweeds can be planted in spring, fall and summer, Kirk-Ballard said. When grown from seed, milkweeds need to be subjected to cold conditions before they germinate, Timmerman said. The seeds can be placed within peat moss or wet sand in a plastic bag and stored in a refrigerator for a few weeks, or they can be sown into pots and placed outside through the winter. This cold stratification process helps improve germination rates.

For more information on monarch butterflies, download the Bug Biz Pest Management and Insect Identification Series fact sheet on monarch butterflies. Visit LSUAgCenter.com and search for publication No. 3782.

The aquatic milkweed is a native Louisiana plant that can be found at many nurseries across the state, and it thrives in all regions of Louisiana. Photo by Anna Ribbeck/LSU AgCenter