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June 11, 2024

LSU AgCenter's Weekly Message

Many Milkweeds Support Monarchs

Many people know of milkweeds because they’re hosts of monarch butterflies. Butterflies often need two types of plants. They need larval hosts, or plants to eat while they’re caterpillars, and they need nectar plants to sip sugary nectar from when they’re adults. It’s while gathering nectar that they unintentionally spread pollen from plant to plant, thereby serving as pollinators. (Some butterflies acquire sugary liquids from other sources, such as tree sap and rotting fruit.)

While most butterflies will take nectar from many types of plants, many only eat leaves of certain plants while they’re caterpillars. Monarch butterflies use milkweeds as both larval hosts and, along with other plants, nectar sources. Part of the special relationship between monarchs and milkweeds are chemical compounds called cardenolides. These are toxic to most animals but tolerated by monarchs. As the caterpillars consume milkweed, cardenolides build up in their bodies, and this deters many insects and other animals from eating them.

Within the past 40 years, losses of milkweed habitat in the US and of oyamel fir forests in Mexico (where migratory monarchs from the eastern US overwinter) have contributed to smaller monarch populations.

Milkweeds are in the dogbane family (Apocynaceae). Garden centers often offer tropical milkweed (Asclepias curassavica; AKA bloodflower or scarlet milkweed) for sale. This milkweed grows easily in a wide range of soil conditions and often spreads by seed. Flowers have red, orange, and yellow coloration.

Tropical milkweed has gotten a bad rap among some. Converse to what one might expect, the leaves of tropical milkweed tend to stay green for longer in the fall than those of native milkweeds. In coastal Louisiana (approximately USDA Hardiness Zone 9B and warmer), the plant is likely to be evergreen. This encourages monarchs to remain in the US instead of migrating to Mexico. When the butterflies don’t migrate, there is more opportunity for a protozoan parasite called Ophryocystis elektroscirrha to build up in their populations. For this reason, gardeners who grow tropical milkweed are encouraged to cut the plant back to close to ground level in the fall.

While tropical milkweed is most readily available in garden centers, a number of milkweed species are native to Louisiana, and many of these are found in one or more of the Florida Parishes. Some grow well on drier sites, while others are better adapted to wetter ones.

Butterfly weed (A. tuberosa) has orange, yellow, or red flowers and grows on well-drained sites. It’s not easy to transplant this plant from established stands because of its fleshy taproot, but it can be grown from seed or from root cuttings.

Other milkweeds that one might find on well-drained sites in the Florida Parishes include clasping milkweed (A. amplexicaulis), pinewoods milkweed (A. humistrata), pineland milkweed (A. obovata), Michaux’s milkweed (A. michauxii), white milkweed (AKA redring milkweed, A. variegata), whorled milkweed (A. verticillata), green comet milkweed (AKA green-flowered milkweed, A. viridiflora), and green antelopehorn (AKA spider milkweed, A. viridis). Despite the name, common milkweed (A. syriaca), another milkweed of well-drained sites, is not widely distributed in Louisiana. It’s only reported from a few parishes in the northern part of the state.

On wetter sites, you might find aquatic milkweed (A. perennis) or fewflower milkweed (A. lanceolata). Longleaf milkweed (A. longifolia) may be found on both well-drained and moist sites.

Swamp milkweed (AKA rose milkweed, A. incarnata) is a pink- to white-flowered species that’s found on wet sites in other parts of Louisiana. While it’s not likely to be found growing naturally in the Florida Parishes, this is one of the milkweeds that’s more readily available for purchase as plants or seeds.

Besides tropical milkweed, two other non-native milkweeds are balloon plant (Gomphocarpus physocarpus) and giant milkweed (Calotropis gigantea). Both species grow quite large and can be grown as annuals in Louisiana. Giant milkweed sometimes survives the winter in southern Louisiana.

Let me know if you have questions.

Click here for previous LSU AgCenter's Weekly Messages

Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).

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