When artist Shawn Hall took a close look at the flowering plants in Crescent Park, she noticed something that made her curious.

“I was looking at the milkweed and there was a monarch caterpillar on it, even though it was the dead of winter,” Hall said. “It didn’t seem right to me, so I began studying the phenomenon and learned I was far from the first to realize that something wasn’t right.”

What wasn’t right, as Hall put it, was the fact that the monarch butterflies were flitting about on the banks of the Mississippi River in Bywater in January, rather than hibernating in central Mexico. As she began to research the matter, she learned a startling fact: The number of monarchs wintering in Mexico has plunged 95 percent since 1996, giving rise to a nationwide effort to “save the monarch.”

Most efforts to regenerate the monarch population — decimated by the eradication of milkweed, the only food monarch caterpillars eat — have encouraged gardeners to plant milkweed. Crescent Park has 300 milkweed plants, according to Hall, but that isn’t all good news.

“What I learned is that it matters what kind of milkweed you plant,” she said. “Tropical milkweed can be a problem because it doesn’t die back until there is a hard freeze, so it encourages monarchs to stay in the Gulf South rather than continue on to Mexico.”

Tropical milkweed (Asclepias curassavica) is now considered a villain by some monarch supporters, largely because it interrupts the natural migration process and because it is believed to spread OE, a protozoan parasite that causes monarch deformities and weakens them.

An article in Science Magazine stated that “monarchs who stayed in the southern United States for the winter were five to nine times more likely to be infected with OE than migrating butterflies were.” Tropical milkweed seems to be the culprit in enticing the monarchs to say put.

The answer may be to plant native milkweed instead of the tropical variety, but that may be easier said than done. Linda Barber Auld, known as “The Bug Lady” for her educational programs designed for schoolchildren, said it’s hard to find native species at nurseries and garden centers.

“For years, all they sold was tropical milkweed,” she said. “I’ve been working with them to get them to start selling native plants and native seeds. But natives are challenging to propagate from seed because they have to be cold stratified (a process necessary for germination), which you don’t have to do with tropical milkweed. And the native plants grow very slowly.”

Nonetheless, Auld’s “Project Monarch” plans to distribute seeds of five species of native milkweeds to garden centers between New Orleans and Lafayette. Her goal is to get the seeds in the hands of every nursery in the state, she said.

Before the controversy arose about tropical vs. native milkweed, her project was responsible for disseminating 120,000 tropical milkweed seeds.

Kathy McNamara, horticultural manager at the New Orleans Botanical Garden, is ambivalent about promoting native milkweed over tropical.

“The whole native versus tropical milkweed seems to have polarized monarch supporters as much as democrats versus republicans. Seriously. The more reasoned ones admit that a lot more research needs to be done,” she commented. “We’ll continue to grow and offer tropical milkweed at our (Pelican Greenhouse) sales. We’ll grow natives and offer them but … they don’t seem to do that well so I don’t want to promote them as what people need to grow here.”

Hall said that righting the problem of monarchs staying put in the winter and becoming infected with OE is as simple as reading the label on any milkweed plant you plan to purchase for the garden and making sure it’s one of the native varieties. But what if you have a thriving butterfly garden filled with tropical milkweed? Do you remove it?

“From what I have learned, you don’t have to as long as you keep it cut back to about 6 inches in the winter,” Hall said. “Keeping it cut back encourages it to stay healthy.”

Auld, who has been raising butterflies since 1983, doesn’t believe that tropical milkweed is as harmful as some others if it is properly managed.

“By all means, plant natives when you can but tropical if you can’t,” she said. “If you plant it, they will come.”

A final piece of advice: Plant a minimum of six milkweed plants — whatever the variety — to ensure an ample food supply for the very hungry caterpillars that the milkweed will need to feed.