Is Tropical Milkweed Bad For Your Butterflies? What You Can Do

Tropical milkweed is a harmful plant that can trick both humans and monarch butterflies, and is contributing to declining monarch populations.

Bright flowers of a tropical milkweed plant
(Image credit: Jacky Parker Photography / Getty Images)

The plight of monarch butterflies is well known. The population of the monarch’s host plant, native milkweed, is diminishing, which makes it difficult for the butterflies to survive. As a result, many gardeners have been planting milkweed to try to help the monarchs. But one type of milkweed that has been advertised as being good for monarchs - tropical milkweed - is in fact a problem for these butterflies.

Before you start planting tropical milkweed seedlings, you need to understand the ways in which this plant upsets the migration of the species.

What's the Difference between Tropical vs. Regular Milkweed?

Not every milkweed species is exactly the same. Tropical milkweed versus native milkweed – exactly what are the differences?

Tropical milkweed (Asclepias curassavica) is an attractive plant, with blossoms and gorgeous green foliage. In mild climates, tropical milkweed doesn’t go dormant. It thrives in spring, summer, fall and winter.

Native milkweed plants, on the other hand, die back after they bloom. They also have flowers and lush leaves, but once winter arrives, they go dormant. This difference proves critical when we consider the relationship of the plants to the butterflies.

The Benefits of Growing Tropical Milkweed in Your Garden

Tropical milkweed is not only an attractive plant, but it is so easy to grow. Compare this to native milkweed which is difficult to propagate. It requires cold stratification of seeds and doesn’t constantly germinate. The tropical variety has impressive blossoms that ornament the garden from June through November, putting on a fabulous display. Tropical milkweed plant care is minimal, and, in warmer areas, it either thrives all year long or returns every year as a perennial.

With all of these benefits, the tropical milkweed plant makes a very good impression on gardeners who want to assist the monarch. You may wonder why you should plant the more difficult native milkweed when this tropical variety is both prettier and easier to take care of. Here’s why.

Why Tropical Milkweed is Bad for Monarch Butterflies

But despite all this, tropical milkweed is bad for the monarch population. It interferes with monarch migration and reproduction in several ways.

First, in the north, tropical milkweed grows at the same time of the year as native milkweed. This can confuse the butterflies and make them believe that it is breeding time rather than migration time. For example, in Minnesota, the presence of tropical milkweed prevents monarchs from starting on their normal fall migration to Mexico. While it takes several generations of monarchs to fly north from Mexico, these butterflies fly all the way back to Mexico in one generation. Because this flight requires so much energy, the adults pass into reproductive diapause, meaning that they do not mate. But if the monarchs encounter tropical milkweed, they can break out of their diapause, lay eggs, and stay in the area. When a cold snap comes, the butterflies, caterpillars and eggs will die.

Another problem occurs in mild weather areas where tropical milkweed survives the winters. Monarchs that visit the plants deposit a protozoan parasite called Ophryocystis elektroscirrha (“OE”) on the milkweed leaves. Monarch caterpillars hatching out on the plant eat the leaves together with the OE. Over time, the levels of OE become toxic, which is not true for native milkweed. Although monarchs deposit OE on native milkweed as well, the plant always dies back after flowering, killing off the OE every year. As climate change continues to warm the plant, tropical milkweed may successfully overwinter in more and more locations, enlarging the scope of the OE issue.

Is it Possible to Grow Tropical Milkweed Safely?

There may be safe ways to grow tropical milkweed. This plant can be safely grown in colder-winter areas, since it will die back with the first frost. In warmer areas, gardeners might try growing it in greenhouses. But as the world climate continues to warm, the better, safer option is simply not to plant it and plant native species instead.

Wildlife-Friendly Alternatives to Tropical Milkweed

If you love the look of tropical milkweed but have been convinced not to plant it, don’t worry, there are wildlife-friendly alternatives. If you want a native species that is drought-tolerant, try yarrow (Achillea millifolium). It has lovely yellow, red, pink, or white blossoms. The Zinnia (Zinnia elegans) is another option. It is an annual and there are many cultivars in different colors. Another option is tickseed (Coreopsis), a native perennial with gold and red flowers.

Frequently Asked Questions

Is tropical milkweed invasive?

Yes, tropical milkweed is invasive. It escapes cultivation and establishes in the wild.

Is tropical milkweed a perennial?

Tropical milkweed is a perennial plant in USDA zones 8 - 11, growing as an evergreen in many warm areas. It is an annual in colder zones.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.