The popularity of tropical milkweed, also known as Mexican Milkweed or Scarlet Milkweed (Asclepias curassavica), a non-native species of milkweed, has skyrocketed due to its ease of propagation and attractive flowers and foliage. However, when planted in areas with temperate climates, it doesn't die back in the winter, which can lead to problems with monarch migration. They tend to stay where the food source is and won't travel.
Monarch butterflies that visit the plant can carry a parasite called Ophryocystis elektroscirrha, or OE, which can be deposited on the plant's leaves. As caterpillars hatch and feed on the plant, they ingest the OE, which can have harmful effects on monarchs' migration success, body mass, lifespan, mating success, and flight ability.
As stated on Pollinator.org, unlike native milkweeds, which die back after blooming and take the parasite with them, tropical milkweeds remain evergreen throughout the winter, allowing OE levels to accumulate on the plant. This can result in successive generations of monarch caterpillars being exposed to dangerous levels of OE.
We have heard it said that Tropical/Mexican Milkweed is ok as long as you cut it down in the winter. And that is correct. But the problem comes when it's not cut back or contained and it spreads. If you don't cut it back, or when your neighbors don't cut it back, or when a future homeowner doesn't know and doesn't cut it back that's when the problems occur so it's best not to compound a problem.
If it gets into an area where it naturalizes then it can't be cut back. Then you've just created a problem and it's best not to plant it in the first place. It is recommended to use native milkweed instead of tropical. Asclepias incarnata, Asclepias perennis and Asclepias tuberosa are all native milkweeds.
Purchasing Milkweed
If you are not buying from Blackcreek Nursery, make sure you are purchasing the native varieties. Native Milkweed is always the best option. At Blackcreek Nursery and Garden we will ONLY grow and sell the native milkweed suitable for our area. Cultivars you will want to ask for are Asclepias incarnata, Asclepias perennis, or Asclepias tuberosa.
References & Resources
Georgia Field Guide for Milkweed
Monarchs & Milkweed for Georgia Gardens (State Botanical Gardens Georgia)