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Sunday, October 6, 2019

Save Monarchs. Cut Back Tropical Milkweed Now.

I know the weather doesn't seem like it, but it IS fall here in the Austin area. According to the Journey North database, the Monarch winter migration to Mexico has started and should pass through here any day.

Monarchs are a beloved insect and much attention has been paid to their declining numbers. In an effort to help them, many homeowners have rushed to plant milkweed in an effort to supply larval food to their hungry hoard of caterpillars. These good intentions mean that retail nurseries have rushed to supply plants to meet demand.

Unfortunately, the easiest milkweed to propagate is the tropical variety, Asclepias curassavica. While it does serve as a host for the caterpillar, the trouble comes because it is planted outside of its native Mexican range and can disrupt the Monarch migration. According to Monarch Joint Venture, in parts of the U.S. that don't have winter freezes, the year-round presence of tropical milkweed allows monarchs to breed throughout the winter. "These year-round tropical milkweed patches foster greater transmission of the protozoan Ophryocystis elektroscirrha (OE), increasing the likelihood that monarchs become infected with the debilitating parasite. Therefore, we recommend that tropical milkweed (Asclepias curassavica) should be cut back in the winter and fall months in the southern U.S. and California, and should be gradually replaced with native milkweeds as they become available."

The trouble is not just from the parasite. Here in Austin the Monarchs face food shortages. Native milkweeds have gone dormant for the summer and will not start to grow until moisture returns. Caterpillars who have not pupated will either starve or die in the cold. Adults don't fair any better. Most native nectar producing plants have gone to seed, limiting the adult food supply. Adults are also in danger of dying from the cold since they have not reached their winter home.

So what is to be done? You can help by doing the following:

If you have tropical milkweed in your garden, cut it back now to about 6 inches in height and keep cutting it back every few weeks as leaves re-sprout. It will die completely back if we get a freeze and will sprout again in the spring. Going forward, plant only native milkweed. For the Austin area, the Ladybird Johnson Wildflower Center recommends the following:

Personally, I've had the most luck finding Asclepias asperula (antelope horns) at local nurseries. I've also ordered plants online from Monarch Watch.

Be patient with them. I've found them to be like bluebonnets - they don't want to be fussed over. I let them think they are a weed stealing a little moisture and growing in the soil between the pathway and raised vegetable beds. I remind myself that milkweed grows with abandon along MoPac and persists even with occasional mowing.

There are a lot of things in this world that we are powerless to change. This we can do.


  1. Must be a very cool experience to see the migration. We had a Painted Lady migration this year (only happens every 10 years or so in our area). Beautiful ephemeral creatures.