Thursday, October 10, 2019

Help the Monarch Migration - Cut Back Tropical Milkweed October through February in Central Texas

Monarch on Gregg's Mistflower. Photo by Sheryl Williams

Monarchs are a beloved insect and the news of their declining numbers have caused alarm. Many homeowners have planted milkweed in an effort to supply larval food for the caterpillars. These good intentions mean that retail nurseries have rushed plant supplies to meet demand.

Unfortunately, the easiest milkweed to propagate for the retail nursery trade is the tropical variety, Asclepias curassavica. While it does serve as a host for the caterpillar, planted outside of its native Mexican range it can disrupt the Monarch migration. According to Monarch Joint Venture, in parts of the U.S. that do not have winter freezes, the year-round presence of tropical milkweed allows monarchs to breed throughout the winter. This is a problem because winter larvae are more likely than migratory Monarchs to become infected with the debilitating parasite Ophryocystis elektroscirrha (OE). The infection spreads by spores from infected adults to the eggs and milkweed plant. In parts of the south, like Central Texas, it is possible for some newly hatched butterflies to reach the overwintering sites and spread the disease to the migratory population.

Therefore, Monarch Joint Venture recommends that tropical milkweed (Asclepias curassavica) be cut back in the winter and fall months in the southern U.S. and California, and homeowners should consider gradually replacing them with native milkweeds as they become available. The native milkweeds usually die back on their own and don’t pose a threat to the migration. If the native varieties are not available, it’s okay to plant Tropical Milkweed, just know that it takes more careful maintenance and should be cut back October through February.

You can help Monarchs reach their winter destination by doing the following:

If you have tropical milkweed in your garden, cut it back October 1 to about 6 inches in height. Keep cutting it back through February as leaves re-sprout. It will die completely back if exposed to freezing temperatures and will likely sprout again in the spring. 

If possible, plant native milkweed. For the Austin area, the Ladybird Johnson Wildflower Center recommends the following:
            It’s also important to have a nectar source for the traveling butterflies. Some examples of native bloomers are Gregg’s mistflower (Conoclinium greggii), Shrubby boneset (Ageratina havanensis), and Fall aster (Symphyotrichum oblongifolium)

            You can check on the Monarch migration to Mexico on the Journey North database.