Q: I planted seeds of what I thought was a milkweed (Asclepias). The plants look somewhat like milkweed, but they are close to 4 feet tall with no sign of flower buds to confirm their identity. There are leaf buds at the axils, which I don’t see on other milkweeds. What is this plant? I would like to have milkweed plants for Monarch butterflies.
A: What you have here is not a milkweed. It is Tall White Aster, Symphyotrichum lanceolatum, and the good news is, it is actually what the Monarchs need more at certain times of the year than Asclepias. More on that in a minute, but first, a few notes and a caution about planting milkweeds.
As many people know, milkweeds are essential host plants for Monarch butterfly caterpillars. We commend people for adding milkweeds in their gardens to support butterfly conservation!
Twelve species of Asclepias are native to (all or part of) Maryland. There are other types of milkweeds you may find in garden centers or by way of purchased of seeds. One type you may see recommended for Monarchs is Tropical Milkweed, Asclepias curassavica. It is relatively easy to propagate and blooms late into the fall, so it is promoted as beneficial to migrating Monarchs.
However, we do not recommend planting A. curassavica, and neither does the Xerces Society. What the Monarchs need while they are here in Maryland is the milkweed that is native here. It provides what they need during the appropriate phase of their migration. A. curassavica is not native to most of North America.
The primary concern is not that a few butterflies will stay here too long in the fall because the A. curassavica stays green for so long, or that a few monarchs would get a parasite (OE disease) by aggregating around this particular species. The concern is that these are alien milkweeds, and no one has tested them for impact on our native milkweed populations.
If A. curassavica could hybridize with our natives, or if its seeds or plants carry a plant pathogen, there is a small but distinct possibility of irreversible harm to our native milkweed populations. If something bad happened to our native milkweeds, it could be devastating to all the Monarchs that migrate through the eastern United States. So we encourage people to hold off on the alien milkweeds. Asclepias tuberosa, A. syriaca, and A. incarnata are better choices.
Now, about that aster! Recent studies suggest that the cause of Monarch decline is not a lack of milkweeds at all, but a shortage of goldenrods and asters to provide nectar to fuel the adult butterflies’ return journey to Mexico (Agrawal, 2017). Tall White Aster can be a bit aggressive, but not awful. It spreads by short rhizomes. It is tolerant of compact soils and de-icing salts. If you would like to prevent these asters from reaching full height of about 6 feet, you can trim them back to about 3 feet in early July. Trimming some but not others prolongs the bloom season. And, it’s a monarch favorite!
By Sara Tangren, Ph. D., Sr. Agent Associate, Sustainable Horticulture and Native Plants, and Christa Carignan, Coordinator, University of Maryland Extension, Home & Garden Information Center
Monarch Butterfly Nectaring on Tall White Aster
References and Resources
- Monarch Butterfly Habitat Needs | United States Department of Agriculture Forest Service
- Monarchs and Milkweed | Anurag Agrawal, 2017 | Princeton University Press
- Recommended Native Plants for Maryland | University of Maryland Extension
- Butterfly Garden Plant List | Department of Environmental Protection, Montgomery County, Maryland
- Pollinator Plants: Mid-Atlantic Region | Xerces Society