Milkweeds are Different Underground: The Difference Between Milkweed Plants
Here is a great write-up from Wild Ones that focuses on the most popular milkweed plants in our area (most popular with us as gardeners/habitat stewards and with the Monarchs :):
Milkweeds are different underground
Different milkweeds (Asclepias) have slightly different flowers, though they have many similarities. But there are more prominent differences below ground. These differences aren’t visible to you (or to the monarch butterflies), but they do make a difference in how you select and maintain your milkweeds.
Swamp milkweed’s (A. incarnata) roots are the most conventional looking; they simply radiate out from the center as a clump. Additional stalks the plant develops as it matures are connected to roots, so it’s possible to divide the plant in a conventional way. They will often reseed, too, but in a modest way.
Butterflyweeds (A. tuberosa) have taproots, which helps them thrive in dry, poor soil. Taproots cannot be divided! They will occasionally reseed, but generally not as often as we’d like.
Common milkweed (A. syriaca), on the other hand, spreads underground by rhizomes. That’s why you’ll see plants popping up here and there, forming a colony. Although this may not be convenient in a formal garden, they can be accommodated if you reserve part of your landscape for more natural landscaping, perhaps pulling up extra stalks. And they’re a monarch butterfly favorite!