I don't think you have to worry. I live in an area of heavy alkaline clay soil and my A. incarnata does just fine. Many of these plants are "weed" plants that grow just about anywhere. Unless you have one of the rarer species I wouldn't worry about it. Just be sure of where you are going to plant them because they don't like to be moved.
Cheers,
Elisabeth

Ditto what Bananas said. They'll grow pretty much anywhere.

Martha

A. tuberosa can be difficult to get established. It will grow and thrive in clay soil that has been amended with some compost. But it will not do well in a location that gets wet often, and tends to stay wet. It should be located in a sunny spot. If there is a drought, its OK to water this plant, but once a week is generally sufficient. It has a really deep tap root.

I also have A. tuberosa all over the place; I did start it in a sandy bed, but it has volunteered itself in loamy beds with lots of compost. All areas it's in have good drainage though and lots of sun.

The A.incarnata likes damp soil--it generally a wetland plant, but it does just fine in a typical garden bed too. It too has volunteered itself in all of my beds, and does fine in a hot, dry bed.

Sounds like your plants will be fine. Have fun with your new flower bed and with butterfly gardening. Hope you get lots of Monarchs your way.

I grow both. My yard is heavy clay soil. I have to water the Swamp milkweed (Asclepias incarnata) daily from late June thru August to keep it blooming. The Butterflyweed (Asclepias tuberosa) needs regular watered about once a week to keep it growing or it goes stagnant. If I had better soil structure I don't think I would have to water nearly as much.

I've grown both in my raised beds, so I don't think they are particular to soil type.