You'd most likely be wasting them if you move them. If you do try to get some from the field, I would look for the smaller ones and then only get two or three to see how it goes (assuming you have permission) and wait until the weather cools off.
I don't know if they root well in water but with butterfly weed, I think you'd be better off in the long run, have stronger plants, if you just start them from seed. Would you be able to collect seeds from the nearby field or will it be mowed?

Butterfly weed is one that germinates well using the "winter sowing" method, and even though mature plants transplanted from a field tend to die, seedlings transplant pretty good.
To winter sow means to start the seeds outside in containers with clear vented lids. I like to use those big foil lasagna pans that come with clear plastic lids. Some people use milk jugs. If you want to know more about that, there's plenty of info in the FAQ section of the Winter Sowing forum.

I started some with winter sowing just this year. They are growing marvelously. I would strongly suggest trying that.

Any of the milkweeds are very tough to move unless very young - short roots.

FataMorgana

And I germinated many plants this Spring without messing with WS. Thirty days in the fridge is all it takes.

I have successfully grown A. tuberosa from seed in peat pots which negates the transplant issue since you plant the pot and all, the root is not disturbed. I like to plant a trap crop of dill somewhere nearby to distract butterfly larvae that enjoy feeding on butterfly weed.

kalia, caterpillars that eat Asclepias (monarch) can't substitute dill.

They grow well from seed. That is the cheapest and easiest way to get this plant. You could even harvest the seed from the ones behind your house for free.

The 2nd best method is root cuttings. The taproot is literally as big as a carrot.

Interesting purpleinopp, thanks for your comment. I know these are different plant families and so understand your comment....but each year I plant a trap crop of annual Apiaceae family plants and the same caterpillars that munch my butterfly weed are all over them!

The larvae of black swallowtails look similar to those of monarchs and prefer to feed on members of the parsley family.

I have seed for A. tuberose if you'd like to try growing it from seed. You could try starting some now using the "summer sowing" method, which is just like wintersowing, only done during the summer months. Once the cooler fall temperatures arrive, you can transplant seedlings of hardy native perennials into the garden and they'll settle in to resprout in the spring. Or, you could save the seed for wintersowing or direct sowing this fall.

Incidentally, I've transplanted A. tuberosa successfully, but I've always done it in early spring, just as it is beginning to poke up from under ground. And I did take a pretty generous root ball, and the plants were usually only one or two years old.

Hope this helps. If you want seed, e-mail me.

Martha