Will it thrive in the flower box and if at the end of the season I bring the box into the garage for the winter, will it survive till next spring?
Butterfly milkweed is pretty durable, you might not need to bring it in. What growing zone are you in?
(PS: Butterly Milkweed is also known as Pleurisy Root, and is used as an antiviral. If you want to, you can grow the roots as a cash crop. Harvest in their 3rd year.)
I have a book called “Hedgemaids & Fairy Candles”, The Life & Lore of North American Wildflowers by Jack Sanders. I highly recommend it.
For milkweed, he says:
Milkweeds are slow spreading and have not been really serious field pests. Farmers though, dislike them because those roots are so hard to eradicate. Moreover, common milkweed has a mother plant, with the deepest roots, which sends out underground runners that sprout other plants nearby. Unless the mother root is killed, colonies cannot be prevented from starting and spreading.
Many people would just as soon have a patch of milkweed. They are handsome plants. Because of the deep roots, successful transplantation of mature plants is difficult. Attempt it only with small offspring of the mother plant in the spring. Better yet, grab a few pods in late August or September and plant the seeds in fairly dry soil that gets plenty of sunlight Seeds may be planted in the fall or spring.
Something else interesting from the book:
Recent research has found that many common milkweeds contain potent, often poisonous substances known as cardiac glycosides (digitalis from foxglove is a cardiac glycoside) It is probably these powerful cardiac glycosides that make monarchs unpalatable. Birds almost immediately become nauseous and vomit for up to a half hour after eating a monarch. Most birds will simply bypass the monarch(and the look-alike viceroy) in the future. Some crafty birds, however, will catch a butterfly and sample a bit of the wing to see if it tastes bad, letting it go if it is a monarch. In Mexico and Central America, where our monarchs spend the winter, there are birds such as grosbeaks and orioles that have learned which parts of the monarch contain the smallest doses of poison and eat only those parts.
There is one butterfly milkweed with a latex sap which can blind you if it accidently splashes into the eye. Be careful if you are tempted to use the blooms in cut flower arrangements.
I have zero experience with milkweed. I am sorry that I don’t have an answer to your question. Maybe someone else will pick up your question and be able to help.
TIK, do you know anything about this issue? Thanks