Enjoying the comments here.
If you’re ever in Arizona, Butterfly Wonderland in Scottsdale is a must see. Prior to entering the massive butterfly pavilion, there’s a 3D film about the life cycle and migration of Monarchs.
For all the plants mentioned by others here, Monarchs can see them from very far away even if it’s a tiny patch in a sea of otherwise unfriendly territory. The most interesting thing to me was how the migration works across three generations, and the sizes of Monarchs are very different depending on the migration task ahead.