Frostweed
Named for the ice crystals that form on its stems when frozen, frostweed (Verbesina virginica) is a major fall-blooming nectar source for monarchs. By fall it's usually at least three feet high and has dense clusters of snow-white flowers. Blooming typically coincides with peak migration.

"Today I saw more Monarchs than ever before. I would estimate a good 300+ butterflies, although they were flitting about and impossible to count. They are very keen on the Frostweed we have in abundance on our 50 acres."