The past couple years even local native MW in Texas did not die back for many folks I know. In Southern Florida many Monarchs are year round residents. There is also a growing body of evidence that "Tropical" milkweed is in fact native to Texas and much of the Southern portion of the US. Many plants do not realize there are borders.
Anyone worried about Monarchs not migrating can cut ALL Milkweed to the the ground in late Fall. Pity the poor Monarchs who for whatever reason choose not to migrate.
