We've been talking about protecting the one without buds to see if it can make it through a winter, so it's good to know that the efforts would be worthwhile.

Some plants just die off anyway when the short daylight/low temperatures get too much for them. My basil is starting to go now (I need to do that squash with barley and basil oil recipe, which requires ridiculously large quantities of basil leaves!) and nothing I could do will keep it going through the winter. Indoors basil dies back too.

Cosmos may well just rot away from the root in cold, damp soil. But you've got nothing to lose by trying...