Does anyone know if winter will kill off the garden plants?

Hmm, well in TS2, their health would deteriorate unless you built a greenhouse around them. Of course, produce gardening was introduced in Seasons too, so it's not like you needed to build a greenhouse around pre-existing gardens. In TS3 they just go dormant, but greenhouses were separate from Seasons.

My money's on plants going dormant during the winter. I feel like this would upset fewer simmers who've grown elaborate gardens pre-Seasons.