Would it be useful to have a fall/winter gardening tag? We've got a handful of questions that this would fit.
How about just off-season instead. This would be for anything related to off season gardening and planting. It could also relate to non-growing season lawn maintenance and some other useful things.
I would prefer bstpierre's suggestion of having fall-gardening, winter-gardening in addition to wax eagle's off-season. That way, a question on planting kale/carrots could be tagged just fall-gardening (because that's the season for them) and if you're planning on just growing plants off-season (e.g., perennials which would've taken off great if planted in spring, but too bad you forgot and now want to plant them in the fall) you could use both tags.
They shouldn't be synonyms of each other either, because I think they represent different concepts.