I found when I went to Granite Falls in early winter for two days the first day there were some plants just turning (like the chamomile in the Hermit's garden showed several faint flowers but would not allow harvest or getting a shoot), and by the next day the same plant was only s tiny snow mound. You should still be able to find/interact with the winter or all-season plants that are mature enough to harvest from or take a shoot. Check this page for a list of winter plants:
carls-sims-4-guide.com/skills/gardening/ I was able to find both morel (reg and false) mushrooms, poison fireleaf, muckleberry and huckleberry during that trip (there were other plants also, but I already had them from other worlds &/or the seed packs, so I didn't collect those). ...Plus I found a LOT of insects during that (winter). (Blue Morpho Butterfly, Walking Stick, Luna Moth, Lady Bug, Dragon Fly, Mosquito, Jewel Beetle, Will-o-the-Wisp, Katydid - most of these multiple times - and an Eggplant Frog. Some you find at night.
carls-sims-4-guide.com/collections/insects.php Go to the big forest - and into the hermit cave/area. So don't lose heart, there is still a lot there in winter. Not sure if it would be different if you had 'blizzards' enabled, as you would get more snow then maybe? I'm waiting to go back in the summer next to get other plants and insects that I didn't find there. It helps to bring a few other sims that you can call from the cabin to hunt for the items when foraging. (I look, then I distribute who I send to fetch the items.)
The best way to get the flowers is the seed packs, though there are many around the worlds. This is a great list of where to look, though they will not be mature in worlds you haven't played, so you may need to play in the world for a bit:
carls-sims-4-guide.com/skills/gardening/plant-locations.php Happy hunting!
