I bought a package of wildflower seeds for this past season, and was very pleased with the results. For next year, in case the plants reseeding themselves don't produce a thick flowerbed, I plucked a lot of the seed heads. Do most flower seeds need to overwinter in freezing temps before they'll sprout in the spring, or can I just keep them in a dry dark place before planting in spring? I've heard of some seeds needing to be kept over the winter in the freezer in order to germinate. Any help would be terrific.