You are correct in that Osmocote works best in the spring as the weather warms but your spring blooming bulbs do not need fertilizing over the winter. The flower is already formed inside the bulbs. The time to feed is after the flower blooms as the yellowing foliage is making next year's flower. If you are not going to replant the bulbs then there is no need to fertilize them. Bulbs in containers are most often enjoyed as annuals which means they bloom in the pot then are discarded after they flower. Then they are replanted with fresh bulbs annually. In this case they need no fertilizer. Keep growing, Marianne Binetti