Guys here keep 3 medium boxes for brood first, and in near the end yield period they limit laying to 2 mediums. So bees over winter with 2 mediums. That is a way they can harvest all honey off. After final harvesting they start to feed hives.
In spring they put a third box back with foundations. It reduces early swarming.
That is one style...
Wet winter...if there are extra space in beehive, the winter food sucks moisture and frames are easy to take mold. Dense beecover keeps the hive warm and keep the stores dry. (relative moisture)
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