Mullein is such a wonderful medicinal plant to have around. It's been said to have magical powers! Like, planting it next to your home will keep lightening from striking...ha! OK enough of my folk lore.
You should harvest their leaves in their first season and the flowers as they open. Pluck each flower from its calyx,(the small, green enclosure located at the base of the flower). Spread the leaves and the flowers on a screen to dry in a well-ventilated spot, away from any moisture. If the deliecate flowers are dried improperly, they will spoil and mildew. Once dried, seal the flowers in airtight jars and the leaves in plastic bags, and store them away from direct heat and light.
The leaves and flowers can be harvested from late spring to late summer.