As catnip is a member of the herb family, dry catnip using the same methods followed to dry other herbs. Harvest catnip for drying in the late morning or early afternoon to lessen the chance of the herb becoming moldy before it fully dries. Plant oils that cats love peak when the catnip plant flowers. Gardens suggests snipping each stem at the base of the plant to encourage rapid re-growth.
Set up a drying rack in a cool, dark spot. Spread the catnip across the drying rack, leaving about an inch or so of room between each stem to allow for plenty of air circulation. Allow the herbs to dry until a leaf easily crumbles when rubbed between the fingers. At this point, the catnip is ready to use or store.
Gardens also suggests an alternate method for air-drying herbs. Harvest the catnip in the same manner, and then tie the stems together in small bunches. Hang the bunches with the leaves and flowers facing down in a cool, dark area. According to CookGardenSew, a food dehydrator can also be used to dry catnip. Use an empty mesh sheet placed over the leaves to keep them flat in the dehydrator and prevent the leaves from blowing around.