When growing catnip from seed, you'll need to properly prepare the seeds. Catnip seeds are tough and need to be stratified or slightly damaged before they will sprout. This can be done by first placing the seeds in the freezer overnight and then placing the seeds in a bowl of water for 24 hours. This process will damage the seed coat and will make it much easier for the catnip seeds to sprout. After you have stratified the seeds, you can plant them indoors or outdoors. Thin them to one plant per 20 inches after the sprout.