Xena does not go outside very often anymore. I have a lot of flowerbed square footage. A couple spots are designated for nip. Like any member of the mint family, it reseeds freely and finds it's way around. I usually allow the volunteers to reach flowering before uprooting and hanging to dry. In places I desire it, I cut back at flowering. Xena really prefers the fresh flowertops. I use the dried material to restuff catnip toys and give away the surplus. There are a couple neighborhood outside country toms who investigate the plantings and they are welcomed to help themselves, but they usually just mark the area.
I also use a little of the catnip for myself. The main way I use it is as part of one of my jerky marinade recipes. It can also serve as a substitute for various other herbs. Experiment and you'll be surprised. Use in subtle quantities and you just might purr a little.