Even in states with medical or recreational weed, the grow laws are a crazy hodgepodge. Last I knew, in California you can grow 6 plants, but there's no limit to how large they can be. So some growers manage to grow marijuana trees!
If you have a medical need recommendation from a doc, you can grow up to 99 plants. You just have to claim you need that many. And of course, state narcs can arrest you if you sell it without a "weed farm license". But you can give it away an ounce at a time. Transporting more than an ounce is still illegal, unless you have that farmer license.
Medical users can register with the state, and get a handy little laminated card to show the cops. But you don't need to register, and doing so might generate some legal problems for you. Like prohibited occupations, owning guns, etc.