For your own safety and piece of mind, I'd run 10/3 copper wire in pipe. Now you can add a small distribution box with 2 distinct circuits. As long as your yard is secure, you could fish the 10/3 through pipe conduit or even inside an old garden hose, just burry it 4-6 inches below ground. This will protect from rocks and rodents. You can always pull it up if you change plans later and save the wire. This connects to your main service panel. 2 breakers @ 20Amps, one for the red wire and one for the black wire. The white wire will be the neutral, and bare wire is ground. That will be plenty of power, enough to add a few extra outlets outside of grow for yard work, radios etc. Good luck...