In my experience, water works great, but only for a few days at most. And you also have the issue with needing long stems--if it's that bushy, any leaves under water will get yucky.

Personally, I would make the basil/oil/garlic part of the pesto now and freeze that. Garlic is cheap, and you can save the spendier pine nuts and parm until you're ready. I do that at times anyway, as the nuts/cheese pick up freezer taste. But, since I've taken to freezing my pesto in 4oz canning jars, they seal well enough to avoid picking up freezer odor.