Planting defaults

This fixes the problem reported by Jo in https://www.pivotaltracker.com/story/show/43277457

I also decided to sort gardens alphabetically (this came up when writing the test, and seemed like a good idea generally).  I asked for opinions on the list but everyone's asleep.  Maia (who is sitting at my kitchen table) agrees with me that alphabetical makes more sense, for what that's worth.