You can now create categories.
Go to your Lists control panel:
http://garden.org/lists/manage...
Then on the right side, click on "ยป Add categories" and off you go.
You can create as many categories as you like, edit and delete them, and change the order in which they will appear.
For now, that's all you can do. Soon I will add the code to let you file your plant list items into those categories.