Travis,
A plant data overhaul is already in the works that will improve your custom columns.
You can, however, add those in yourself by customizing the plant size preference columns. If none of the plants placed in a project have a "size" set for a particular column, that column won't appear in a schedule. So really you can do that right now.
Plant Sizes Preferences
You can already reorganize groups in the plant preferences. Click on the Plant Groups button in Planting Preferences. You can add, edit, and delete groups from there to add your own "Evergreen" Group (although our default preference set does already have an "Evergreen Trees" Group included).
Plant Groups (Subcategories)
If you're thinking about something else, feel free to clarify further (it might be best to make a new forum post about it, though, since it's separate from this post's topic).
-Amanda