I've scanned through this thread, and just have one other thing to suggest (and if someone has already mentioned it and I missed it, please forgive me!)...
Instead of treating the garden as an Area of Focus (20,000 feet); or as a Someday/Maybe, have you thought of treating it as a 30,000 foot Goal? While this would strictly be something you intend to accomplish in 1 to 1 1/2 years, I find it a useful distinction. It keeps things a little more active than putting "Prepare Garden" on a Someday/Maybe list, because to me, if something's on S/M, I don't have active projects associated with it. However, you can have subprojects that are both active and S/M associated with a goal (YMMV)!
30k GOAL: Prepare Garden
S/M: Buy new edging
Project: Prepare Soil