Maybe, instead of an extra layer, a site map that lists related forums together.
Like:
gardening
plants
regional gardening
not exactly gardening
A lot like this - I think i'll suggest it to Dave:
http://garden.org/forums/
My theory is that, as the number of forums grows they will become more focused with fewer "live" threads each.
For example, "seeds" is too broad for my taste, but I tend to be a 'splotter" rather than a "lumper".
This would be much too sub-divided:
Seed Trading
Seed Collecting
Ninja Seed Snatching
Polite Seed Snatching
Seed Cleaning
Seed Storing
Mailing Seeds
Buying from Seed Vendors
Becoming a Seed Vendor
Seed Identification
Breaking Dormancy (Stratification)
Winter Sowing
Seed Germination Indoors
Direct Sowing Annuals
Direct Sowing Biennials
Direct Sowing Perennials
Direct Sowing Tropical Perennials
Finding a middle road will presumably be an ongoing journey, like dividing and pruning a perennial.
Corey