Basic idea: a utility belt with a required Gardening level that gives additional sidebar slots.

Belt of the Master Gardener
Requires Gardening level 50
+5 sidebar slots

I think this would need to be balanced by some kind of additional mod that either prevents it from being used in combat, or a sufficiently negative mod that all but guarantees it won't be used in combat like a "+50% damage from all sources" mod.

This could fit into the set of class-specific belt Toolcrafting recipes.