Examples:
Tractor: You can hitch any gardening machine to the tractor, which passes it over your yard.
Machines the tractor can pull:
Sower: Add seeds to the sower. In one pass it plants them in the right gardens in no particular order.
Sprinkler: Must be engineered (put together) with a watering can. Waters all gardens in a pass.
Duster: Add guano to the duster. Fertilizes all gardens once. Requires 2 passes for full guano effect.
Harvester: Harvests all sprouted crops/herbs. Player collects them from the harvester after the pass.
Plow: Cleans all dirty plots in a pass.
A machine uses one fuel unit for each plot during a pass (3 large gardens = 45 plots = 45 fuel)
The amount of fuel the tractor uses depends upon the size of your yard.
So that's my idea of how to implement multi-plot functionality, but really any method would be welcome. I just like this method because it would make gardening less boring and would make the fuel-making skill more valuable.