Energy costs for cleaning/watering plots and planting seeds is only reduced after learning Croppery II. Croppery III doesn't reduce these energy costs further.
Adding skill descriptions for your convenience:
Croppery I: "Anyone who possesses a Garden in their back yard would be wise to learn Croppery because anyone who knows Croppery will yield twice as much crop product from a plot (along with benefits to mood and XP)."
Croppery II: "Advancing your Croppery skill to the second level means you'll use less energy planting, watering and harvesting your crops. Every so often you will have a 'super harvest', yielding double the amount of crops from a plot, and once in a Blue Moon, the land gives you a little super fun happy bonus present."
Croppery III: "Croppery III makes you a veritable master of the garden, further reducing the energy costs tending crops through their lifecycle and giving the highest chance of surprise bonuses (or bonus surprises). 'Super harvest' at this level yields triple the amount of crops from a plot.