It looks like the back-end doesn't check energy levels before letting the action, such as hoeing, watering, harvesting, happen.
Even when my energy hit zero I could carry on picking my rice and potatoes. This is with the game client closed, and just the garden open in the browser.
If the client was still open I expect I'd get sent to hell thus moving me away from the garden and therefore preventing any more gardening actions. An interesting loophole.