I got thinking the other day, when we level up our characters we refill our life, mana and energy. But when we get to max level we have to wait or by energy potions to refill potions. I was thinking that when we level up in gardening it also refills our energy.
I'm not completely sure about this, but I think when you level up in gardening your energy refills. Since it doesn't have anything to do with your actual level as a wizard, It wouldn't affect your energy level.
Have you used the free energy elixer they've given out yet? It restores your energy to full. :)