I managed to win, but only through a method of going directly for the lighthouse and ignoring the garden completely. When playing the game the optimal way by recharging only at 1 bar, feeding the lighthouse only when it doesn't waste excess energy, it is impossible to win.
I think with a bit of rebalancing to the plague mechanic (it comes up too often and makes you run out of energy dealing with it!) it will be perfect.
I really love this game though! Everything about it is cute- aside from the sadness that comes with things dying.