In the eco-parking mini-game, you receive fertilizers as a prize. You can then use it on the farm side to speed up the ripening time. 

To do that, tap on the slot with a plant and choose a green bag a little lower. Then just drag it to the plant you need to speed up. One bag lows the time for 10 minutes.

Pay attention: this action doesn’t need confirmation and while dragging on the other plants, the bags of fertilizers will be used. This action is also irreversible.