Everyone loves delicious food, and your crops are not an exception. Feed them by using fertilizer, and they will thank you with instant growth. Tap garden beds with crops and then drag the fertilizer icon to the garden beds  to harvest these crops immediately. You can get fertilizers as a reward for some quests; you can also make it in the Salt Crusher (you can win the Salt Crusher in Saltvic Mines).
Once the Crusher has produced 100 packs of fertilizers, it vanishes.