The fact that seedlings can disappear is so broken. if you made the grow time longer, that would make the crops more scarce. Or make it where you have to grow it in the ground and use fertilizer to make it grow in the area. in order to continue growing after harvest, you have to "spread" some more fertilizer on the plants. You must also water the plants, or set up irrigation [trenches or pipes] a good use for small pipes. this may make the player need to set up the farm near water and put in a pump and pipe system. This would be a great addition to the game. If the player creates a "well or pool" the water gets used up and will need to be refilled at times. Basically one glass of water per plant per day would be reasonable. This would be a very good way to slow down the proliferation of crops. It would also attract zombies if powered. so you would need to have some defense on your farm.