Fertilizer is an item used to fertilize plants in the Zen Garden, making a plant grow bigger by one stage. It can be bought at Crazy Dave's Twiddydinkies and is available when the player unlocks the Zen Garden at the end of Level 5-4 in Adventure Mode. It costs $750 and is purchased in sets of five. The player can have only up to 20 at a time. The player's young Zen Garden plants will ask for them three times after three to six waterings so that they can be mature. A single plant needs three fertilizations until it is fully grown.
While watering plants (when needed) gives the player silver coins, using fertilizer on their plants (when asked) will give them one or two gold coins (three gold coins in the Mushroom Garden or Aquatic Garden) or two diamonds (one for Marigolds). The player will know when their plant needs fertilizer when it has a little callout with a picture of it. After fertilizing, the plant will not do anything for about half an hour or so until they need to be watered again.