Slow Release 16-9-12 Granular Bonsai Fertilizer. Apply once every 5-8 months. Fertilizing or feeding your Bonsai regularly is essential to keep your tree healthy and strong. Because Bonsai are planted in pots they need to fertilized in order to replenish the soil 's nutritional content. Instructions: Lift the moss of your bonsai and uniformly apply the fertilizer on the surface of the soil. Use approximately 2 heaping tablespoon per 10 inches of pot size.