If your petunias are green and healthy but short on flowers, bone meal can help. It’s a slow-release source of phosphorus and calcium, and phosphorus is the nutrient most linked to blooming. Mix a small amount, roughly a tablespoon for a medium pot, into the top layer of soil and water well. It takes weeks to break down, so apply it once at the start of the season and again partway through. Keep it away from pets, which are often drawn to the smell, and use it sparingly, since too much can build up in the soil.