You usually only need to apply fertilizer once or twice per year, especially when you use slow-release fertilizer. Never apply fertilizer when you expect heavy rainfall. Why? Most fertilizer will be flushed away, and that's money and pollution down the drain. Be sure to apply fertilizer evenly and avoid overlapping. Clean up any spills. Put a plastic bag or tarp on the ground where you pour out the fertilizer, so any spills can be caught and used.