The best time to fertilize most permanent plants is March and April. Lawns also need a late summer or fall application. I also fertilize anytime I plant new plants. Although there are many specialized fertilizers available, I use two types for almost all my outdoor plants. I use lawn fertilizer for my lawn, trees, and shrubs, and a general purpose fertilizer for flowers and vegetables.
With the change in Washington fertilizer law, lawn fertilizers no longer contain phosphorus, the middle nutrient element. Typical lawn fertilizer formulas have about twice as much nitrogen as potassium. A typical balance of nitrogen-phosphorus-potassium would be 15-0-10 or 20-0-10. Organic fertilizers will have a lower percentage with about half as much fertilizer. It is also important to have a small percentage of iron (1 to 3 per cent or less is enough).
Since nitrogen is the most soluble of nutrients, it is also desirable if at least part of the nitrogen in lawn fertilizers is of the slow release or timed release type. This is often accomplished by coating or encapsulating the nitrogen pellets. This allows the nitrogen to be released over a longer period, reducing the need for frequent applications.
Although many tree and shrub fertilizers have different nutrient balances, I have found that lawn fertilizer works equally well, and sometimes better.
General purpose fertilizers usually contain about a 1-1-1 ratio of nitrogen- phosphorus-potassium. Typical is 12-12-12 or 16-16-16, with organic fertilizers more like 6-6-6. These ratios do not have to be exact, just so there are approximately equal amounts of the 3 major fertilizer nutrients. Micro-nutrients such as iron, zinc, and manganese are also beneficial. I have found that general purpose fertilizers work equally well as flower and vegetable fertilizers.
General purpose fertilizers are also available in a coated or time-release type. I use them for all my containers and sometimes in flower beds. I generally prefer granular over liquid fertilizers because they are cheaper and longer lasting.