If you’ve fertilized at planting time as recommended with a slow release fertilizer, you shouldn’t have to fertilize annuals again during the season. With annuals, the flowers are the thing, and overfertilizing can lead to lush foliage growth, which is really beside the point. The primary exception is container-grown plants, which usually need to be fertilized with water-soluble fertilizer every couple of weeks to maintain a colorful show.
Annuals have shallow root systems and so require a regular supply of water. Avoid overhead watering if possible, which can stain some types of flowers (such as petunias), and make them look unattractive. It also can contribute to a buildup of botrytis fungus, which can affect plants such as zinnias, geraniums, and marigolds. For best results, use a soaker hose or a drip irrigation system, or direct your watering can right at soil level.
A great place to get information around slow release fertilizers is http://www.annualsandperennials.com
Having the proper nutrients in your garden’s soil is important for healthy perennials. Fortunately, most perennials do not need large amounts of fertilizer if the soil has been properly prepared.
If over-fertilized, perennials will produce excessive soft growth with very few flowers. This can also cause perennials to lodge or open up too early.
If you test your soil to find out exactly what it is deficient in, you can use a customized fertilizer. Most of us will not do this, so a good rule of thumb in fertilizing is: 1 pound of nitrogen per 1000 square ft. This is precisely why it is so useful to use slow release fertilizers.