The answer to your question depends on the formulation of fertilizer you decide to use. Water-soluble fertilizers are intended for frequent application as you water. Follow the recommendations on the product label for best results. Slow-release fertilizers are designed to gradually break down and supply fertilizer over several months.
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For northern regions, one application per growing season will usually suffice. In southern areas with longer growing seasons, a second application may be needed. The third formulation of fertilizer is granules. It’s a good idea to mix some granular fertilizer into the soil at planting time. To keep annuals blooming well, reapply granular fertilizer every 6 weeks through the growing season. Use one pound of 5-10-5 or 5-10-10 or similar analysis for 100 square feet of bed area.