Cut flowers are heavy producers—you are removing large amounts of plant material every time you harvest. Consistent feeding replaces those nutrients and keeps production high.
Work compost into beds before planting (see soil preparation). Then supplement with a balanced fertilizer (such as 10-10-10) every 3–4 weeks during the growing season. Liquid fertilizers applied through a watering can are the easiest for small gardens. Granular slow-release fertilizers are more convenient for larger plantings.
Heavy feeders like dahlias, lisianthus, delphinium, and peonies benefit from regular feeding. Light feeders like cosmos, nigella, and yarrow actually perform worse with too much fertilizer—they produce leaves instead of flowers. Sunflowers and zinnias are moderate feeders.
Too much nitrogen produces lush foliage at the expense of blooms. If your plants are green and leafy but not flowering, cut back on nitrogen. A fertilizer higher in phosphorus (the middle number) promotes flowering.