The right watering and fertilizing schedule can help ensure that your bulbs put on the best possible show.
Bulbs need water during their season of active growth. For most kinds, this period begins soon after planting and continues until the foliage dies back (either after flowering has finished or in autumn). If you must supplement rainfall, water deeply enough to penetrate the root zone; the roots grow beneath the bulb. A layer of mulch helps retain moisture. (Bearded iris are an exception; they will rot if mulched.)
In addition to applying fertilizer at planting time, give your bulbs a feeding of high-nitrogen fertilizer at the start of the growing season, to enhance the quality of the current season's flowers.
After bloom ends, much of a bulb's stored nutrient supply is gone. If it is to perform well the next year, those nutrients must be replenished. To make sure the bulb has enough food to store, you can do two things.
First, always leave the foliage on the plant ― even if it begins to look unsightly ― until it has yellowed and can be pulled off easily. As long as the leaves are green and growing, they continue to manufacture food for the plant and for next year's flowers.
Second, apply a complete fertilizer as the flowers fade, using a 10-10-10 formula or a "bulb food" high in phosphorus and potassium. Because phosphorus and potassium must reach the root zone to be fully effective, you'll need to get the fertilizer as close to the roots as you can. In an established planting, scratch the fertilizer lightly into the soil to help it move deeper; then water thoroughly. Or, if there's enough space between bulbs, dig narrow trenches (up to 8 inches deep) close to the plants, taking care not to damage the roots. Scatter fertilizer in the trenches, cover with soil, and water.
If an established bulb planting has begun to bloom sparsely, the cause is probably overcrowding ― and that means it's time to dig and divide. You'll also need to divide bulbs if you want to make more plantings of a favorite kind. Because each of the five bulb types increases in a different way, techniques for division differ as well.