The most important time for fertilizing bulbs is when they're first planted. For best results dig out an area to the depth of four inches or deeper if the bulb you are planting requires a deeper depth. Incorporate organic matter and a high phosphorous fertilizer into the soil in this root zone. One-half pound of super phosphate (0-46-0) per 100 square feet is adequate. It is important to incorporate it into the root zone of the bulb as phosphorus fertilizer does not move through the soil as other nutrients do. The addition of organic matter improves soil aeration and drainage, both important in the health of bulbs. Note that research has shown that phosphorus from bone meal is only available to plants in soils that have a pH below 7. Bone meal is not regularly recommended for Colorado gardens.
A light application of a balanced fertilizer may be cultivated into the soil around the plants following bloom but before the foliage has withered. In the spring as bulbs finish flowering, allow the foliage mature fully. Remove only the faded flowers. The remaining foliage will manufacture food for the bulb as they mature or "ripen." When the foliage is withered and yellowed it is an indication that the bulb is dormant and unable to utilize any supplemental nutrients.
High-nitrogen fertilizers are not recommended as they encourage lush foliage at the expense of flowers.
If flowering declines, check your cultural conditions. Bulbs grow best in sunny areas, but sunny sites can become shaded with the growth of trees as gardens mature.
Bulbs also should be watered throughout the year in periods of drought.
For more information, see the following Colorado State University Extension fact sheet(s).
- Vegetable garden: Soil Management and Fertilization
- Choosing a Soil Amendment
- Spring-Planted Bulbs, Corms and Roots
Updated Wednesday, June 10, 2015