Irises are garden showstoppers in spring, when their large, ruffled blooms appear in many colors atop green stalks. Hundreds of iris species and cultivars exist, from the classic bearded iris (Iris germanica), hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10, to smaller types such as the stinking iris (Iris foetidissima), hardy in USDA zones 6 through 9 and named for its unpleasant odor.

Fertilizer Types

When fertilizing irises, be careful about the type of fertilizer you use. Fertilizers high in nitrogen encourage leaf growth but don’t do much for flowers and also lead to bulb rot. This includes commercial fertilizers as well as high-nitrogen composts, or those that were produced with a lot of green material such as leaves and grass clippings. When choosing a fertilizer, opt for superphosphate, or choose a fertilizer where the first number in the NPK ratio is lower than the second two, such as 5-10-10 or 6-10-10.

Planting Time

When preparing beds for iris planting, add fertilizer. Ideally, you should take a soil test to see which nutrients -- nitrogen, phosphorous and potassium -- you need, and fertilize accordingly. In the absence of a soil test, the North Carolina State University Extension recommends you use ½ pounds low-nitrogen fertilizer such as 5-10-10 per 50 square feet of iris bed. Till the fertilizer in to the planting depth, which should be about as deep as the iris rhizome.

Annual Fertilization

Before fertilizing irises, examine them to see if they need it. If they regularly perform well, with strong, green growth, they probably do not require fertilization. Older iris varieties will likely need less than modern varieties, as their blooms are smaller and less abundant. If your irises do need some help, add ½ cup 5-10-10 fertilizer per iris clump after the plants have have finished flowering.

Other Considerations

Generally irises prefer full sun, although some varieties tolerate partial shade. While they like a medium amount of water, irises need well-drained soil and can rot if left in standing water. Plant irises either in spring or in late summer to early fall. Bearded iris, for instance, prefers planting between July and September, giving it enough time to establish before winter. Leave the top of the rhizome slightly about soil level. Divide clumps every three to five years, or whenever clumps become too dense.