Best nutrition for your iris plants Improves iris bloom Improves health of iris plants Low-Nitrogen fertilizer specially formulated for growing Iris. Apply one month before and one month after bloom. Planting new Iris or dividing your existing Iris beds this summer? Give these newly planted rhizomes a boost with our 6-10-10 Controlled-Release Iris Food. The 4-lb bag typically covers approximately 100 newly planted Iris when applied as directed.