Like all plants, your iris will do better when their nutritional needs are met, and that usually means fertilizing them every now and then. Old-time wisdom suggests doing it when your tulips are blooming and we’ve found that to be a good indicator here. This is also a good time for doing a soil test, as over-fertilizing can cause long-term problems, but if you haven’t fertilized in a while, you’re probably safe doing it this spring. Some growers recommend a 6-10-10 fertilizer, while others prefer a balanced (10-10-10) one, but as long as it’s slow-release and not high in nitrogen (the first number), most fertilizers will work just fine.