Cem, I made a big mistake when I first got into irises by over-watering and over-fertilizing after I received them in the summer (it's in the 100's here), and I almost lost about 75 of the new ones! I had potted them up and we were preparing to leave town for a week, so we put them in an area where the sprinklers would hit them for 15 minutes each day. But too much nitrogen and lots of water in warm weather promote BACTERIAL SOFT ROT, which can kill a rhizome pretty quickly.
Oh yes, and I should mention that I am talking about bearded irises only. I don't have any of the other types and I'm sure their care is probably quite different.
Bearded Irises need only a low-nitrogen fertilizer, and I only fertilize them with a granular fertilizer twice a year. However, when I plant my rhizomes in the ground I put a handful of bone meal in the hole and mix it in. Then I fertilize in the fall (Sept-Oct) and again in very early spring (Feb. here) I get my fertilizer at WalMart for about $3.50 per box, and it's a very low nitrogen fertilizer, 6-10-10. Nitrogen is the first number of the three there. Then I sprinkle the fertilizer around the rhizome, making sure that I don't let any of the fertilizer actually touch the rhizome, scratch it in, and then water. And last year I also added a foliar feeding of MiracleGro, the kind you attach to your hose, to give them a boost. But that was a little later in the spring, when the foliage was getting lush and green (about mid-March here).
Bearded irises are somewhat sensitive to water, too, especially as the weather warms up. I water only in the early morning or late in the afternoon, when they are no longer in direct sunlight, because it gets so hot here. But in the early spring when they really start growing, as long as it doesn't get too hot I water them a lot more at any time. Bearded irises go dormant in the summer months, so just maintenance watering is fine then. But if you want the foliage to stay green all summer they need about 1/2" of water each week.
And then, of course, there are the rebloomers. They require fertilizer applications right after each bloom cycle and regular watering year-round, or as long as they will keep reblooming. Because it gets so hot here I have picked irises that rebloom over the fall and winter months rather than during the summer. So I don't start fertilizing and regularly watering them again until about September.
I hope I haven't confused you, and perhaps someone else can explain it better. I'm sure you'll settle into a routine that works best for you, and if you're in an area that regularly gets rainfall you won't need to worry about the watering like I do. Good luck with them, and I know that when you see them blooming you'll know it was well worth it.
