In the past, I believe I've been guilty, as many others are, of loving my Irises to death. Basically people over do it when it comes to supplementing their growing medium with too much compost, manure, and other non organic fertilizers. By the time the irises get planted in the new bed, they have to deal with a severe over dose of nitrogen. I have been guilty of this in the past and managed to destroy several expensive new introductions in only a month or two.
When I moved to my new house in 2001, I decided to employ the tough love approach to my existing collection that I brought from our old garden. I actually felt guilty not preparing what's considered to be appropriate growing beds for them. I just planted them in the loamy sand/clay/composted leaves mixture that is part of the forest floor. I added nothing else to it except a few doses of bone meal.
When my 100+ rhizomes arrived in EARLY July from Sutton's, I did the same thing. I made an effort to ensure the drainage was adequate and that there was enough sand in the main bed. The rest of the growing medium was decades of composted leaves on clay from the forest floor. These new rhizomes were planted directly into the bed with only a sprinkle of bone meal and perlite mixed in.
Because I plant in early to mid July, I ensure that I'm available to water the new rhizomes thoroughly at least once around 5:00 PM and a second time before bed at around 11:00 PM every day for 2 weeks. After this, they're pretty much on their own. Planting early also gives new additions a chance to develop a deep root system in preparation for the winter. I have a hypothesis that planting in hot weather encourages tap roots to grow deeper in search for water once they become established after the two week watering in period. Don't be fooled, once the roots decide to grow, they can easily grow over an inch a day. I observed this with my winter window ledge experiment where I grew dormant mart rhizomes in transparent cut off 2 liter coke bottles. I found that irises could be quite happy and grow explosive root systems. Having them in the plastic coke bottles gave me a window to view this.
The amazing new for 2002 was that I had a 100% survival rate with only 2 not looking so good. One of them was a popular Black iris and the other was a rhizome that my son's friend kept stepping on. To sum everything up for a zone 4b gardener I'd suggest great drainage as a major factor. Go easy on nitrogen rich supplements like compost or manure that isn't fully composted. Use bone meal as a main fertilizer and only use Miracle Grow or similar items if you feel the soil is missing nutrients and ensure that your solution is not too concentrated. I've been guilty of not reading the instructions properly and having rot problems in my rebloomer bed. Too much nitrogen cause a plant not to bloom if you over do it.
With arctic like Winters, chilly Springs, Texas like Summers and Falls that approach like a charging tiger, growing Irises here in Lafontaine , zone 4b, has and will always be a challenge. This is what I had found so far that works for me.
Good luck