In the bulb forum I have previously mentioned a method that can boost hardiness and bloom of the Dutch Iris. As I noted that a few members in colder zones have problems with getting repeat performance from these irises, I though I'd start a discussion here as well. Some may object that I'm in USDA 7b and should have no problems with Dutch Irises, but keep in mind that the USDA zones really aren't relevant for the Scandinavian climate. We are far north and the winters are long and the summers cold and short.
One of the problems with the Dutch Iris is that they starts to grow very early. In a mild winter this may not be much of a problem, but if the foliage is damaged in the winter, it means less energy to build a new bulb and less bloom in subsequent years. The solution to this is quite simple. They need late planting.
Plant them at the same time as tulips should be planted in your area. Wait until the soil temperature is at 10℃/50℉. Here that usually means late October or mid November, your area may differ.The idea is to give them enough time to root, but not allowing too much foliage growth.
I do the same to prevent freeze damaged on fall planted garlic. It's not a 100% certain method as if we are unlucky and the weather turns mild and warm, we could still get some foliage growth, but at least it will be less than if we plant too early.
Come spring they will need to be fertilized lightly a couple of times with a fertilizer suitable to your local conditions. Lower nitrogen numbers is usually a good thing.
Once the foliage yellows the bulbs usually need to be dug. They need warm and dry conditions as this improves winter hardiness and bloom. Moisture will stimulate early root growth. I like to stor mine in dry sand in a covered, ventilated cold frame, however in warmer areas this may prove to be too warm, so then you may need to bring the bulbs inside or in a more shady position. In all cases they should be stored away from ripening fruits and the area must be well ventilated. At this time we should also divide the bulbs as Dutch Iris reproduce like rabbits and leaving them crowded would eventually lead to declining bloom.
The downside to this method is that it's a bit more work with it and unfortunately late plantings means the bloom can be later as well. For reference, last year my Dutch Irises bloomed at the same time as my Tall Bearded. For these reasons I might not keep growing the Dutch Irises as the extra work and the late bloom is a negative for me, but the method as such has been a success so far. I have had much better bloom on my own bulbs, than I had with freshly bought stock. That said, an extra cold winter still may cause problems, so obviously I can't guarantee success.
Plant late, at the time it's correct to plant tulips in your area.
Dig and divide them annually. Store dry and warm