Ian, no need to worry about starting new threads, if you have a question, it's perfectly fine to ask it!

I'm sure there are a lot of different ways to go about fertilizing irises, but I think that it's a good idea to have your soil tested before you add anything. That's just my personal point of view. If you know what's in your soil, and what's not in your soil, you can add what it needs. That being said, if you have an iris bed that's been there for a long time, it's most likely going to need a boost now and then. For those beds I usually add a bit of a balanced fertilizer, that's one with all three numbers the same. I add it in the spring after the plants are coming out of their winter dormancy and the leaves are beginning to grow. The larger the number, the less you use. For example if it's 13-13-13, then you'd use less then if it's 10-10-10. Bloom booster is OK to use, but it's kind of high priced to treat a lot of irises. Some people like a 5-10-10, and that's fine, it's hard to come by that here, and often only available at a higher cost. I've never had an issue with using a balanced fertilizer causing too much Nitrogen and thus low bloom. I'm sure others have differing things that works for them, once again, each garden is unique and we all have to do different things depending on our soil conditions and climate. It takes some trial and error to find that sweet spot that works for your garden.