Yes -- I do think that the bonemeal helped the plants. And it is absorbed slowly -- so I think it helps them for at least two years. I did not intentionally compare Irises with and Irises without bonemeal -- but the plants that got it when planted were much more vigorous than the plants in another bed that did not. But as in all things -- it could have something to do with something else. Difference in soil, drainage, amount of direct sun, etc.
Here's what I suggest. Buy a few less plants this year, and buy a bag of bonemeal instead and try it. It's not that expensive -- and a bag goes a long way. See if the bonemeal makes a positive difference for you, with your growing conditions.
I think that Irises are probably the most forgiving plant on the planet -- and if you don't fertilize at all, you'll still get flowers in the spring. But you won't get the quantity and size of blooms maybe, nor the amount of increases that you would if your plants are fed. I think that's probably true for every growing thing. JMHO.
For myself -- my goal is to fertilize with one product each year. The bed that got bonemeal last year will get alfalfa pellets this year. And maybe next year I'll try a commercial fertilizer. That way I can see how each product affects things in my growing conditions. And I won't spend a fortune doing it.
