bonemeal is a fine organic fertilizer and additions of it won't hurt, but its a "incomplete" fertilizer. In my experience if you need phosphorus in the soil, get your soil tested, and then use a complete formulation -- ie 5 -10-10, or 6-8-12, or even a 10-20-20
and incorporate it into the soil/iris bed prior to planting. Phosphorus moves very VERY slowly in the soil because of its binding ability with soil ions and cations, and in cased of very high ph or low, it doesn't move at all.
Also iris use NPK (nitrogen, phosphorus, potassium) ratio's differently depending on time of year. For example, in the spring the ratio is 1-1-1 meaning the iris clump is sucking up all three nutrients at an even rate. Immediately or soon after bloom the balance starts to shift to a 1-2-2 ratio, ie the iris is using more phosphorus and potassium and less nitrogen. During late summer and fall, the balance completely shifts to a 0-2-2 or similar ratio as the iris matures its rhizomes and hardens its cell walls in preparation for freezing temps.
I use a 16-16-16 with nitrogen evenly split between a fast release and a slow release. Also most important of all, is that bearded iris seem to be heavy users of trace elements -- ie zinc, manganese, molybdenum, etc. If you can incorporate a fertilizer brand with these minor elements, you'd be amazed at the performance increase.
disclaimer:
The above is observations from my personal 43 years of growing bearded iris and is offered as "food" - lol -- for thought ONLY. Your individual soil, climate and environment can differ greatly. A good source of info is a local iris society, and the members experiences. Please pardon the length of my post
