Bone-meal is very slow acting. It is good for long term health of plants when initially planting. I personally like MagAmp as a booster for rhododendrons and azaleas. It is hard to find, but if you can find it, it is a good slow release phosphorus rich fertilizer with magnesium. The standard phosphorus specific fertilizer is superphosphate (0-20-0). There is also a (0-45-0) tripple superphosphate formulation on the market.

Superphosphate applied in late Fall generally increases flower bud formation the following Summer. So anything applied now will probably only have a small effect on the bud set for next year but have more of an effect on the budset for the following year.