When is the best time to fertilize with bone meal for hydrangeas?
I do not prune my hydrangeas unless I know the dead stmes.
Unlike nitrogen, phosphorus moves slowly through the soil. It can be added anytime since the effect is not immediate. It should be worked into the top 2 inches of soil so it doesn't run off in a heavy rain and pollute waterways. Where I live, soils are naturally high in phosphorus and only farmers and those growing in containers need to add a source of phosphorus. Too much phosphorus is not good for plant growth. You may want to test the soil every few years to guide use of amendments. Compost is also a decent source of phosphorus and is in a form that plants can absorb easily.