Plants that are grown especially for their flowers need extra phosphorus. Phosphorus encourages blooming as well as strong roots and disease resistance. Fertilizers come in several forms. There are also liquid forms that can be injected into the soil or poured on. The fertilizer needs to match both your needs and the needs of the plant.
Dry fertilizer is inexpensive.
If you are spreading it on the surface, dry fertilizers are easy to apply. If you are drilling it in, it will take more of your time and labor. Spikes are quick and easy to use, but they are a little more expensive and their nutrients aren't as evenly spread around the tree. Root feeders are easy, but it takes quite a bit of time to move them from site to site to get good coverage and their nutrition doesn't last as long. There isn't just one right answer.