Using phosphorus to get shrubs to bloom is a beloved myth that's widespread and cherished. But it won't help.

I will agree and disagree with the last post at the same time. True, it is not a magical way to force blooming. However, plants that lack P will be stunted and won't bloom. Phosphorus is also important for root development. So if your plant looks stunted and isn't blooming I think you can benifit the use of superphosphate. I tend to use two applications a year on plants that bloom alot like encore azealas, endless summer hydrangeas, and knock-out roses to name a few. The high bloom production can deplete P in a plant. Here is a good link.

Here is a link that might be useful: Info on fertilizers

Only a professional laboratory soil test knows for certain if your soil needs supplemental phosphorus.

True, I believe in soil testing yearly. It is very important in talking about phosphorus because it's not available to ph levels under 6 and not much over 7. I usually use small amounts of superphosphate when I apply. I know that P is the largest pollutants in waters causing algea blooms etc. Without going deep into a soil class. But taking a soils class in college I know the important need of still using P because of the fact that not all the P is available to plants. It's a bit hard to explain without diagrams. That's the other thing on a soil test. The results may come back that none is needed when some of the P isn't even available. Notice when you plant grass that starter fertilizers are high in P because of the need for new seedlings when they may not be able to absorb the P in the soil. I'm going to leave this all of my opinion I will offer because I know some who read posts will go into talking how organic is the only way to go and I'm about using organic products but when you need results, sometimes you need to pull out the big guns!

I have to wonder if debating superphosphate here would in any way solve Julia's no-blooms problem - which I suspect is more a matter of winter protection than nutrient deficiency. She's had a couple of Zone 8 responses for her Zone 5, and here's my third -

The most common reason for failure to bloom in a harsh winter area is bud kill due to freezing. What kind of winter protection are you providing?