Pruning trees, shrubs and clematis is a garden art form. We worry so much about taking so much off that the plant will look unsightly. Unfortunately, the opposite is true - not enough pruning promotes an unhappy plant.
I see so many beautiful specimens around me that have been pruned at the wrong time sans blooms the following year, or not pruned at all with a few paltry blooms at the top of the shrub.
As soon as your lilac, magnolia, viburnum, forsythia, bridal wreath have bloomed in the spring, prune, shaping the top of the shrub and removing one third of the old growth each year to the bottom of the shrub. This has to be done pretty quickly as the shrubs begin to set flowers for the following year. One year, I waited too long to prune the Magnolia which hung over the walkway and I had very few flowers the following spring. I have already trimmed back my Viburnum Carlesi. Hydrangeas have pruning requirements also, some growing on old and new wood, some growing only on old wood. I happen to have the types that I can cut down quite a bit each year and get beautiful new blooms the following summer and late summer.
My Hydrangeas are Endless Summer, Unique and Limelight. I shape my Limelight in a semi-circle when I prune and it is a gorgeous specimen come August. Unique., I cut down to about two feet and it is spectacular in the late summer. With Endless Summer I wait to see how much growth is coming and then trim whatever is not viable.
Don't be afraid to heavily prune damaged or overgrown flowering trees. My daughter's Crabapple is over fifty years old and we pruned it heavily last year because it had more suckers than anything else. I pruned my Pagoda Dogwood because it had been so heavily damaged by the Cicadas and grew in all different directions. Both trees have had the best blooms ever!
I think I have finally figured out my Little Henry's Garnet (Sweetspire)- tons of little beginning flowers this year. In one of my previous posts I noted that I had to bring out the big guns for this shrub and feed it a systemic. I noticed an improvement almost instantly. I also cut it down to about 12" high. I will do the same this year after it blooms and see if this is the answer to a very finicky shrub for the six plus years that I have tried to grow it.
Clematis, we all learn the hard way with this lovely plant, meaning if you cut it down in the spring and it is not a type 3, you will get very few blooms or they will be very small. I went through this many years paying no attention to the different types. Guess what, type 3 is easy, but some beautiful blooms are passed up if we don't also try the type 1 and type 2. It is a challenge remembering which is which. One final note, you have to fertilize if you want maximum bloom, acid based for Hydrangeas and other acid loving shrubs, general fertilizer for others, equal numbers if possible 5-5-5 or 10-10-10, not too high in nitrogen or it will again cut down on the blooms