There is only one set of flower buds each year.
Flower buds at the top of the tree open first.
In addition, flower buds have been set for next spring.
If there is not enough, the leaves keep growing and flower buds never set.
That means the tree has less energy for making flower buds.
The flower buds form on the old wood, so you don't want to cut it back by more than about half.
To set flower buds, it has to form a strong plant while days are cool and short.
Plants need time - months - to form flower buds.
The process from flower bud to ripe seed takes about five weeks.
Or sometimes both the flat leaves and the flower buds appear together.