Answer: Plants fail to flower for various reasons. One of the more common reasons is plant immaturity. Trees, in particular, must reach a certain age before they begin to flower.
If the plant is old enough, the growing conditions may be too poor to allow flowering. Plants that require full sun, for instance, may grow but fail to flower in the shade.
Cold winter temperatures may kill the flower buds. This often happens to forsythia, flowering plum and peach. Nothing can be done to prevent this type of injury.
Improper pruning may cause failure to flower. Some plants bloom only on last year's wood. Cutting the plants back severely removes all the flowering wood.
Overfertilizing with nitrogen can sometimes cause plants to grow only leaves and stems. Such plants will be quite large but without flowers.
If none of the above seems to fit as a reason for non-flowering, you might try pruning your tree this winter to force the development of new wood (which should set flower buds). Of course, pruning in winter will ensure the removal of any flower buds developed this summer and will result in non-flowering next spring. But, based upon your experience with declining flower production, it may be acceptable to you to miss one more flowering spring in favor of an abundance of blooms the following spring.
Best wishes with your ornamental plum!
Q&A Library Searching Tips