The weather at blossoming time may play an important part, either directly or indirectly, in fruit production. It may be so miserable that bees do not venture out to do their job as pollinators, or heavy rains, when the flowers are open, may wash the pollen away.
Another point that must be recognized is that in many cases there is what is known as "self-incompatibility," which demands cross-pollination to insure fruit production.
Then there are dioecious plants, which produce male and female flowers on separate plants. Holly is an example.
Pruning must not be done at the wrong season. As an example, most varieties of Hydrangea macrophylla ("French" Hydrangea) produce their flowers only from the tips of the shoots made the preceding year; if these are cut off early in the spring or if they are killed by low temperatures during the winter, they cannot blossom.
Pruning during the late summer may result in sappy growth which can be affected by low temperature; similarly, the same effect may be produced if the subject is grown in soil that is too rich. For example, Cytisus scoparius (Scotch Broom) and Kolkwitzia amabilis (Beauty Bush) when grown in poor sandy soil may come through the winter with no damage, but if grown in rich soil may be killed to the ground.
Summer pruning may hurry up the blossoming of some varieties of Apple and Pear, especially in the case of espaliered plants.
If, after five or six years, Apples show no sign of blossoming except for notoriously laggard varieties such as Baldwin and Northern Spy, which are slow in coming to bear, something must be done about it. When trees are growing vigorously the elimination of nitrogen in the fertilizer program and/or the planting of a cover crop of Winter Rye, Rye Grass, or Buckwheat in August may, by checking the exuberant vegetative growth, cause the trees to produce blossoms. If neither of these measures is successful root pruning should be tackled.
This is done by digging a trench 1 1/2 to 2 feet deep halfway around the tree, 3 to 6 feet from the trunk, and cutting off the large roots encountered. The trenches should not be continuous. First make a circle around the tree and divide it into four or six equal parts, alternating trench with undisturbed areas. Mix superphosphate, 1/2 pound to each square yard, with the soil before filling it back into the trenches. Avoid pruning the top of the trees when they are dormant, and if they still persist in making luxuriant shoot growth, the other half of the root system should be pruned.