Virtually all broad-leaved trees bear fruit of one kind or another. But when gardeners speak of “fruit trees,” whether deciduous or evergreen, they’re referring to trees with edible fruits. There are a variety of aspects involved in fruit trees producing fruit.
Although the flower is a crucial element in the process of fruit development, many other factors are just as important because without them the flowers might never form. Fruit trees must have all their growth requirements met in order to produce flowers and, eventually, fruit. Adequate light and water, proper soil conditions -- including nutrients -- and favorable temperatures all work in concert to shepherd the tree to that stage. After all the conditions that get the fruit tree to the flowering phase are met, the process of fruit development can then begin.
Fruit develops from flowers, so aside from the overall plant at large, flowers are the starting point in growing fruit. Some kinds of plants have male and female flowers on separate individual plants. Others have separate male and female flowers on the same plant, and still other plants have flowers with male and female parts contained within the same flower. Many fruit trees in question here have this last type of flower -- the kind botanists call perfect flowers.
In any case, there can be no fruit production until flower pollination occurs. Pollination is the flowering-plant equivalent of fertilization. Male flower parts produce pollen, and female flower parts receive the pollen. The shorter the distance the pollen has to travel increases the likelihood of pollination.
Some fruit trees are what botanists and horticulturists call self-fruitful. This means that the female part of the flower can receive pollen from the same variety of tree and be successfully pollinated. Only one tree is required for the production of fruit. Others are partially self-fruitful. These kinds can receive pollen from the same variety and bear fruit but are only mildly productive if only one tree is planted. Production of fruit increases when another variety is planted. Self-sterile kinds will only bear fruit if cross-pollinated by different varieties.
After pollination occurs, if conditions remain favorable, an embryo begins to grow, eventually progressing and developing into the mature fruit.
Many other factors can influence fruit formation, some positively, some negatively.
Pruning and Shaping
Fruit tree growers often practice one of many pruning methods to encourage the tree to produce so-called fruiting wood. Left to its own devices, a fruit tree is likely to develop many non-fruiting shoots and branches at the expense of fruiting wood.
Pest and Disease Control
Pests, including insects and rodents, may weaken or kill the tree, or even attack the fruit directly. Some form of control is often necessary to help ensure good fruit production.
Temperature and Nutrients
Unseasonable frosts can damage blossoms and reduce or even prevent fruit production. In such a case, the backyard grower might have the option of covering trees for these brief but damaging events.
Like all plants, fruit trees require nutrients. And, as with other plants in which the fruit is the gardener's objective, too much nitrogen can spur shoots and leaves rather than flowers. A low-nitrogen or a slow-release nitrogen fertilizer is generally best for fruit trees.