Here is an article I wrote recently on this topic. Maybe your friend will find something to explain her fruitless pear trees.
If you have backyard fruit trees that you’ve nurtured along in anticipation of a delicious home-grown harvest, only to be disappointed by a lack of fruit, there are a number of possible causes. Maybe you have older trees that have been reliable bearers and they’ve suddenly decided not to produce fruit. What could be the problem?
With new, young fruit trees, it could just be their age. Most fruit tree nursery stock is sold when the trees are only one to two years old. Bearing age ranges from 2 to 7 years depending on the cultivar, rootstock and tree vigor. Dwarf trees will generally begin to bear sooner than standard size trees, with the semi-dwarfs falling in between. A tree that is growing at a moderate rate will bear earlier than one growing either too rapidly or too slowly. Growth rate is affected by environmental conditions, soil fertility, and moisture availability.
Plant fruit trees in a sunny location with enough space to avoid root competition with other nearby plants and trees. Competition from weeds or grasses can be reduced using cultivation, mulch or properly labeled herbicides. Avoid excess nitrogen fertilizer. This stimulates vegetative growth at the expense of flower bud production. Overfertilization is one of the most common causes of reduced flower bud production in the backyard orchard. This is due to the application of high-nitrogen fertilizer to lawn areas around the tree. Fertilizer recommendations for fruit trees are ¼ lb. of nitrogen per tree just after planting followed by reducing or eliminating added fertilizer until the tree begins to bear. Once trees begin to bear, if lawn fertilizer is applied that is adequate; if not, apply 0.1 lb. nitrogen per inch of tree trunk diameter. Broadcast the fertilizer over the root zone. To evaluate whether you should increase or decrease your fertilizer rate, note the length of new shoot growth during the previous season. The length of new growth should not exceed 18-20 inches. Of course, a lack of nitrogen and other nutrients that reduce tree vitality will also decrease flower bud formation, fruit development and fruit quality. A soil test can be used to obtain accurate fertilizer recommendations.
Excess pruning can delay the onset of flowering in young trees and stimulate vegetative growth at the expense of flower bud formation in bearing trees. In young non-bearing trees, prune only as needed for developing a strong, desired framework. In bearing trees, adjust the amount of annual pruning based on the length of terminal shoot growth as with fertilization rate. Prune out water sprouts.
If your fruit trees have an abundance of blossoms but fail to develop fruit, the most likely causes are related to the weather and pollination. Open blossoms can be injured by freezing temperatures. In some cases, the blossoms will still look normal but will not be able to form fruit because of injured parts. Small backyard trees can be protected if overnight freezing temperatures are expected. Cover the trees with plastic sheeting, old bed sheets, cheesecloth or similar materials. The cover should reach the ground to be most effective. Another alternative is to use sprinklers. Turn the spray on when the temperature reaches the low 30s. Ice will form on the tree surfaces, insulating the tissues from temperatures falling below freezing. This occurs because as water freezes, heat energy is released. The sprinkler MUST be kept on until the ice melts on its own or more severe injury can occur.
The other important considerations affecting fruit production relate to pollination. First, many tree fruits are self-unfruitful and need another cultivar as a source of pollen. This is true (with a few exceptions) of apple, pear, sweet cherry, Japanese plum and some European plums. Peach and apricot are self-fruitful and don’t require another pollinizer tree. When purchasing new fruit trees, it is important to understand each cultivars requirements. When selecting cultivars, consider the bloom period. For example, an early blooming apple will need a pollen producer that is either an early or midseason bloomer so that the bloom periods will overlap sufficiently. Some apple cultivars are poor pollen producers and need to be grown with two other cultivars to ensure a good pollen supply for all. These include Baldwin, Gravenstein, Stayment, Winesap, and Rhode Island Greening. One exception to the self-unfruitful apples is Golden Delicious, which is self-fruitful. If you’re just interested in growing one main variety of apple, plant one pollinizing tree for every 8-9 trees.
A few tree fruits require individual male and female trees; in other words female trees have only female flowers and bear fruit while male trees only have male flowers that produce pollen. In Connecticut, these include the hardy kiwi and persimmon. A good rule of thumb is to have one male tree for every six females.
Okay, now we have plenty of healthy blossoms and a good source of pollen; the next ingredient is the pollinator. The most important pollinators of fruit trees are bees. Minor pollinators include flies, wasps, beetles, butterflies, moths and other insects. Fruit trees that require cross pollination should be spaced in close proximity to each other without crowding to increase pollination. The better a flower is pollinated, the more seeds the fruit will have and it will be larger and more uniform. Cold, rainy or very windy weather during bloom will reduce bee activity which will result in reduced fruit set.
Some fruit trees, including apple, have a tendency to bear a large crop one year followed by little or no fruit the next year. The flower buds of the fruit trees are set during the summer before they open. If there is a lot of fruit developing on the tree, there are simply fewer nutrients available for flower bud formation. This tendency can be countered by practicing fruit thinning. For apple and pear, thin to one fruit for every three to four spurs or 4-7 fruits per yard of branch within 30 days after bloom. Thinning peaches and plums isn’t necessary to get a crop every year, but for larger fruit, thin to a spacing on the branch of 6-8 inches. Sweet and sour cherries, apricots and peaches will be able to produce flower buds while carrying a heavy crop.
Pest and disease management are important in maintaining the vigor of the tree and in the development of quality fruit. Some diseases affect the blossoms resulting in a reduction of yield while others affect the fruit as it develops. In general, diseases of the tree may reduce its vigor, decreasing its ability to put nutritional resources into flower bud formation and/or fruit development. Insect pests can decrease fruit production because they reduce plant vigor by feeding on the leaves and reducing photosynthesis or by feeding directly on the flowers or developing fruits.