Growing Fruit Trees in Maine - Lack of Fruitfulness
Fruit trees fail to bear fruit for four general reasons: failure to form flowers, flower bud death from winter injury or frost, lack of pollination, and insect damage to the fruits. Observations during bloom and early fruit growth enable us to determine which of these cases has caused a lack of fruitfulness.
Fruit trees will not bear fruit until they reach a particular age. The age at which a tree begins to bear depends on the species, variety and rootstock. Generally, apricot, peach, plum and cherry begin to bear fruit three years after planting. Pears generally begin to bear fruit six to seven years after planting. Apples are the most variable in how long it takes to reach bearing age because of the diversity of the rootstocks available. With dwarfing rootstocks, apples can bear fruit the year after they are planted. In contrast, trees on standard rootstocks will take as long as ten years to produce fruit.
Failure to form flowers is also caused by biennial bearing. Apples, pears and some types of plums have a biennial bearing habit, meaning that every other year the tree produces only a small number of flowers. This is followed by a year with profuse bloom and a large crop of fruit. Biennial bearing is a complex phenomenon caused by the presence of fruit at the same time that next years’ flowers begin to form. In summer, prior to the season in which they bloom, the first stage of flower development occurs inside the young developing buds. The undeveloped flowers remain very small until new growth begins the following spring. A large number of fruit present on the tree in early summer will inhibit flower formation. Biennial bearing can be alleviated by removing some of the fruit in late spring, a practice known as fruit thinning. To be effective in promoting flower formation, fruit thinning needs to be completed within three weeks of bloom or by mid June in Maine. The sooner thinning is done after bloom, the more effective it will be in preventing biennial bearing. Hand thinning does not completely prevent biennial bearing, but will lessen it to an extent depending on how early it is done and how many fruit are removed.
Lack of sunlight and severe disease infestations are two other reasons why trees fail to form flowers. Trees need full sunlight and will fail to produce flowers if planted in full shade. Severe outbreaks of disease such as apple scab cause defoliation. Without leaves, the tree does not have enough energy to form flowers. Damage to the leaves at the time that flowers form will lead to poor flowering the following spring.
Extremely cold temperatures can kill flower buds, especially those on cherry, apricot and peach trees. Following a cold winter, some or all of the flower buds may be dead even though the rest of the tree survived. Peach, apricot and Asian plum flower buds are killed by temperatures below -15°F. Cherry flower buds usually survive winter but become sensitive to freezing temperatures in early spring as they resume growth. With cherry, the pistil or the part of the flower that grows into the fruit may be dead even though the rest of the flower appears healthy. The pistil is located in the center of the flower, and appears black or brown when dead and green when alive. Sweet cherry flower buds frequently die from the cold temperatures that occur in spring.
In spring when trees are in full bloom, flowers have lost all hardiness and are killed by temperatures below 28ºF. Avoid planting trees in low spots where cold air settles. These are “frost pockets” where freezing temperatures during bloom are likely to occur. The extent of flower death can be severe with all the flowers killed when the temperature drops well below 28ºF. When the temperature remains close to 28ºF, some of the flowers may survive. Frost damage does not occur frequently in Maine unless trees are planted in frost pockets.
Another reason for lack of fruiting is poor pollination. Apple, apricot, plum and most sweet cherry varieties need cross pollination by another variety in order for fruit to develop. Cross pollination is discussed in the section on pollination requirements.
Bees transfer pollen from one flower to another and are essential for fruit production. When weather is cold throughout the bloom period, there may be a light crop because most bees only work in warm weather. Bees are the main pollinators of fruit trees, so insecticides should never be sprayed on trees that are in bloom.
Insect damage is a common reason for fruit loss in unsprayed orchards. Soon after bloom, insects damage fruit causing many of the fruit to be shed. The plum curculio attacks developing apple, apricot, plum and cherry fruit but peach and pear to a lesser extent. European apple sawfly attacks apple fruit during a similar period. Unsprayed orchards lose many of their fruit because of these two insects.