Pollination

Almost all temperate fruit species require pollination and seed set to produce commercial crops. A few cultivars of apples and pears are capable of setting fruit without seeds, while some species (such as Malus hupehensis and Malus sikkimensis) set seed apomictically; in other words, seed is produced vegetatively from a source other than the zygote. Some fruit crops require cross-pollination (apples, pears, most sweet cherries) while others are self-fertile (most peach cultivars, sour cherries, most apricots). European and Japanese plums vary in their requirements for cross-pollination by cultivar, with about half being self-fruitful. For apples, most cultivars will pollinate most others, with two key exceptions. Triploid cultivars, such as 'Mutsu' and 'Jonagold', produce nonviable pollen and therefore should not be considered pollinizers. Also, closely related cultivars (such as parents, siblings, or sports) may not work well as pollinizers for each other. 'Golden Delicious' has historically been regarded as having some degree of self-compatibility, but recent research shows the self-fertilization potency of this cultivar to be quite low. In addition to producing viable, compatible pollen, a pollinizer must flower at the same time, or slightly prior to, the cultivar for which it is intended to provide pollen. Typically, pollen is viable only for a short period of time, a matter of hours, and, therefore, synchronous flowering is important.

Successful pollination depends on both adequate and timely polli-nizers (the source of the pollen) and pollinators (the agents of pollen transfer). Many nut trees rely on wind pollination, but the pollen of most fruit species tends to be heavy and not suited to wind pollination. As a result, pollinators are especially important. Pollination requirements of the crop should be considered at the time of orchard planning. For those fruit requiring cross-pollination, several approaches may be taken, but the most common is to avoid planting large blocks of a single cultivar. Bees tend to fly up and down rows rather than across rows, especially in orchards where trees form a continuous canopy rather than discrete trees. Orchard blocks of any one cultivar should be no more than five to six rows wide—this provides the best compromise between ensuring good pollen dispersal on one hand and efficient orchard management on the other. In some of the larger fruit-growing regions of the world, production is based mainly on just one or two cultivars, making large production areas of one cultivar desirable. To facilitate adequate pollination in such orchards, pollinizer trees can be planted throughout the orchard (usually every third tree in every third row). Crab species are frequently used for this purpose in apple orchards, with Malus floribunda and Malus 'Profusion' being popular choices due to their profuse flowering and commonality of flowering time with many commercial cultivars. Alternatives to pollinizer trees are hive inserts, which are packs of pollen that may be purchased and placed inside the bee hives so that bees are coated with suitable pollen upon leaving the hive, and bouquets of different cultivars placed in buckets of water and spaced around the orchard. Obviously, both hive inserts and bouquets require additional management time and are expensive to maintain and, consequently, are seldom used in commercial orchards.

The honeybee is the most prevalent pollinator for most fruit crops, although a number of other insects may play a secondary role. The level of pollination required differs among the various fruit crops, from a single-seeded fruit, such as a peach or a cherry, up to a kiwi-fruit with well over 1,000 seeds per fruit. Many factors determine the number of hives required to achieve optimal pollination, but two to eight hives per hectare is the generally accepted norm for tree fruit crops. Hives should be distributed at several locations around the orchard and introduced soon after the first flowers have begun to open.

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