Urban Orchards : The Birds and the Bees and Fruit Set

The following is an article by Duane Greene of the UMass Dept. of Plant and Soil Sciences
The Birds and the Bees and Fruit Set in Tree Fruit

Fruit set in many commercial orchards was light this spring*, and presumably this may also be true for fruit trees in many home gardens. Since this may be a recurring problem it seems appropriate to discuss factors that influence fruit set in tree fruit. Availability of appropriate pollen and the transport of that pollen to the flowers will be emphasized because you have some control over these factors that influence fruit set.

First, an elementary sex education lesson in fruit trees. Pollen from a flower must be carried by insects to the stigma of a flower. The pollen germinates, grows down into the ovary where sperm in the pollen tube fertilize the ovule(s). If pollen is not transferred to the stigma of a flower or if fertilization does not take place, the flower will soon turn yellow and fall, and no fruit will develop. If fertilization of the ovule does occur, the ovary (fruit) soon starts to grow and eventually it may develop into a mature fruit, Therefore, any factor that influences the supply of appropriate pollen, the growth of the pollen tube, or the transport of pollen to the stigma will prevent fruit set.

Pollen Source
The source of pollen in most cases influences whether fruit set occurs. Many fruit are self-incompatible, which means that the pollen from that variety can not serve as a source of pollen for fertilization. Another variety of the same tree species must be planted to serve as I source of pollen. For example, assume that you want to plant a Delicious apple tree. If you want to get good fruit set you must also purchase another variety of apple, such as Mclntosh, to plant near the Delicious. Pollen from the Delicious can be used to pollinate the Mclntosh tree, and the Mclntosh pollen can be used to pollinate the Delicious. It is important to note that successful crosspollination does not occur between species. For example, a pear will not pollinate an apple.

Not all fruit are self-incompatible. In these cases you only need to purchase one variety, since the pollen produced by that tree will be viable and fertilize that tree. One can become quite confused and frustrated at a nursery or garden center trying to figure out, not only what type of fruit tree to purchase, but also the variety, and whether you need to purchase more than one tree. The bad news is that there is no logical way to figure out this dilemma. The good news is that you can look it up, or buy two different varieties and don't take the chance. Table 4.2 lists the most common fruit trees and the percent that are self-compatible and self-incompatible. Let me give you an example of how you can use this chart. Pears are 2% self-compatible and 98% self-incompatible whereas peaches arc 98% self-compatible and 2% self-incompatible. With 98% of the pear varieties, it will be necessary to purchase a second variety to serve as a source of pollen. Conversely, with peaches there is a very high probability that if you purchase just one tree, it will be self-compatible and only one variety will be necessary for good fruit set.

Compatibility of pollen of major tree fruit

Species

Self-compatible (%)

Self-incompatible (%)

Apple	4	96
Apricot	79	21
Pear	2	98
Peach	98	2
Prune	50	50
Plum	2	98
Sweet Cherry	4	96
Sour Cherry	37	63

Commercially, we recommend that pollinizers be planted no more than l00 to 150 feet away. If your neighbor has a pear tree, it may be sufficiently close to serve as the pollen source if you purchase and plant a different variety of pear.

Pollen must be carried between trees. Honeybees and bumblebees generally carry pollen from one tree to another as they forage for nectar in the flowers. They are primarily responsible for cross pollinization of self-incompatible fruit trees. Bee activity is high on warm sunny days and sufficient ' pollen transfer between trees can easily be accomplished in one day or less. However, if the weather is rainy, windy, or the temperature is low, bees do not work well. If these weather conditions occur during the whole bloom period, insufficient pollen is exchanged and fruit set will be light, if it occurs at all. You can substitute hand pollination for bees in this situation. This is accomplished by taking open flowers from one variety and gently brushing them individually on open flowers of another. The pollen from the flower will collect on the sticky surface of the ovary. It will soon germinate, grow down into the ovary, and within 4 to 6 days fertilization will take place. You will soon note the fruit increasing in size.

In conclusion, most trees are self-incompatible and it will be necessary to plant at least two different varieties. If weather is bad during the bloom period you can act as a surrogate bee and hand pollinate flowers to assure adequate fruit set.
Duane Greene
UMass Dept. of Plant and Soil Sciences

*article published in 1998