Based on a short course by Richard L. Norton, Regional Fruit Specialist with Cornell
Cooperative Extension (NY), edited and minor additions by Dave Green.
Nothing in fruit production is more important than fruit set, yet it receives so little
attention. This is a course to provide the grower with a few of the basic reasons why a
flower may fail to set fruit. It would appear that all of the brief discussion statements
below can be grouped into three catagories:
1.) Lack of pollination due to lack of pollen or to a lack of honeybees and pollinating insects.
2.) Pollen does not provide fertilization. This may be due to sterility of the pollen grain or egg cell. Also, pollen may not be able to penetrate the style due to incompatibility between two varieties.
3. Seed abortion. This may be due to nutrition and low temperature.
IMPORTANT PRINCIPLE: The more seeds present in the fruit, the bigger the fruit,
and and the more symmetrical the shape. Seed numbers have a profound influence on both the
size and shape of the fruit. There is a direct correlation between the number of bees
and/or pollinating insects and seed numbers in the fruit.
-All apple varieties should be considered self-unfruitful; therefore, cannot
effectively pollinize themselves. Even such so-called "self-fruitful" varieties
as Golden Delicious, Rome, Wealthy, Baldwin and Jonathan, etc, need another variety nearby
to serve as a pollen source (pollinizer) to set consistant crops. No apple variety is
sufficiently self-fruitful to be dependably productive when planted alone.
-Because of poor pollen (triploid), R. I. Greenings, Mutsu, Jonagold, Baldwin and
Stayman, etc. are worthless for pollenizing other varieties. Therefore, if a triploid
variety is to be planted, a pollen variety must be planted with it, then a second
pollenizing variety must be planted to pollenize the pollenizing variety.
-Red strains or sports or a variety will not pollenize the parent variety or other red
sports of that variety. Example: no Red Delicious strain or sport will pollenize another
Red Delicious strain or sport. The same is true for McIntosh, Rome, Golden Delicious, etc.
-Biennial varieties are not reliable pollenizers. In their off years, when they fail to
bloom, there will be no pollen supply to pollenize the annual varieties. Is is suggested
to set a third variety in such blocks.
-In our northeast climate, Red Delicious often fail to set adequately, Red Delicious
requires an abundance of cross-pollenation. For this reason, we urge growers to set a
pollenizer row next to every Red Delicious row. If you want consistant Red Delicious
crops, this practice is very important.
-Red Delicious and Empire have a very sensitive blossom to frost injury. Such varieties
should not be set in low spots, only on sites with good air drainage. Note: Starkrimson
(Bisbee) Red Delicious appears to have considerable frost hardiness compared to most Red
-We suggest never to plant more than four rows of a main variety followed by two rows
of a pollenizing variety for top yields.
-In high density apple orchards, bees tend to fly down the row in contrast to flying
from one row to another. In such hedgrow plantings, we suggest grafting in, for example,
Winterbanana, Golden Delicious, etc. in the row of a red colored variety.
-Cross pollenizing varieties should bloom at approximately the same time the main
variety blooms. do not expect Golden Delicious (late bloomer) to pollenize Idared (early
bloomer). In most years, there is sufficient overlap between early and late bloomers to
give adequate pollination. However, in some years, the early bloomer may be in petal fall
before the late bloomer comes into bloom. Note: For some unknown reason we take our best
Red Delicious (mid-season) crops from orchards that have Romes (late season bloomer) as
-Pruning plays an important role in pollination. Blossoms on older spurs are first to
open, and followed in sequence by blossoms on three, two, and one year laterals. Prune to
ensure a balance of fruiting wood of different ages in the tree. If the flowering time is
spread, this will increase the chances of some flowers meeting favorable weather. Pruning
provides light within the tree, and bees will fly in areas where flowers are more shaded,
if the tree is pruned.
-Tree nutrition helps set apples. Pollen from trees short of nitrogen is markedly
inferior in its ability to bring about fertilization, to pollen from a tree with adequate
levels of nitrogen. Trees that are deficient in nitrogen often flower profusely, but the
blossom is weak and fruit set is usually light.
-Weather at blossom time plays a dominant role in pollination. As temperature falls to
28 degrees F., ice formation within the flower tissue can cause injury to fruit finish. At
27 degrees and below, the styles and ovules can be killed, preventing fertilization. Low
termperatures (below 41 degrees F.) can cause the pollen not to germinate and pollen tube
growth is very slow below 51 degrees. Wind plays no significant role in cross
pollinating tree fruit. However, it does have an adverse effect in insect activity, that
provides for cross-pollination. If cold weather prevails from green-tip to half-inch green
while the pollen is in the process of production, the quality of the pollen may be
seriously reduced. Hot, dry weather, will reduce the stigma receptivity. When the stigma
is receptive to pollen germination, it glistens with a sugary exudate. Once it starts to
dry and turn brown, it is no longer receptive to pollination. Bee flight is reduced by
rain, wind, and cool temperatures, therefore, pollination may fail even though all other
factors are okay.
The effective pollination period has a direct relationship to crop potential. This is
the time needed for the pollen, once it is deposited on the stigma, to grow its pollen
tube to the ovule before it degenerates. This concept was introduced by an English
researcher (Williams). The effective pollination period varies from year to year, and from
orchard to orchard. For apples, it is several days, and for pears (some varieties), in can
be as little as 24 hours.
-There must be a large enough population of bees to transfer the pollen while the
flowers are receptive. There should be two colonies per acre on pears and one colony on
other tree fruits.
-Move bees into apple orchards when the king blossoms have opened. If they are moved
too soon, they will find other sources of nectar and will not work the orchard properly.
If they are moved too late, they will set up too much of the late and less vigorous bloom.
Move colonies into pear orchards when trees are 30 to 50% in bloom. Due to low sugar
content in the pear nectar, the blossoms are not very attractive to honeybees. However, if
the trees are well into bloom, they will work the blossoms for a period until they find a
more attractive source of nectar. If there is only a small amount of bloom, they will
start working more attractive sources almost immediately. For the same reason, it's a good
idea to mow dandelion bloom as the bees are brought in.
With sweet cherries, move colonies in a day or two before the first blossoms open. The
flowers have a short period of viability. Sweet cherries must be pollinated quickly. If
bees are one day late, the crop can be reduced considerably.
-If the bloom is light, bee colonies should be distributed throughout the orchard in
small groups. If bloom is heavy, colonies can be placed in larger groups.
-Protect the bee colonies from wind. Always place the colonies in sunny locations. If
no natural windbreak is available, erect a temporary wind shelter. Bales of straw, old
doors, pallets, tarps, etc, are suitable. It's best to place the colonies where the
morning sun hits the entrances. Make certain there is an uncontaminated water supply
nearby. Keep hives off the ground with pallets, old tires, or concrete blocks. Beehives
setting on the ground leads to stress from dampness and lack of ventilation.
-If dandelion bloom is heavy, mow as bees are placed. Heavy bloom is competition for
the apple bloom. Light dandelion bloom will just help feed the bees.
-Bouquets: If pollenizers are lacking, flowering branches from cross compatible
varieties (crab apples are also excellent) can be placed in barrels or pails of water and
will provide an emergency source of pollen. This is a short term solution, and pollenizers
need to be included in long term pollination planning.
-Beehive inserts placed at the hive entrance and loaded with pollen from a pollenizer
variety can be useful. However, improper handling of the pollen can quickly destroy the
viability. It must be kept cool (thermos chest) and out of sun until it is placed in the
insert. The insert needs to be fed on intervals of a half hour to an hour. Lycopodium
powder (spores of club moss) is sometimes used to dilute the pollen, although this is not
recommended by some, who feel that this irritates the bees, and causes them to rub it off
their bodies before flying to the blossoms.
We have some varieties that are self-compatible, and can set crops without cross-pollination. There are also self-incompatible varieties that need to be pollenized by another variety. There is no intersterility in apricots; any variety can be used as a pollenizer.
Nearly all sweet cherries are highly self-unfruitful, and require cross pollination.
The exception to this statement is Stella. It is self-fruitful, and can pollenize any
other sweet cherry variety.
Bing, Lanbert, Emperor Francis and Napoleon are inter-sterile, and cannot pollenize
each other. Van or Windsor are good pollenizers for these varieties and can in turn, be
pollenized by any of them. Sam is a satisfactory pollenizer for all the above varieties,
including Van. Chinook and Ranier are effective pollenizers for Bing and can be pollenized
satisfactorily with pollen from Bing, Sam, and Van. There are two other groups that are
inter-unfruitful - Black Tartarian and Early Rivers and the other grouping - Abundance and
All commercial varieties are self-fruitful and can be planted in solid blocks. However,
self-compatible cherries still require the presence of honeybees and other insects to
effect pollination. Without honeybees, fruit yields can be reduced.
All peach varieties, with the exception of J. H. Hale, and June Elberta, which are male
sterile, are fully self-compatible and require no pollenizer variety. As with sour
cherries, bees are needed, even though the pollen is only moved within the flower.
However, fewer hives are needed than for fruit that requires crossing.
Bartlett is one of the main varieties, and seems to be affected to some extent by
climate. On the west coast, it can develop without fertilization of the ovule
(parthenocarpic fruit) and is seedless. This is NOT true in the northeast, and the number
of pollenizers in pear plantings should be as large as possible, preferably on a
Bartlett and Seckel are inter-sterile; they will not pollenize each other. Kieffer is
not usually a good pollenizer for Bartlett, since pollen from the late bloom of Kieffer is
sometimes aborted. Flemish Beauty and Dutchess d'Angoviene appear to be self-fruitful.
Although the Italian prune and the Stanley prune are considered self-fruitful, these
vareities benefit greatly from cross-pollination. All other European varieties should be
considered self-unfruitful, requiring other varieties for pollenizing.
Japanese type varieties must be pollenized by other Japanese type varieties, since they
bloom earlier than most European varieties. Although Shiro will pollenize Burbank, Burbank
will not pollenize Shiro - and thus, these two varieties should not be planted together,
without a third pollenizer.
POLLINATION AND HARVEST DATES
for standard apple varieties in the Rochester, NY area:
|Variety||Bloom Date||Annual or Biennial||Pollen Viability|
|R. I. Greening||10-5||Late||B||Not Viable|
|Red Delicious||10-12||Medium||A*||Very Good|
|Golden Delicious||10-25||Late||A*||Very Good|
|Mutsu (Crispin)||10-28||Medium||A||Not Viable|
*These varieties tend to be somewhat biennial, but can be kept annual
most years through good management. This means fruit thinning must be used most years.