Diagram of apple flower.
Pollinating Fruit Crops
Michele R. Warmund
Department of Horticulture
Most fruit crops require pollination to ensure that fruit sets. Pollination
is the transfer of grains of pollen from the anthers (male floral part) to
the stigma (female floral part) of a flower (Figure 1). Pollen grains get
caught on the sticky surface of the stigma, germinate and produce a tube that
grows down the style and unites with the female cell in the ovary. This union
is called fertilization. After fertilization occurs, seeds develop and the
Honeybees are the most important natural carriers of pollen. As the bee
flies from flowers on one tree to those on another in the orchard, pollen sticks
to its body hairs. The bee rubs off the pollen onto the stigma and transfers
additional pollen from the anthers as it visits the flowers. A honey bee may
visit 5,000 flowers a day. Home plantings of fruit crops generally have enough
wild bees for adequate pollination. However, in commercial orchards, beehives
are generally placed in the orchard when the trees are in bloom to enhance
pollination and fruit set. In some fruit crops, pollen is also transferred
by the wind.
Each fruit crop, and even specific varieties within individual fruit crops,
has distinct requirements for pollination. The following terms are used to
describe the pollination characteristics of fruit crops.
The transfer of pollen between two different species or varieties
The transfer of pollen within a single plant or among several plants of the
- Self-unfruitful or self-sterile
Plants in which very little fruit will set
Varieties that set fruit with their own pollen
Varieties that will not set fruit even when cross-pollinated
Neither of two varieties will fertilize the other
An agent (bees, insects, people) of pollen transfer
The plant species or variety that produces the pollen
To ensure that fruit sets, the pollination requirement of the varieties
of a given fruit crop should be evaluated before planting.
All varieties of apple trees should be cross-pollinated with another apple
or crabapple variety. To attain the best fruit set on apple trees, the king
blossom (the largest and first one to open) in the flower cluster must be pollinated.
Thus, the bloom periods of the pollinizer and the king blossom of the apple
tree must overlap.
In backyard plantings, two semidwarf apple varieties that bloom at the same
time should be planted within 50 feet of each other. Two dwarf apple varieties
with similar bloom periods should be spaced less than 20 feet apart to ensure
the transfer of pollen between trees (Figure 2).
Apple and crabapple bloom periods. The shaded area represents the time of bloom.
Although some apple varieties, such as Lodi, Liberty, Empire, Winesap, Jonathan,
Jonagold, Gala, Golden Delicious, Rome and Granny Smith may be listed as self-fruitful,
they will set more fruit on an annual basis if they are cross-pollinated. Additionally,
some apple varieties, such as Winesap, Stayman, Mutsu and Jonagold, produce
sterile pollen and therefore cannot be used to pollinate other apple varieties.
Many nursery catalogues include pollinization compatibility charts (Figure
3) or recommend good apple varieties to use as pollinizers.
Apple pollination. Except where indicated, varieties listed on the
left can be used as reliable pollinizers for cross-pollination.
Manchurian crabapple, with profuse white flowers, is commonly used to pollinate
early- to mid-blooming apple varieties, while Snowdrift crabapple is used for
mid- to late-blooming apple varieties (Figure 2). When using a crabapple
tree as a pollinizer, it should be planted within a similar distance to an
apple tree as listed above.
In situations where a solitary apple tree is planted, branches of open fresh
blossoms of another apple or crabapple pollinating variety can be placed in
buckets of water and hung in the tree. Another way to ensure pollination where
a single tree is planted, is to top-work or graft another apple variety onto
the existing tree. To top-work an apple tree, 6- to 8-inch sections of branches
of one apple variety are cleft-grafted onto terminal branches of another variety.
In commercial apple plantings, a row of pollinizer trees is often planted
between every four rows of the main variety of trees (Figure 4). If pollinizers
are placed within the row, every fifth semidwarf tree is a pollinizer and each
pollinizer is offset in adjacent rows to stagger them throughout the orchard
block. In high-density plantings of dwarf trees (5 to 6 feet between trees
within the row), apple or crabapple pollinizers may be planted between eight
to ten trees of another variety in the row.
Alternative planting plans to ensure cross-pollination of apple trees.
Beehives are generally placed in commercial apple orchards as the king flowers
open. If hives are brought in before this time, bees may forage flowers of
other broad-leaved plants instead of the apple blossoms. For this reason, dandelion
flowers should be removed by mowing or by herbicide treatment before hives
are placed in the orchard. In orchards where semidwarf trees are planted, one
hive of a medium-strength colony (15,000 to 20,000 bees) is generally sufficient
per acre. Two hives per acre are used in high- density orchards where dwarf
apple trees are planted. Extra strong colonies of as many as 50,000 bees have
been effective in pollinating four acres of semidwarf trees under ideal climatic
Most pear varieties are self-unfruitful. However, nearly all pears are suitable
pollinizers for other varieties that bloom at the same time. One exception
is Seckel, which is not a good pollinizer for Bartlett. Even though Anjou,
Bartlett and Kieffer are partially self-fruitful, they should be cross-pollinated
to produce heavy and regular crops. Pear flowers produce only a small amount
of nectar, which is low in sugar. For this reason, more pollinizers and bees
are needed for pears than for any other tree fruit.
Apricot, peach, nectarine and sour cherry
Nearly all common varieties of apricot, peach, nectarine and sour cherry
are self-fruitful (do not require cross-pollination). However, the J.H. Hale
peach is not self-fruitful but can be pollinated by almost any other peach
variety with a synchronous bloom period except Elberta. Self-unfruitful varieties
of apricots include Perfection, Riland and Rival. These self-unfruitful varieties
can be pollinized by any other apricot variety.
Sweet cherry pollination
Stella, Lapins and Starkrimson are self-fruitful. Most other varieties of
sweet cherries require cross-pollination. Several varieties are intersterile
and cannot fertilize each other. For example, Bing, Lambert and Royal Ann (Napoleon)
will not pollinate each other. Refer to Figure 5 and nursery catalogues for
pollinizer recommendations. In commercial plantings, beehives should be placed
in the orchard on the first day of bloom
Cherry pollination. Except where indicated, varieties listed on the
left can be used as reliable pollinizers for cross-pollination.
Most European plums (e.g., Bluefre, Blue Ribbon, Earliblue) either benefit
from or require cross-pollination from another European variety. However, European
or prune-type plums, such as Stanley and Damson, are self-fruitful.
Japanese plums, such as Santa Rosa, Burbank, Redheart, Shiro, Methley and
Ozark Premier, require pollination from another Japanese or an American-Japanese
hybrid. Follow nursery recommendations for suitable pollinizers.
Pollination of small fruit crops
Grape, strawberry, raspberry, blackberry, gooseberry and currant plants are
all self-fruitful. However, blueberry varieties require cross-pollination for
fruit set. Thus, varieties that bloom at a similar time should be placed within
rows or planted in adjacent rows.
Poor fruit set or low yields are often caused by poor pollination or frost
during the period when trees are in bloom. Some of the common reasons for pollination
problems can be the lack of a suitable variety for cross-pollination. Pollination
failures also occur when bloom periods of two varieties used for cross-pollination
do not overlap. Poor climatic conditions during bloom can also adversely affect
pollination. Bees travel shorter distances during cool (below 50 degrees Fahrenheit),
rainy or windy weather. In areas where native bees have been infected with
tracheal or varroa mites, growers should rent beehives from commercial beekeepers
that have strong colonies. Do not spray carbaryl (Sevin) or any other insecticide
that could harm bees during the bloom period. To prevent bees from foraging
flowers of dandelions or other weeds, mow the orchard or control the weeds
with a herbicide before the fruit plants bloom. Wait to move the beehives into
the orchard until the fruit crop has started to bloom and remove the hives
as flowers stop blooming.
Beehive inserts can also enhance pollination in commercial orchards. Inserts
are placed at the hive entrance and filled with pollen from a pollinizer variety.
Pollen can be purchased from a commercial supplier and should be kept cool
and out of the sun until it is placed in an insert. Use a teaspoon of undiluted
pollen every several hours. A total of 1.4 ounces (40 grams) of pollen per
acre is usually recommended.
G6001, reviewed November 2002