Acotanc: Tree Crop Pollination in California

Tree Crop Pollination in California

Author: Joe Traynor
E-mail: [email protected]

For more information see:
Organization: Scientific Ag.Co.
P.O. Box 2144, Bakersfield, CA 93303
+1 805-3272631
E-mail: [email protected]
ATCROS Reference: A1627.


California's Mediterranean climate makes it ideal for growing a number of tree crops. With the exception of olive, all the tree crops discussed herein are deciduous (they lose their leaves in the winter and enter a dormant or resting phase before blooming in the spring).

The 2 tables below divide California's tree crops into 2 categories: wind pollinated crops and insect pollinated crops. Wind borne pollen is structurally different from insect carried pollen and the differences can be seen under a microscope. Wind borne pollen can travel well over a mile under proper conditions while the heavier insect carried pollen is lucky to make it 10 feet during a breeze. (except for olive pollen, an oddity, in that it is both wind borne and carried by insects).

Monoecious (M)
or Dioecious(D)
Self fertile (SF)
or Self Sterile SS)
Chestnut200M SS
Pistachio 71,000DSS
Walnut 191,000MSF


  • Non bearing acreage is usually 5 to 10% of bearing acreage
  • Monoecious plants bear male and female flowers on the same plant
  • Dioecious plants bear male and female flowers on different plants
  • Self Sterile crops must use pollen from a different variety to set
  • Except for Filbert, most of these crops bloom in April, May
  • Filbert is also known as Hazelnut
  • The state of Oregon has the most Filberts (30,000 acres)
  • Texas and Georgia have the most Pecan acreage
  • 99% of the Pistachio acreage is the Kerman variety

Crop Bearing
BloomtimeBee rental
per colony
Almonds500,000mid Feb.$45 55 
Apples 35,000March April0-25 
Early cherries4,000 mid Feb.$45-55 Dormex or calcium nitrate
sprays give early bloom*
Normal cherries15,000 March10-25
Pears19,300 March $10-25Includes 15,000 ac. Bartletts**
Plums42,000 mid Feb.$45-55 
Prunes86,000 March0-15  

*early cherries command a premium price
**Bartlett pears in California usually set without pollination

Dates - A Unique California Crop

There are about 5,000 acres of date trees in California, confined mainly to the desert area in and around the Coachella Valley. The dioecious date tree is usually pollinated by hand (1 man can cover only an acre a day) or by a machine that blows pollen to the female flowers (a machine can cover 20 to 40 acres a day).

Two Curious Cases - Walnut and Filbert

Too much pollen reduces the set on walnut by causing pistillate flower abscission (PFA). Some varieties (e.g., Serr) are more susceptible to PFA than others. It has been conclusively demonstrated that if too much walnut pollen is available, pistillate flowers can abscise. This phenomenon puzzled California researchers for years because emphasis was always placed on the importance of maximizing pollination for walnuts (and most other tree crops). Walnut growers solved the problem by reducing catkin numbers prior to the pollination season. It is possible that other wind pollinated tree crops could have a similar problem. No such adverse effect (from excessive pollen) is known for insect pollinated crops; in fact, it is felt that the more pollen grains that land on the stigma (of insect poilinated flowers) the better because it has been shown that individual pollen tube growth is stimulated by surrounding pollen grains.

Filberts are unique in that pollination occurs in late January, but fertilization (the joining of the pollen tube and ovule) does not take place until May or June. Following pollination (the deposition of pollen on the stigma) the pollen tube grows to the base of the style and remains there in a resting stage while the ovary matures.

Bee Rental Prices

The wide range of bee rental prices (for insect pollinated tree crops) is due, as one might expect, to supply demand. 500,000 acres of almonds (and 42,000 acres of plums) using about 2 honey bee colonies per acre creates a tremendous demand for bees in February. When almond bloom is over (by mid March) pollination prices plummet due to reduced demand.

When it is considered that a honey bee colony can be purchased for $50, the question arises: why don't growers buy, rather than rent, bee colonies? The answer is, that it takes superior beekeeping skills + about $50 per colony to keep colonies alive throughout the year. Mite control, trucking and feeding are the main costs. These costs, combined with depressed honey prices (due to global competition) have caused a decline in U.S. bee colony numbers. Most U.S. beekeepers are losing money.

Present and Future considerations

With a global economy, both U.S and California farmers are going through very difficult times. 1 million acres of apples in China has severely damaged the U.S. apple industry. Global competition from other tree crops has made them less profitable. Two crops, almonds and pistachios, have fared better than most. The unique climatic requirement for almonds (winters cold enough to break dormancy, but not so cold as to damage flowers ) can be met in only limited areas in the world. California pistachio growers have enjoyed good domestic markets due to sanctions agains Iranian pistachios. Improved U.S. relations with Iran could depress the pistachio market (in 1999, Iran reportedly produced 154 million pounds of pistachios while the U.S. produced 122 million pounds).

As California farmers remove acreage of less profitable crops (including wine grapes, which receive stiff competition from Australia and other countries) almond acreage has increased. A significant question is whether increased global demand for almonds will maintain almond prices at their current levels.