Written by Geoffrey Njue, former SDSU Extension Specialty Crops Field Specialist.
Proper pollination is needed for optimal fruit set and production of tomatoes. The female parts of the tomato flower (style and stigma) are enclosed inside the male parts (stamens and anthers). When anthers mature they open and release pollen. For successful fruit development sufficient quantity of pollen must be transferred to the stigma for the fertilization of the ovules. The size and weight of the tomato fruit correlates directly to the amount of pollen transferred to the female part of the flower. Pollen released without vibrating the flower is not sufficient to produce high yield of good quality fruit.
In field-grown tomatoes the air movement (natural wind) is sufficient to vibrate the flowers to achieve good pollination. This is not generally true in the high tunnel because there is not sufficient natural wind to vibrate the flowers. High tunnel tomato growers should therefore pollinate their crop by other means.
High Tunnel Pollination Methods
There are several means by which growers can use to pollinate their crop. To achieve successful pollination these should be applied during the middle of the day when the humidity in the high tunnel is 60-80%. Tomato pollination/fertilization is also influenced by temperature and humidity. Low night temperatures (below 55° F) inhibits pollen development and high day temperatures (90°F and above) leads to flower abortion. High humidity conditions (above 80% relative humidity) cause pollen grains to stick together, resulting in poor or nonexistent pollination.
There are four means by which growers can pollinate their high tunnel tomatoes:
Tapping the Support strings or Wires
Tapping on the support strings or wires help to agitate the plant and vibrate the flower cluster. Tap on the support strings to vibrate the flower cluster every other day or at least three days per week. When using this method work on one row at a time to avoid missing some clusters. This method is sufficient for small growers.
These vibrators can be purchased from any greenhouse supply store. You vibrate the flowers by touching the stem of the flower cluster for a few seconds. The vibration releases enough pollen to fertilize the ovules. Do not touch the flower itself to avoid creating a hole on the developing fruit. Work on one row at a time to avoid missing some clusters. Pollinate the flowers every other day on sunny days.
Household leaf blowers operated at normal speed with air flow directed to the flower clusters can be used to pollinate high tunnel tomatoes. Direct the air stream at individual clusters 7-8 feet down the row to avoid blowing pollen away from the flowers. Work on one row at a time to avoid missing some clusters. Use the air blower at least three times a week. Air blowers take half the time to pollinate the plants compared to battery operated vibrators. However the quality and yield of fruit is lower (smaller fruit size and weight) because either less pollen is deposited on the stigma or pollen is blown away by the air stream. Gasoline-powered air blowers should not be used in the greenhouse to avoid ethylene toxicity on the plants.
Bumble bees are excellent pollinators for greenhouse and high tunnel tomatoes. Bees vibrate the flower as they collect pollen for feeding. Bees are active from sunrise to sunset and at low temperatures (as low as 41°F), and windy and cloudy conditions. Excellent pollination results in larger size and heavier fruits. Using bumble bees in small high tunnels can lead to over pollination and injury to the flowers because even the smallest hive has more bees than a small high tunnel can sustain. Bumble bees are available from most Biological Pest Control Companies. Bumble bee colonies are shipped to growers in completely maintenance-free hives. The housing is made of solid, recyclable cardboard with a moisture resistant coating. The hive has two flight openings. Work with the company representative to determine the size of the hive that will work for the size of your high tunnel.