Karl Foord, Extension Educator - Horticulture
Tomatoes are heat loving plants that require a long frost-free season and full sun. The flowers have optimal temperature ranges and are sensitive to extremes. The flowers will abort and drop from the plant when extremes are experienced. In early spring when night temperatures drop below 55 degrees F, the blossoms can drop. In summer when daytime temperatures exceed 90 degrees F or when nighttime temperatures remain above 75 degrees F, the flowers can abort. In addition drought stress can also cause flower abortion.
The Tomato Flower
The tomato flower is hermaphroditic containing both male and female organs (Figure 1). The flower is self-fertile but not self-pollinating. The tomato anthers fuse to form a tube that surrounds the style. Pollen is released from pores located on the inside of the tube as shown in Figure 2. The flower requires vibration to shake the pollen grains from the anther pores to the stigmatic surface.
Sufficient pollen movement to achieve maximum fruit set is usually achieved when plants are grown outside and subject to vibration by wind. One reason this works is because the stigma is receptive for a relatively long period of time e.g. from 1-2 days before anther dehiscence to 4-8 days after dehiscence.
Plants grown in glasshouses do not receive enough vibration and are usually pollinated by purchased bumble bees. Bumble bees effect "buzz" pollination by vibrating their flight muscles without moving their wings (Photo 1). This serves to shake the pollen from the anthers where it falls on the stigmatic surface and the body of the bee.
Honeybees will not visit tomato flowers because the flowers have no nectaries and the structure of the flower makes access to pollen difficult.
Find ways to protect your tomato plants from low temperatures in early spring with either water jackets of various types, or row covers to hold ground heat around the plant.