Most varieties are not capable of cross-fertilisation. The exceptions are currant tomatoes, the potato-leafed types, and beefsteak varieties with double flowers. All these have a stigma which protrudes from the anther tube, making it accessible to insects. The stigma is visible with a hand lens.

To prevent crossing a paper bag or a piece of fleece can be tied around tomato trusses when they are in flower. It can be removed after the fruit has set.

Cut ripe tomatoes in half and squeeze out the seeds and juice. Allow the seeds and pulp to ferment in a warm place for three days, to release them from the jelly that surrounds them, before filling up the container with water and stirring vigorously. Allow the seeds time to settle on the bottom and then pour away the liquid on top. Rinse and repeat until only good seeds remain. Strain away any remaining water and spoon out the seeds on to a plate to dry. They will stick to a pice of paper. Place the plate in a well-ventilated place out of the sun and, from time to time, stir the seed to break up the clumps and allow even drying. After several days when they are completely dry, put them in an airtight container. They will keep for about six years.