Growing tomatoes for seed is not very different from growing them for food. The plants should be started indoors, but it is not necessarily an advantage to start them really early. Here in Vermont you can start tomatoes as late as the third week in April. Smaller plants will suffer less from transplant shock than larger ones, and will catch up very quickly once they are set out into the garden.
When growing any crop for seed be sure your growing season is long enough for the seed to mature. In the case of tomatoes, the seed is ripe when the fruit is ripe, so you just need to know that there’s enough time for the tomatoes to ripen.
Plant tomato sets out after all danger of frost. This usually means around the last week of May. Set them about 3 feet apart in fertile, well-drained soil, in full sun. To encourage a strong root system you can lower the plants into the ground up to the first set of true leaves. The plants will need support in the form of either a tomato cage or a trellis. Don’t over-fertilize or the plant will produce too many leaves and less fruit. Once the fruit is set and starts to ripen, you can prune out some of the leaves at the center of the plant to allow light and air in.
Tomato seed is ripe when the tomato is ripe. There are chemicals in the gel surrounding the seed that prevent them from germinating inside the tomato. To neutralize these you need to ferment the seed. Squeeze the seed and its gel from the ripe tomato into a small container. There should be enough moisture to keep the seeds wet, but the seeds shouldn’t be more than about 1/4 inch deep in the dish. Set the dish in a warm place out of direct sunlight. A screened porch is a good place since the dish will attract fruit flies. Make sure the seeds don’t dry out. After a couple of days there will be a layer of mold on the surface of the dish. The gel will be dissolved so that the seed will be easy to clean. Transfer the seed to a sieve with a screen that is smaller than the seed. Hold it under running water and press on the seed to remove any particles. Once the seed is clean, tap the sieve, wipe the underside with a cloth to remove as much moisture as possible, then transfer the seed to a ceramic or plastic dish or a pie plate. Don’t use a paper towel or plate since the dry seed will stick and be hard to remove. Separate the seeds as much as possible to hasten drying. Let the seed dry completely in a well-ventilated spot out of direct sunlight before storing it in a cool, dark, dry place.
Carefully stored tomato seed can remain viable for up to 10 years, but it is best to grow it out at least every 3 to 5 years.