Why save tomato seeds from year to year when buying packaged ones each year is so convenient? Because it's easy and satisfying, especially when the seeds you save are heirloom varieties. September, when tomato production begins to wind down in Western gardens, is a good time to harvest seeds. Before you start, here are a few guidelines.
Preserve seed only from nonhybrid (open-pollinated) tomatoes. They produce offspring just like themselves, with only slight variations. On the other hand, hybrid tomatoes, which include most modern varieties, produce offspring that won't necessarily look or taste the same as the parents.
Preserve seed that hasn't been cross-pollinated. All tomatoes are self-pollinating, but a few kinds (currant or potato-leaf types like 'Brandywine') can be cross-pollinated by some insects. If you're not growing currant or potato-leaf types, or you're growing just one of these in addition to other types of tomatoes, you can save seed from this year's harvest.
To prevent cross-pollination in the future, cover flowers with a bag made from cheesecloth or spun-polyester fiber (available at nurseries) before blossoms open. Tag the covered flower stem with brightly colored yarn. Remove the cover when fruits are developing.
Harvest fruits when they're thoroughly ripe and soft. Tomato seeds are enclosed in a gel sac; to remove the sac and to help destroy seed-borne diseases, put them through a fermentation process.