Tomato seeds sit inside a wet gel. The gel slows sprouting. Gardeners ferment the seeds in a jar for a few days to break the gel down. The method costs nothing and works with a jar, a spoon, and a sieve.
Start this in early fall. In most of Canada, tomatoes need to come off the plant before hard frost. Seeds from fruit picked in September and early October give you time to dry them before winter.
Pick the right tomatoes
Choose fruit from a plant you want to grow again. Look for a healthy plant with fruit that tastes good and ripened well in your season.
Pick tomatoes that are ripe. Skip green fruit that never colored, since its seeds are not mature. Skip fruit from plants with leaf spots, blight, or wilting.
Check the seed source. Seeds from open-pollinated and heirloom varieties grow into plants like the parent. Seeds from F1 hybrids do not. The plants that grow from them can differ from the parent plant in size, shape, and flavor. If the original packet says F1, keep the seed for your own curiosity and leave it out of a swap.
Think about crossing
Tomato flowers mostly pollinate themselves. Most varieties come true from saved seed. A few types have flowers where the center sticks out past the petals. Bees can reach those flowers and carry in pollen from a neighboring plant.
Grow two varieties apart from each other if you plan to save seed. Another option is to cover a flower cluster with a small mesh or paper bag before the flowers open. Write “possible cross” on the packet if another tomato grew close by. The next gardener will want to know.
Ferment the seeds
Cut the tomato in half across its middle. Squeeze or scoop the seeds and gel into a clean jar or cup. Eat the rest of the tomato.
Add a splash of water. The mix should look like a loose slurry. Write the variety name on a piece of tape and stick it to the jar. Ferment one variety per jar.
Cover the jar with a paper towel or a cloth and hold it in place with a rubber band. Air needs to get in. Keep the jar at room temperature, out of direct sun. A kitchen counter works. Stir the mix once a day.
A layer of white or gray mold will form on top within a few days. That is normal. The smell turns sour. After two to four days, the gel is gone and the good seeds have sunk to the bottom. Cooler rooms take longer.
Do not leave the jar for a week. Seeds can sprout inside it. Stop when most seeds have sunk and the gel no longer clings to them.
Rinse and sort
Skim off the mold. Fill the jar with water and stir. Good seeds sink. Pulp, scum, and empty seeds float. Pour the floating material off slowly. Repeat until the water runs clear and only the heavy seeds remain.
Pour the seeds into a fine sieve and rinse them under cool running water. Rub them gently with a fingertip. The seeds should feel clean and a little rough, not slimy.
Dry the seeds
Tip the seeds onto a glass or ceramic plate. Paper towels glue them down, so avoid them. A coffee filter works if the seeds are spread thin. Put the plate indoors, away from sun and heat.
Stir the seeds once a day to keep them from clumping. Break up any clumps with a fingertip. Most seeds dry in one to two weeks. A dry seed is stiff and does not bend. If a seed still feels soft or sticks to the plate, give it more time.
Damp seeds mold in storage. If you doubt they are dry, wait a few more days.
Skip the fermenting if you need to
Some gardeners rinse the gel off in a sieve and dry the seeds right away. This is faster. It works for seed you will plant next spring in your own garden. Fermenting is the better choice for seed you plan to store for years or give to other people. The sour stage also kills some disease organisms that cling to seeds.
Package the seeds
Put the dry seeds in a paper envelope. Label the envelope with the variety name, the plant type, the year, and the town and province where it grew. Pencil or a waterproof pen works best. The full list of label details, plus advice on storage and mailing, is in how to package and label seeds for a swap.
A small pinch is enough for a swap packet. Ten to twenty seeds give another gardener a few plants and a spare. Tomatoes produce a lot of seed, so most gardeners have plenty to share.
Keep the packets in a jar or tin with a tight lid. Store the jar somewhere cool, dark, and dry. A closet shelf is better than a garage or shed. Tomato seed stays alive longer than onion or parsnip seed, though germination drops as the seed ages.
Seed Swap and your tomato seeds
Seed Swap does not have a swap list yet, and there is no way to post one yet. Both are coming. Tomato seeds you ferment and dry this fall will be ready when they arrive.