Saving tomato seeds by fermentation, step by step

A tomato pleased you this year. Good taste, fine harvest, it held up well in the garden. You would like to grow it again next year without buying a packet. You need a glass, some water and a few days. The method surprises people a little, because you let a mould form. In short, you scoop the seeds with their gel into a glass of water. They ferment for 2 to 4 days. You rinse and sort them. They then dry for 5 to 7 days. Only one condition really matters. The variety must be open-pollinated.

Is your variety open-pollinated?

An open-pollinated variety (heirloom, or conservation variety) gives plants that look like the mother plant. An F1 hybrid comes from a controlled cross between two parents, made by a seed company. Its seeds do not reproduce the plant. According to Oregon State University, they give a mix of types, most of them worse than the parents. The University of California extension says the same.

The tomato varieties in the Vegiwise shop are open-pollinated. Marmande, Cœur de bœuf, Triomphe de Liège and Potager de Vilvorde all suit. If you do not know the variety of a tomato, try anyway. You will see the result the following summer.

The tomato is one of the easiest vegetables for saving seeds. Its flowers carry both male and female parts. It pollinates itself and does not need insects to set fruit (Seed Savers Exchange).

Choose the plant, then the fruit

Choose the plant in July, while you can still compare it with its neighbours. Tie a ribbon on the plants you keep so you can find them again.

• A vigorous, healthy plant. Rule out a weak or sick plant. Your seeds could pass on its weakness (UC).
• Fruit true to the variety in size, shape and colour.
• The taste, the hold or the earliness you like. You choose your own "best".

When the time comes, pick a fully ripe fruit, neither green nor overripe, with no rot or deformity (UC, Oregon State).

A few fruits are enough to start (Seed Savers Exchange). To keep a variety going year after year, the organisation advises collecting from 5 to 10 plants. UC recommends several fruits, from several plants if possible, to keep diversity.

Two varieties grown side by side can be crossed by an insect. Seed Savers Exchange advises separating them by 3 to 15 metres (10 to 50 feet). Other sources give between 2 and 15 metres. UC offers another solution. Cover a cluster of still-closed flowers with a cloth or paper bag, never plastic, for a few days. Then mark the cluster with a label.

Fermentation in four steps

Get a glass, water, a fine sieve, a plate and a pen.

1. Extract (5 minutes). Cut the fruit in half. Scoop the seeds and their gel into a glass labelled with the variety name. Add 60 to 120 ml of water. The rest of the fruit is for eating.
2. Ferment (2 to 4 days). Cover the glass with a piece of tulle or a cloth. Put it out of the sun, at room temperature. Oregon State gives 72 hours between 22 and 27 °C. Above 35 °C, the seeds get damaged. A white film forms on the surface. This is normal, it is even the signal you want. Move on to rinsing as soon as it covers the surface well. Fermenting too long can make the seeds sprout or rot.
3. Rinse and sort (10 minutes). Gently remove the film. Fill the glass with water, stir and let it settle. Healthy seeds sink. The pulp and empty seeds float. Pour off the water with the pulp and the empty seeds. Repeat until the water stays clear. Rinse one last time in the sieve, under the tap.
4. Dry (5 to 7 days). Drain. Spread the seeds in a single layer on a plate, in the shade, in a dry, airy room. A cardboard plate suits well because it absorbs water. On a ceramic plate, stir the seeds with a finger several times a day. Avoid kitchen paper, the seeds stick to it (UC). A dry seed snaps instead of bending (Seed Savers Exchange).

Why ferment rather than just rinse

Each seed sits in a gel that stops it from sprouting inside the fruit. Fermentation destroys this gel. It also helps separate the good seeds from the bad ones. And it reduces some of the diseases carried by the seed (UC).

The Californian extension admits it plainly. You do not have to ferment. Without fermentation, you peel off the gel by hand and the sorting is less reliable. You also lose the reduction in seed-borne diseases. Fermentation does not disinfect everything, though. Never take seeds from a sick plant.

Store, label, test

Once the seeds are fully dry, store them.

• Put them in a paper envelope, then in an airtight jar. Keep the jar somewhere cool, dry and dark. A sachet of silica gel absorbs leftover moisture.
• Label the envelope with the variety and the year of harvest. Also note in a notebook the original plant, the harvest date and your impressions.
• The fridge or the freezer work, in an airtight container. Let it come back to room temperature before opening it, to avoid condensation (UC).

A tomato seed keeps for a long time. The University of Illinois table gives about 4 years. Seed Savers Exchange speaks of 5 to 10 years, cool and dry. Count on 4 to 6 years.

Before sowing an old packet, do the test. Lay 10 seeds on a damp paper towel, in a closed bag, somewhere warm, for 7 to 14 days. If all 10 sprout, sow normally. If 7 to 9 sprout, sow a bit more densely. At 6 or fewer, renew your stock (University of Illinois).

Your homemade packet can also go in the Vegiwise app. It tells you when to sow it where you live.

And other vegetables?

The tomato is easy because it pollinates itself. Other vegetables need more care. The ranking below is based on the Oregon and California university leaflets.

Saving your seeds for your own garden is no problem at all. Selling or swapping seeds on a regular basis follows European rules, which have changed in recent years. Find out before you start. Our guide Open-pollinated seeds, F1 and heirloom varieties takes stock.

Further reading

• The collection: Tomate Lovers
• The Start page: Tell us which seeds you have, and we tell you when to sow each one where you live
• Three other guides: Open-pollinated seeds, F1 and heirloom varieties, The Triomphe de Liège tomato and Sowing calendar for Belgium

Photo: Mustafa Akın, Pexels