Why Save Your Own Tomato Seed

There is something quietly satisfying about closing the loop on the plot: a tomato you liked enough to save now paying you back next spring in seedlings. Beyond the creeping cost of seed packets, saved seed adapts. Select year after year from plants that ripen early, crop hard and shrug off blight in your particular corner of the country, and you gradually build a strain suited to your soil and your weather rather than to a breeder's trial ground.

It only works properly with open-pollinated varieties. Heritage and heirloom types come true from seed. F1 hybrids will not — their offspring split back into all manner of shapes, colours and flavours, and most will disappoint. If your best tomatoes are F1s, enjoy them and buy fresh seed next season instead.

Choosing Which Fruit to Save From

The seed is only as good as the plant it came from, so be ruthless. Saving from a sulky, disease-ridden specimen because it happens to have a few fruits left is a false economy; you will simply grow more sulky plants next year.

• Pick from your healthiest, heaviest-cropping plants, and from the fruit you would choose to eat.
• Take fully ripe tomatoes from the second or third truss rather than the first. Early fruit often carries smaller, less vigorous seed.
• Avoid anything with blossom end rot, splits, mould or signs of disease.
• Save from at least three separate plants where you can, to keep some genetic diversity in your strain.
• One or two fruits is plenty — a single good tomato can yield fifty or more seeds.

If a tomato is not quite ripe, leave it on a sunny windowsill for a few more days. The seed inside continues to mature after picking, though green fruit picked in October will never give you the same quality as a properly ripened one in August.

Fermenting the Seed

Tomato seed sits inside a gel sac that stops it germinating inside the fruit. Left alone, that gel also encourages mould, so the traditional fix is a short fermentation. It mimics the fruit rotting on the ground, dissolving the gel and knocking back seed-borne troubles such as bacterial canker and bacterial spot.

• Cut the fruit in half across the equator and squeeze or scoop the seeds and gel into a small jar.
• Add roughly twice as much water as seed pulp. Do not fill the jar more than half full.
• Cover loosely with a lid or a piece of cloth to keep flies out, and stand it somewhere warm — a kitchen windowsill or airing cupboard at around 18–24°C is ideal.
• Stir once a day. After three to five days a layer of mould will form on the surface and the good seed will have sunk to the bottom.
• When the mixture smells distinctly sour, it is done. In cool weather it may take a week; in a heatwave, two days is enough.

Watch it closely in warm spells. Leave it too long and the seed will start to sprout in the jar, which ruins it.

Rinsing and Drying

Once fermented, add plenty of water, swirl gently and pour off the mould, the pulp and any seeds that float — these are usually hollow and useless. Tip the rest into a fine sieve and rinse under a cool tap until the seeds are clean and no longer slippery.

Shake off the excess water and spread the seed thinly on a china plate, a coffee filter or a sheet of greaseproof paper. Do not use kitchen towel, as the seeds weld themselves to it and you will lose half of them trying to peel them off.

Leave them in a dry, airy room out of direct sunlight for two to three weeks, stirring gently every day or two to stop them clumping. They are properly dry when they snap between your fingernails rather than bending. Any trace of moisture left in storage means mould, and mould means a wasted year.

Storing and Labelling

Label as you go, not from memory in February — every saved tomato seed looks identical once dried. Write the variety, the date and any useful note, such as "early, good flavour, from plot 3".

• Keep seed in small paper envelopes, which breathe, rather than sealed plastic bags.
• Stand the envelopes in an airtight tin or jar with a sachet of silica gel to soak up any lingering damp.
• Store somewhere cool, dark and steady — a cupboard in an unheated room is ideal. Avoid sheds that swing between damp and baking.
• Labelled and dried properly, tomato seed germinates well for five or six years, and often longer.

Keeping Varieties True, and Sowing Next Spring

Tomatoes are largely self-pollinating, so saved seed usually comes true. Crosses do happen, though, especially between varieties growing close together and particularly with potato-leaved and beefsteak types, which have more open flowers. If you are growing several varieties in a small greenhouse, either leave a good gap between them, grow only one variety per season, or tie a small paper bag over a truss of flowers for a few days to keep the bees out. It is not an exact science, and a surprise cross now and then is half the fun of saving seed.

Come late February or March, sow your own seed in a warm propagator exactly as you would a bought packet, pricking out the strongest seedlings. Sow a few more than you need to allow for the odd dud, and give the spares to a neighbour on the allotment. That, more than anything, is what saving seed is for.