Common Ground Suffolk, How to save seeds from your garden

Best time June – October

Drying time 2 – 4 weeks

Difficulty Easy to moderate

Viability 1 to 5+ years

Most seeds can be saved with nothing more than a paper bag, a dry windowsill, and a cool cupboard. The process is straightforward, but there are a handful of points where it is easy to go wrong, especially around timing, drying, and knowing which plants are worth saving from in the first place.

Start with the right plants

The most important rule in seed saving: only collect seed from open-pollinated or heirloom varieties, never from F1 hybrids. If the packet says “F1”, the seeds you save will not produce plants that match their parent. The offspring of F1 hybrids are highly variable, often weaker, and will rarely resemble what you grew. This is not a fault in the saved seed; it is simply how first-generation hybrids work.

Open-pollinated varieties have been bred over many generations by selecting for consistent traits. Their seeds come true: you grow the same plant, year after year. Heritage and heirloom varieties are a subset of this, often with more distinctive flavours and appearances than modern commercial types.

Self-pollinators and cross-pollinators

Some plants are far easier to save seed from because they fertilise themselves before insects can carry pollen from a neighbouring plant. Tomatoes, French beans, runner beans, peas, and lettuce all fall into this category. Even if you grow several varieties side by side, there is very little risk of the seeds crossing.

Cross-pollinators require more care. Brassicas (kale, broccoli, cabbage, Brussels sprouts, and the rest of that family) will freely cross with any other brassica in flower nearby. Courgettes can cross with squash and pumpkins if they flower at the same time. To save pure seed from these plants, grow only one variety of each per season, or use hand-pollination and bagging to control which flowers set seed.

Knowing when to harvest

The main seed saving season in the UK runs from June to October. Ripe seeds announce themselves clearly: pods and seed heads turn from green to brown or black and become dry and papery; seed heads rattle when shaken; the plant itself often begins to die back.

Unripe pods will not ripen once they are picked. Harvest too early and the embryo inside the seed will be underdeveloped and will not germinate.

Always harvest on a dry day. Seeds that are wet when collected are far more likely to rot or mould in storage even after drying. If wet weather is forecast and your pods are nearly ready, cut the whole stem and hang it upside down in a dry shed or garage to finish drying naturally.

Dry seeds: pods, heads, and capsules

Most garden plants produce dry seeds. Beans, peas, sunflowers, sweet peas, poppies, borage, calendula, and most brassicas and root vegetables (if allowed to bolt and flower) all fall into this group. The technique is the same for all of them.

Wait until fully dry on the plant – pods or seed heads should be brown and papery, seeds rattling loosely inside.
Harvest into a paper bag or tray on a dry day and bring indoors immediately.
Shell or thresh the seeds out – rub pods between your palms, tap seed heads on a hard surface, or shake in a jar.
Winnow to remove chaff – pour seeds from one bowl to another in a light breeze, or in front of a fan on its lowest setting. Heavier seeds fall; lighter chaff blows away.

Wet seeds: tomatoes, cucumbers, and squash

Seeds from fleshy fruits need to be separated from the surrounding pulp before drying. The method differs between tomatoes and most other crops.

Tomatoes: fermentation required

Tomato seeds are surrounded by a gel-like coating that inhibits germination. This coating must be removed before the seeds can be stored or sown successfully. Fermentation removes it naturally, mirroring what would happen if the fruit rotted on the ground.

Scoop seeds and gel from a fully ripe tomato into a glass or jar with a small splash of water.
Leave at room temperature for 2–3 days. A layer of mould or white fuzz will form on the surface; this is normal and expected.
Check viability: healthy, viable seeds will sink to the bottom. Non-viable seeds, gel, and debris float. Pour off the top carefully.
Rinse the sinking seeds several times under cold running water until the water runs clear, then spread on paper to dry.

Courgettes, squash, and cucumbers

These do not require fermentation. Scoop seeds from a fully mature fruit (for courgettes and squash, this means a fruit that has been left on the plant well past eating size and has developed a hard outer skin). Tip the seeds into a sieve, rub away any remaining fibres under running water, and spread on a tray to dry.

Drying your seeds properly

Thorough drying before storage is the single most important step. Moisture left in saved seeds leads to mould in the packet and dramatically shortens how long they remain viable. Spread seeds in a single layer on paper towels, a mesh tray, or a ceramic plate in a warm, airy place away from direct sunlight. Leave them for two to four weeks; thick squash seeds take considerably longer to dry than small flower seeds.

To test whether seeds are fully dry, try to snap one between your fingers. A properly dry seed snaps or crumbles cleanly. A seed that still contains moisture will bend or flex without breaking. Do not use an oven or any direct heat source to speed things up: even gentle heat above 35°C can damage the embryo and kill germination.

For very small seeds such as poppies or carrot, a paper coffee filter works well as both a drying and storage container. Pour the seeds in, fold the edges over, and label the packet.

Storing your seeds

Seeds need to be kept cool, dry, and dark. Heat and fluctuating humidity are the two main causes of loss in home seed collections. The RHS recommends storing paper packets inside an airtight container with a sachet of silica gel to absorb any residual humidity. Silica gel is available inexpensively from craft suppliers and hardware shops.

Use paper envelopes rather than plastic bags. Paper absorbs residual moisture; plastic traps it.
Store the envelopes inside sealed glass jars or airtight tins with a silica gel sachet.
Keep in the fridge if possible (below 5°C is ideal) or in the coolest, darkest cupboard available. A warm kitchen worktop or garden shed will significantly shorten viability.
Label every packet with the plant name, the variety name, and the year harvested. Seeds look almost identical in storage and memory is unreliable come spring.

How long do saved seeds remain viable?

Viability varies considerably by species, even under identical storage conditions. The figures below assume reasonably good storage (cool, dry, dark).

1–2 years: onions, leeks, parsnips, parsley, sweetcorn
3–4 years: beans, peas, beetroot, carrots, kale and other brassicas, most annual flowers including sweet peas and cornflower
5 years or more: tomatoes, cucumbers, squash, pumpkins, calendula, nasturtium

Onions and leeks are notoriously short-lived: save fresh seed every year and do not rely on packets more than 12 months old. At the other end of the scale, tomato seeds stored correctly at very low temperatures have shown average germination rates above 58% after more than 35 years of storage.

Testing viability before you sow

If you are unsure whether stored seeds are still viable, test them before committing a whole bed. Lay ten seeds on a damp piece of kitchen paper, fold it over, seal it inside a plastic bag, and place somewhere warm (around 15–20°C). Check after one to two weeks.

Eight or more germinating: good seed, sow normally. Four to seven: still viable but sow more thickly to account for lower germination. Fewer than four: viability is too low for reliable results and fresh seed is a better investment.

How to Save Seeds from Your Garden