How to Test Whether Old Seeds Will Still Grow

Before you rely on a tin of seed from last year, check it. The simplest test is a jar of water: put medium to large seeds in a jar of water, and viable seeds will sink while dead seeds float to the surface¹. For smaller seeds, sow a pinch on damp kitchen paper or in a small pot of compost and see how many come up. Either way, the point is to find out now, while there is still time to buy fresh seed or change your plan, rather than at sowing time.

The second thing to settle is what kind of seed you have. Seed saved from F1 hybrids will not grow true to type, so if you want to save your own seed and stop depending on a seed company, you need to start from open-pollinated varieties². Garden Organic's guidance is blunt on this: "You will need to save seed from open-pollinated varieties (not F1 hybrids)"².

Once you have open-pollinated seed, saving it comes down to two simple rules. Garden Organic states them as: "Preservation of seeds has two simple rules: Remove undesirable individuals and prevent cross-pollination"². Everything else, from bagging flower heads to leaving a carrot in the ground over winter, is detail hanging off those two rules.

Start with open-pollinated varieties, not F1 hybrids

An F1 hybrid is a first-generation cross between two parent lines, bred so the offspring are uniform. That uniformity is why the catalogues sell them: modern F1 sweetcorn varieties are described as ideally suited to growing in most of the UK, where older varieties were difficult⁵, and many of the highest quality celery varieties are F1 hybrids bred for good yields in our climate⁶. Hybrid courgette varieties are also described as often having better disease and pest resistance⁷. None of that makes them a bad choice for eating.

It does make them a dead end for saving. Seed saved from an F1 hybrid will not grow true to type², so the plants you raise from it will be variable and unpredictable. If your aim is a seed supply you control, the parent line has to be open-pollinated. The case for open-pollinated varieties is that their variability is an advantage: it allows some degree of adaptable resistance to diseases, pests and stress, and lets future breeders select characteristics². Real Seeds makes the same point in plainer terms, describing open-pollinated seed as a huge living genebank that was very resilient against disease or climate change, so that if your cabbages did not do well, someone down the road had a slightly different one that would cope⁸.

If you are not sure what you have, the packet or catalogue entry is the place to look: F1 varieties are labelled as such. A tomato sold as "Cherry, 'Sungold', F1 Hybrid" is telling you exactly what it is¹⁰. Where the label is lost, treat the variety as unknown and save from a plant you are certain of instead.

Two rules: remove poor plants, prevent crossing

The first rule is to remove undesirable individuals. In practice that means saving seed only from healthy looking plants¹, and sowing more than you need so there is something to rogue out: Garden Organic advises considering sowing extra to allow for roguing out, and notes that some of its guidelines give recommended minimum numbers of plants².

The second rule, preventing cross-pollination, is where most of the work lies, and how much work depends on the crop. Garden Organic splits vegetables into two groups:

With outbreeding vegetables, letting varieties mix widens genetic variability, and rectifying that takes patience and several seasons². That is why isolation is worth doing properly the first time rather than trying to sort it out later.

Some crops need more than isolation. Pumpkins, squashes, marrows and courgettes will all cross readily with each other, and Real Seeds describes hand pollinating one or more fruits as the best, usually only, way to save pure seed on a home scale¹¹. Squash varieties easily cross pollinate, and hand pollination is important if you are saving seed¹¹. Sweet peppers cross very easily too: either grow only one variety, or keep the insects out with a net cage or by bagging individual flowers¹². Parsley is insect pollinated, so flat and curly leaved varieties will cross and you should grow only one type for seed at a time¹¹.

Keeping varieties apart in practice

For beetroot, chard and leaf beet, Real Seeds gives a method that needs no cage: plant at least six plants very close together in a circle with a wooden stake in the middle; as the seed stalks form, growing up to four feet tall, tie them together supported by the stake; then cover the group of flower heads with either a shiny paper bag that will withstand rain, or a bag made out of agricultural fleece, shaking the bag from time to time to make sure pollen is distributed within the bag¹¹.

For other crops the approach is simpler: grow only one variety for seed in a given year, or use distance and physical barriers. Basil is easily cross-pollinated, so if you are growing two varieties together, let only one variety flower and seed¹. Coriander, dill, parsley, basil and chives can all be grown from saved seed¹.

There is a practical limit on how small a gene pool you can save from. For carrots, Real Seeds advises saving seed from at least 40 good roots to maintain good genetic diversity, warning that with too small a genetic pool you will end up with small, poor quality roots in a very few generations¹¹. That is a bigger number than most gardens will manage, and worth knowing before you commit a carrot variety to a saved-seed future.

Some plants are easier to keep going than to isolate. Leeks, onions, carrots and parsnips can be left to over-winter: leave one leek plant, an onion plant and maybe even a carrot and parsnip plant in the ground, and the next year they will flower and you can save their seeds¹. Biennials like these reward a little patience rather than any special kit.

Peas rarely cross, so several varieties can be saved together

Peas are the crop to start with if you want an early success. Garden Organic classes them as inbreeding vegetables that will not cross with each other or cross rarely, though precautions still need to be taken when saving seed². Real Seeds is more relaxed still: "Peas don't in general cross very easily, so you can save seed from several varieties without worry"³.

That means a household can grow, say, an early, a maincrop and a late variety in the same plot and save seed from all of them, provided you follow the first rule and save only from your healthiest plants¹. It also fits the way peas are usually grown: by carefully selecting different varieties and successional sowing it is possible to enjoy peas from early summer to early autumn¹³. Saving from several varieties keeps that spread of harvest dates in your own hands.

The same low-crossing logic applies to French beans, which Garden Organic groups with peas². It does not extend to runner beans, which are outbreeding and need their pollen kept apart from other varieties².

Seed saved each year adapts to your own conditions

Saving your own seed each year allows your varieties to adapt to your own specific conditions, and lets you note any interesting or useful characteristics, such as pest or disease resistance¹⁴. Over several seasons that is a real advantage: the plants that do well on your soil, in your wind, and against your local pests are the ones you are selecting, whether you mean to or not.

This is not a new idea. Amateurs can save their own seed, as has been done for thousands of years, well before the advent of large seed companies and modern agriculture¹⁴. The Heritage Seed Library alone holds over 800 varieties being saved¹⁴. You do not need a laboratory or a polytunnel to join in; you need open-pollinated seed, healthy parent plants and a bit of separation.

There is a supply argument too. Saving your own seed each year allows your varieties to adapt to your own specific conditions, and lets you note any interesting or useful characteristics, such as pest or disease resistance⁹. Amateurs can save their own seed, as has been done for thousands of years, well before the advent of large seed companies and modern agriculture⁹. A household with its own saved seed and a few neighbours doing the same is less exposed to a bad year in the seed trade.

To make saved seed last, deal with the two things that shorten its life: "The two factors that reduce the life of the seed are moisture and warmth"⁴. Keep seed in a sealed container in a cool, dry place. How long it then keeps varies by crop: oregano seed stored this way is described as remaining viable for up to 3-5 years¹⁵, while parsnip is at the other end, with stored seed viable for 1 or 2 years¹⁶. That difference is exactly why testing before you sow matters more for some crops than others.

Testing and sowing what you have

The water test is quick and needs nothing but a jar. Put medium to large seeds in a jar of water: viable seeds will sink, whereas dead seeds will float to the surface¹. It suits beans, peas, squash and similar-sized seed. For fine seed, sow a small sample and count what comes up instead.

If the seed is sound, sow it well. Prepare the ground first: ensure the area is weed free, rake the soil gently so it is fine and crumbly and reasonably level, wear gardening gloves, and read the seed sowing instructions on the packet to work out the sowing distances¹⁷. Remove old plant material and weeds, digging out all the roots and shoots, shaking the soil back over the beds and composting what you can¹⁸. Large seeds can be sown individually using a dibber to make a hole, which is known as station sowing¹⁷.

For an early start, module sowing lets you either delay planting outdoors if conditions are not right, or plan a succession of plantings every few weeks for a constant supply¹⁹. Fill the module tray with potting compost, make sure it is evenly spread and tap the tray on the bench a couple of times to level it²⁰. When pricking out, always hold the seedling by its leaves, never by the stem²⁰. A windowsill propagator helps: fill each cell with seed and cutting compost, water well with a fine spray, and for faster germination place the inserts in the water tray and cover with the lid, propping one side open by about 1cm as plants grow²¹. Keep it bright but out of direct sun²¹.

Outdoors, a sheet of clear plastic over sown ground can bring germination forward; remove it as soon as the seedlings appear²². Water often but do not overwater: test by poking a finger into the compost, and slightly moist just below the surface is right²³.

Where to go next

If you are starting from scratch, the growing your own vegetables guide sets out where to begin, and sowing seeds indoors and outdoors covers timing, depth and germination. For the seed-saving method in more detail, see how to save seed from your own vegetables, keeping vegetable varieties pure and how to store saved seed so it lasts. A seed sowing equipment checklist lists the kit worth having before the season starts.