How to Read a Seed Packet and Actually Trust What It Tells You

There is a particular kind of optimism involved in buying seeds. You stand at the rack, turn a packet over in your hand, and within thirty seconds have committed to growing something you have never attempted before, possibly in a spot where it has very little chance of success. The packet goes into the bag, comes home, and is either sown in haste or tucked into a drawer and forgotten until it surfaces in March looking slightly dog-eared. What most of us do not do is read it carefully, which is a shame, because a well-printed seed packet carries more genuinely useful information than many gardening books.

The difficulty is that some of that information is precise and reliable, some of it requires interpretation, and some of it was written for conditions that bear very little resemblance to the average British back garden. Knowing which is which makes an enormous difference to how your seeds actually perform.

What "Sow Under Cover" Actually Means

Let us start with what the packet says about timing, because this is where confidence breaks down most quickly. "Sow under cover from February" is a phrase that sounds clear enough until you begin asking what "under cover" means and whether your conditions qualify.

In seed-packet language, "under cover" usually means a heated propagator or a consistently warm glasshouse, somewhere that can maintain a soil temperature of at least 18 to 21°C for the germination of tender crops like tomatoes, aubergines, and sweet peppers. If you have an unheated greenhouse, a cold frame, or a south-facing windowsill in a room you keep at 16°C, you do not have "under cover" in the way the packet intends. You have something useful, but it is not the same thing, and sowing in February without the right warmth will give you erratic or disappointing germination, sluggish seedlings, or nothing at all.

A more practical read of the February instruction is: "you could start this in February if you have the right conditions, but you will not be penalised for waiting until March or even April if you lack them." In most UK gardens, tomatoes sown in late March with consistent bottom heat will overtake February sowings made in cold compost, arriving at the same planting-out size and fruiting at roughly the same time. The packet date is a best-case ceiling, not a hard deadline.

For hardy crops, the situation is different. "Sow outdoors from March" on a packet of broad beans or peas is more straightforwardly reliable, because these crops can tolerate cool soil and are genuinely suited to early outdoor sowing. The key is soil temperature rather than air temperature: most seeds need a minimum of around 5°C in the soil to germinate at all, and many do better at 10°C or above. A cheap compost thermometer is one of the more underrated tools in a gardener's kit, and I say that as someone who relied on instinct for years before accepting that the thermometer was more accurate than my instincts.

Germination Percentage: The Honest Number

Somewhere on the packet, usually in small print near the base, you will find a germination percentage. It might read 80%, or 75%, or sometimes rather optimistically 90%. This figure tells you what proportion of seeds in the packet germinated under controlled laboratory conditions, which is worth thinking about before you take it entirely at face value.

"Controlled conditions" means consistent temperature, adequate moisture, and a sterile growing medium. It does not mean cold spring compost in a plastic module tray on a garage shelf. The real-world germination rate from a home sowing is almost always somewhat lower than the figure on the packet, sometimes significantly so if your conditions are marginal or the compost is damp and cold rather than just cool and moist.

What the percentage tells you usefully is how many seeds to sow relative to how many plants you want. A packet rated at 75% germination means that for every ten seeds sown, you might reasonably expect seven to emerge, though eight on a good day and five on a difficult one. Sowing two or three seeds per module and thinning to the strongest is not wasteful, it is a sensible response to the probability the packet has already given you. The gardeners who sow one seed per cell and wonder why they have gaps have, in a sense, misread the data already in front of them.

Heritage and open-pollinated varieties, particularly older ones from smaller seed houses, sometimes carry lower germination percentages than commercial F1 hybrids. This is not a sign that the variety is inferior; it often reflects the different production conditions of small-scale seed growers. Sow a little more generously and the results are usually fine.

Spacing: Written for Fields, Not Raised Beds

The spacing recommendations on seed packets deserve a degree of healthy scepticism, because a significant proportion of them were developed for field or row production rather than intensive bed growing.

Rows 30cm apart with plants 15cm apart within the row is the kind of instruction written for a market gardener working with a mechanical hoe. In a raised bed, or a no-dig growing bed where you are not walking on the soil and do not need the row gaps for access, you can usually grow at closer spacing using a grid pattern. Lettuce that the packet suggests growing 20cm apart in rows can be grown at 20cm each way in a bed, producing a much more efficient use of space without any meaningful loss of quality. Carrots, beetroot, and salad leaves can all be grown at closer spacing than the packet suggests, provided the soil is deep, fertile, and well prepared.

The caveat is this: spacing recommendations for larger crops, particularly brassicas, are not just about access. They reflect the actual size the plant reaches and its competition for light, water, and nutrients. A Brussels sprout plant that the packet says should be 60cm from its neighbours really does need that space. Ignore it and you will get smaller buttons, more disease pressure, and a border of unhappy plants fighting for what little there is. The spacing advice on packets tends to be most reliable where the crop is large and leafy, and most over-cautious where the crop is small, quick-growing, or being used for cut-and-come-again purposes.

Thinning advice, which often appears separately on the packet, is even more frequently ignored than spacing guidance. Gardeners are reluctant to remove seedlings they have raised themselves, which is entirely understandable, but thinning is not killing. It is the difference between a row of reasonable carrots and a tangle of stunted, forked ones. The packet knows this. It is asking you to do something that feels wrong and is right.

F1 Hybrids, Open-Pollinated Varieties, and What It Means to Save Seed

The distinction between F1 hybrid and open-pollinated varieties is one of the most practically important things a seed packet can tell you, and also one of the most consistently misunderstood.

F1 hybrids are the result of crossing two carefully selected parent lines. The resulting seed has excellent uniformity, vigour, and, in many cases, disease resistance, which is why they dominate commercial vegetable production and occupy so much shelf space in garden centres. They grow reliably and perform consistently. The complication arises when you try to save seed from them. If you collect seed from an F1 tomato and sow it next year, the plants you grow will not be like the parent. They will revert toward the original parent lines in unpredictable combinations, producing variable fruit of uncertain quality. This is not a defect; it is how F1 genetics work. But it means that if you want to save seed, F1 varieties are not your starting point.

Open-pollinated varieties, including the many heritage varieties that have been grown and saved by gardeners and small growers for generations, will reproduce true, or close to true, from saved seed, provided they have not crossed with a neighbouring variety of the same species. A heritage tomato such as Brandywine or Black Krim, properly saved and stored, will give you recognisable plants with recognisable fruit the following year. This is why heritage varieties have become so sought after by growers who want to build their own stock across seasons rather than buying fresh seed every spring.

The packet will usually say "F1 Hybrid" clearly if the variety is one. If it does not, the variety is likely open-pollinated, though it is worth checking with the seed supplier if you plan to save in earnest. Growing a mix of both is entirely sensible: F1s for reliable heavy crops where consistency matters, open-pollinated and heritage varieties where flavour, diversity, and the ability to save seed are the priority.

The Date Code and What to Do with Old Seed

Most seed packets carry a "sow by" or "use by" date, which is the producer's indication of the year up to which germination rates are likely to remain at the stated percentage. After that date, the seed does not suddenly die. What changes is that the germination percentage will decline, sometimes gradually, sometimes more sharply depending on the species and how the seed has been stored.

Some seeds remain viable well beyond their stated date. Tomatoes, pumpkins, and cucumber seed stored carefully in cool, dry conditions will often germinate at a reasonable rate for five years or more. Others are notoriously short-lived: parsnip, parsley, and onion seed drop off quite quickly, and last year's parsnip packet is usually not worth keeping, whatever the date says. Sweetcorn and leeks also decline faster than most. Brassicas tend to sit in the middle; a packet of kale seed stored in a tin in a cool room may well germinate acceptably three or four years after its stated date.

Before discarding old seed, run a simple viability test. Count out ten seeds, lay them on a damp piece of kitchen paper or tissue, fold it over, and seal it in a plastic bag or container. Keep it in a warm spot indoors, around 18 to 20°C, and check after the germination period stated on the packet, usually between five and fourteen days depending on the species. Count how many have sprouted. If seven or more have germinated, the seed is still worth sowing, though perhaps a touch more generously than you otherwise would. If fewer than five have sprouted, the germination rate is probably too low to be reliable and the seed is better composted.

Storing seed well in the first place is worth the small amount of effort it requires. The enemies of seed viability are warmth, moisture, and light. A sealed tin or glass jar, stored somewhere consistently cool and dark, is all that is needed. Many gardeners keep their seed tin in the bottom of the fridge, which works very well, with the caveat that the packets should be brought to room temperature before opening to prevent condensation forming on the seed. A silica gel sachet inside the tin will absorb any residual moisture and extend viability further. It is one of those genuinely simple things that makes a difference, probably more difference than the cost of replacing seed you could have kept.

The seed packet is not a perfect document. It was not written with your specific garden in mind, and some of its advice reflects commercial growing assumptions that do not translate directly to a raised bed in Surrey. But it contains far more useful information than most of us extract from it, and most of that information is reliable if you know how to read it. Pull the seed tin out before your next sowing session and have a proper look. You already own the guidance. It just needs a little interpretation.