How to choose vegetable seeds

How we checked what a packet settles

Seed for a kitchen garden is chosen from a packet, a catalogue page or a shop shelf, and the same small set of facts decides whether it grows. Six of them matter, and they do not all come from the same place. Three are printed by the seller. Three are answered by the crop, the weather and the date the seed was packed, which makes them the buyer’s half of the decision rather than the shop’s.

The two groups are read in that order here, because the printed facts narrow the field before the rest of it matters. Nothing below needs a plot: the sowing months, the depth and the thinning rule are taken from the growing guide the Royal Horticultural Society publishes, and the storage lives, germination percentages and soil temperatures come from a university extension publication on seed storage and germination.

The figures are read from those published sources rather than from seed this site grew, with the sources and their check date listed at the end.

What the packet answers

The variety, and what it was bred for

The name on the front is the variety, and two packets of the same vegetable can be bred for different jobs, one for an early crop and another for keeping. The variety decides the flavour and the season, and it is the one line on the packet that a later decision cannot repair.

For a small plot the growing guide’s advice is to grow a selection rather than a large amount of one kind, and to sow the fast crops on a short repeat. Lettuce, radish, carrot and chard are the ones it names for sowing in small amounts at intervals of ten to fourteen days, which keeps a supply coming instead of delivering a glut in one week and nothing after it.

The sowing window, and the frost date behind it

A packet gives a range of months, and the range is written for a climate rather than for a garden. The guide’s own calendar shows the shape of it: December and January are the months to plan rather than to sow, sowing can begin under cover in February with lettuces and other salad crops, and broad beans can go in between November and January for a crop in late May and June.

The other half of the window is the tender crops. Courgette, pumpkin, squash, tomato, sweetcorn and cucumber are started in spring or early summer under cover and moved out only after they have been hardened off, while aubergine, glasshouse tomato, melon and cucumber stay under cover for the season. Reading the printed range against the last frost date of the garden is what turns it into a date.

The depth and the spacing, and what thinning costs

Depth is small and specific. The guide’s rule for sowing into modules is three to five seeds in a shallow depression, covered with a thin layer of grit, and for sowing where the plant will stand it is two or three seeds every few centimetres. Large seeds go deeper, which is why broad beans are given individual containers rather than a tray.

The spacing on the packet is what the row is thinned to rather than what is sown. Once seedlings carry their true leaves the weakest are pinched off at ground level, and the module method is the exception that keeps every seedling in its cell. The guide expects some of them not to make it and advises sowing three to five seeds for each plant wanted, which is the arithmetic behind the packet’s spacing number.

What the packet leaves open

How much of the packet will come up

A germination percentage is a figure from the packing line rather than a forecast for the garden, and it is the minimum the lot was tested against rather than a count of what will appear. The extension publication lists those minimums crop by crop, from 80 percent for lettuce and cucumber down to 60 percent for parsnip, with onion at 70, and it treats each of them as a floor rather than a promise.

For seed kept more than a season the publication recommends a test before the packet is trusted: twenty-five or fifty seeds between damp paper towels, held at 70 to 75 degrees Fahrenheit, counted as they sprout and totalled after two to three weeks. A poor result is a reason to sow more thickly rather than to throw the seed away, since the same page notes that a lot with lowered germination can still be planted safely at a higher rate.

What temperature the seed waits for

Sowing is a race between the seed and the cold, and the month printed on the packet is a rough stand-in for a soil temperature rather than a measure of it. The same publication gives the temperature each crop needs in order to come up, and the spread is wide: cucumber at 95 degrees, onion at 80, lettuce at 75 and parsnip at 65, which is why parsnip is the packet that sits in the ground for a fortnight before anything shows.

The days each one takes follow the same line. Lettuce is listed at two to three days at its optimum, cucumber at two to five, onion at four to five and parsnip at fourteen. A packet sown into cold soil has not failed; it is waiting, and the wait is where the seed is most exposed to rot.

How long it keeps, which the crop decides

Storage life belongs to the crop before it belongs to the gardener. The publication’s table runs from a single season to six years under cool, dry conditions: onion, parsley, parsnip and salsify at one year, okra, pepper and sweet corn at two, carrot, celery, pea, spinach and tomato at three, the cabbage family with beetroot, pumpkin, squash and chard at four, cucumber, lettuce, radish and muskmelon at five, and leek at six.

Two things follow from that. A packet of parsnip or onion is a purchase to repeat every year, and a packet of lettuce or cucumber is not. The conditions also decide how far down the table a given packet gets, since the same page names storage temperature, relative humidity and seed moisture as the factors, and states that seed keeps longer when it is kept dry and cold.

Which point carries the decision

The six points do not move, but which of them carries the decision does.

If the plot is a few pots or one bed

Storage moves to the front, because a packet holds far more seed than a small plot uses and most of it goes into a second season or a third. Buying a smaller amount of the crops that keep badly, and treating the rest of the seed as a stock rather than a purchase, is the change that fits this size. The module method suits it too, since one tray can carry several kinds at once.

If the plot is worked every year

Once the same beds are planted each spring, the sowing calendar and the thinning rule are the points that decide the harvest, and the storage table becomes a list of what to rebuy rather than what to test. Parsnip, onion and parsley come back to the shop every season where leek, lettuce and cucumber do not, and a germination test in late winter tells the difference before the ground is ready.

Frequently asked questions

Sources

RHS (rhs.org.uk), checked 16 September 2026: the vegetable seeds sowing guide for the months that are quiet, the crops started under cover, the repeat interval for the fast crops, the depth, spacing and thinning rules and the module method.

University of Nebraska-Lincoln Extension (extensionpubs.unl.edu), checked 16 September 2026: publication G2090 for the storage life of each vegetable seed under cool dry conditions, the minimum germination percentages and the soil temperatures each crop needs in order to come up.