Broccoli, kale and cabbages
Sprouting broccoli, cabbages, cauliflowers, calabrese, kales and brussels sprouts are all members of the same family (Brassica oleraceae), and will all cross with each other. They won't cross with turnips, swedes, oriental brassicas or mustard greens. In addition, they are mainly self-incompatible – which means that in order to get seed, insects have to carry pollen from one plant to another to pollinate the flowers. Because of this, you can't simply grow your broccoli or cabbages for seed in an insect proof cage to avoid crossing.
So long as you only seedsave from one member of the family in any given year, you can grow as many other brassicas as you like without problems so long as you don’t let them flower.
For absolute seed purity, make sure that there are no other flowering brassicas within a mile of your garden. In practice, fences, trees and tall crops all break up insect flight patterns, so as long as you don't have any immediate neighbours with flowering crops in their garden, you shouldn't have too many problems with crossing. To make it as easy as possible for insects to work your seed plants, make sure that they are laid out in a block, rather than a row, so that bees tend to move from one plant to another, rather than away to other flowers elsewhere.
Keep at least six plants for seed, ideally more. Remove any poor specimens, or any that are not typical for the variety –you can always eat these plants, so long as you don't allow any flowers to open.
All of the brassicas, including cabbages, will throw up a tall flower stalk covered in lots of small yellow flowers. These will then form slender seed pods, which start out green, and turn a straw colour as they mature and dry. Once they start to dry, keep a close eye on them, as they tend to shatter and drop their seed. Its best to cut entire plants once most of the pods begin to look dry, and then leave them to mature further on a sheet indoors. Once they are thoroughly dry, the seeds will come out of the pods very easily; the simplest way is to trample the plants on top of a large sheet, and then sieve out the debris.
You should get lots of seed from even a few plants. The seed will keep well for up to five years so long as it is stored somewhere cool and dry.
Turnips and the oriental brassicas
Mizuna, pak choi, tatsoi and mibuna are all sub varieties of Brassica rapa – the same family as turnip. This means that although they will cross with each other, or with turnips in flower, they won't cross with broccoli or cauliflowers. Although you can only grow one of these vegetables for seed in any year, you can of course grow any of the others for kitchen use, so long as you don't allow them to flower at the same time as your seed plants.
To grow an oriental brassica or turnip variety for seed, you usually need to overwinter the plants. They are naturally biennials, producing their flowers and seeds in their second year of growth. Although spring sown crops may bolt to seed in hot summer weather, this is not ideal for seedsaving, as you may end up accidentally selecting for early bolting in future years. The best solution is to sow your seed crop after midsummer in a polytunnel, where semi-mature plants will overwinter quite happily in all but the coldest parts of Britain. If necessary you can give extra protection in cold weather by putting fleece over plants inside the tunnel. Select at least 6 of the healthiest and most typical plants to reserve for seed, eating the rest over the winter. In spring, the plants will flower, and then form seedpods. Make sure that there is good insect access to the tunnel at this point so that the flowers are pollinated.
The seedpods are green at first, but then gradually dry out and turn a pale tan colour. Once most of the pods are dry and brittle, cut the entire stalks of the plant, and lay out on a sheet somewhere undercover with a good airflow to finish drying off. Then rub and crush the pods with your hands to release the seeds, and separate the seeds from the chaff with a coarse sieve.
Lettuce flowers are self pollinating, and very rarely cross. If you plan to save seed from more than one variety of lettuce, separate them by around 12 foot or plant a tall crop in between the rows.
Select two or three good lettuces from your row, and mark them for seed. It is very important not to save seed from any plants that bolt early, as you want to select for lettuces that stand well. Heading lettuces may need a little help for the flowering stalk to emerge; slitting the heads partially open with a knife works well.
Once the lettuces have flowered, the seeds will ripen gradually, starting in about a fortnight. Harvest seed daily to get the maximum yield, shaking into a bag. Or wait until a reasonable number of seeds are ready and then cut the whole plant. Put it head first into a bucket, shaking and rubbing to remove the seeds. If you leave the whole cut plant upside down in the bucket somewhere dry, slightly immature seeds will continue to ripen over the next few days.
Most of what you have collected in the bucket will be white ‘feathers’ and chaff. To sort the seed, shake it gently in a kitchen sieve. Some seeds will fall through the sieve, with the rest collecting in the bottom. The feathers and chaff will rise to the top, and you can pick them off. There’s no need to get the seed completely clean; a little chaff stored and planted along with the seeds won’t cause any harm.
If the seed feels a little damp, dry it further on a plate before labelling and storing. Lettuce seed should keep for around 3 years, provided it is kept cool and dry.