No. 18
BRASSICAS
Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Kale, Kohlrabi, Swede, Turnip
Brassica oleracea & B. napus varieties
Family: Brassicaceae (Cruciferae)
Brassica seed saving guide also available here as a PDF document (344Kb)
Although they appear diverse, all brassicas have been selected from just two common ancestor species: Brassica oleracea has given rise to broccoli, Brussels sprout, cabbage, cauliflower and kale; Brassica napus is the original species of turnip and, possibly, swede. They will all readily cross with one another so must be isolated. They are biennial, being sown and grown one year to flower and seed the next.
Flowers are perfect and cross-pollinating. They are mainly insect pollinated.
Growing and Roguing
General
Kale
- The plants are sown and grown as they would normally for a food crop. They should be spaced more widely for seeding as the seed stalks are tall and branched and can reach up to 180cm high.
- Thinning alternate plants for eating will ensure enough space between most leafy brassicas – about 1m each way.
- Roguing is essential. Most older brassicas are very variable and plants that differ too much from the norm should be removed before they come into flower. Rogue frequently as the characteristics of the plants change as they develop.
- Roguing may leave a few gaps, but these are easily filled if plants are moved in early spring after over-wintering in the open. There will be little check to growth if the plants are thoroughly firmed in and watered.
- They are best grown in a block as this makes caging much easier. You can move mature plants to a special seed production area to fit in with a rotation, or to make room for crops that are to follow. Root brassicas should be stored over winter and the best roots replanted at a spacing of about 1m.
- Growing plants in a block (if you have the space) means you can discard seed from the outer row or rows if you think that crossing may have taken place. Bees do not carry pollen far within a block of brassicas so the central plants are likely to be pollinated by their neighbours rather than by plants from far away. Seed from the central four plants in a block of 36 can be stored separately with reasonable confidence that the variety has been kept pure, while seed from other plants may carry a slight risk of contamination.
- Remove dead leaves as they can wrap around the flowering stalks and promote fungal growth. In the case of cabbages, particularly the tight headed varieties, you may need to cut a cross into the head or remove a large amount of the leaves to allow the flower stalk to grow through.
Cabbage cut across to allow flower to emerge
Spring Cabbage
The heads can be cut and used, letting the flower stalks shoot from the stump. If you don’t use the head there is a potential for it to rot. You’ll need to cut a cross in it to allow flower stalks to emerge.
Cauliflower
Winter and early summer varieties should flower in the same year and cause few problems. Autumn varieties on the other hand are more difficult to obtain seed from as the plants will need to overwinter.
Brussels Sprout
These can be left in situ. The lower buttons and top head can be eaten.
Turnip and Swede
Protect with straw in extreme cold when overwintering. Turnip varieties may flower in the same year if sown early. Discard early bolters.
Pollination and Isolation
Brassica flowers are perfect but are outbreeding, so require insects for a good seed set. Cross-pollination seldom occurs over distances of more than 1500m. The risk usually comes from neglectful gardeners nearby who allow their previous year’s crops to flower, and from oil seed rape which can cross with B. napus; apart from being grown as a crop nearby, rape often appears in road verges and hedgerows and the flowers threaten contamination.
The best seed set is obtained in the open. If there are too many potential contaminant brassicas growing nearby, you can introduce flies as pollinators inside a cage. Buy maggots from an angling shop (buy ‘whites’), put them somewhere warm to turn into castors; then add them to your cage before they turn into flies. Make sure the supplier knows what you intend to use them for as sometimes they are treated so that they do not hatch. They will need to be protected from the rain but free to fly out when they hatch. An old lidded margarine tub with a hole cut in the side works well. They hatch in a few days. Being enclosed in the isolation tent their forays will be limited to the brassica flowers, thus ensuring varietal purity.
Alternate day caging is a good method of maintaining purity of two varieties in a single season, provided there are no local contaminant species. All brassicas are outbreeders, so as many plants as possible should be used to maintain the health and diversity of the variety. We recommend a minimum of 16 plants, preferably many more.
Flowering starts as the days lengthen quite early in the year — usually mid-May. This is the time to start caging, if necessary. Spun fleece, fine mesh, or old closely woven net curtain (draped over the plants with a little support from canes), is placed over the plants of one variety one day, then moved onto the other the next. Pollen does not survive long enough to be carried from one day to the next.
Harvesting
Harvesting kale pods
Once flowering has finished and the seed pods are developing, any cages can be removed and the pods allowed to mature uncovered. The seed stem often grows much further as the seed pods mature. The pods turn yellow then start turning brown and have a tendency to shatter as they ripen. There is a danger that seed will be lost this way, so keep a constant check on the ripening pods. If you have time, break the seeds out of the pods into a paper sack every few days as they ripen to maximise seed production. Otherwise, remove the entire stalk when the majority of the pods have ripened and dried, but not shattered. It is important not to cut the seed stalks too early, while the pods are still green, as the seed will not continue to develop once cut.
Cleaning
Cleaning refers to the removal of chaff and debris, leaving only seed. Cleaned seed keeps better.
Seed should be fully dry for threshing and most of it will shatter from the pods of its own accord. The remainder can be broken out by hand or by beating the pods in the sack and winnowing.
Winnowing is best done outside in a stiff breeze. Pour the seeds steadily from one container to another, allowing the wind to blow the chaff away. Repeat until the chaff is gone and you are left with only seed. Do this over a tarpaulin, in case a sudden gust upsets the container and spills the seed.
Storage
Stored cool and dry, all brassica seed will last for at least five years, giving full germination.
Returning Seed to HSL
It is important that seed returned to HSL is not cross-pollinated. Do not send us seed that you suspect might have crossed.
Seed must be completely dry and fully cleaned. Seed that retains moisture can go mouldy in transit and will have to be discarded. It can take a few days to get to us in the post. Pack it in breathable material (e.g. a paper envelope or cotton bag) and place it in a padded envelope or stout box to protect the delicate seed from impact damage, before sending it in the post.
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