Two species, and why the distinction matters
Brassica oleracea covers the European cole crops: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi. They are biennial — they grow a vegetative plant in year one, overwinter, then flower and set seed in year two.
Brassica rapa covers the Asian greens and turnips: pak choi, mizuna, tatsoi, komatsuna, turnip. Many are annual or fast biennial, and they bolt readily in heat.
Within each species, everything crosses. Across species, they largely do not. That gives you a workable rule: you can grow one oleracea and one rapa for seed in the same year without isolation, but not two of either.
Sowing and the cool-season window
Brassicas germinate in 5–10 days at 15–20 °C (59–68 °F). They are cool-season crops and will sulk above about 25 °C (77 °F), so time them for spring and autumn rather than midsummer.
For leafy types — kale, pak choi, mizuna, tatsoi — direct sowing is fine and they will crop within 30–50 days. For heading types, start in modules and transplant once the seedling has four true leaves, setting it deeper than it grew in the module so the stem is buried.
Kohlrabi is the fastest and most satisfying: 55–65 days from sowing to a swollen stem you can eat, and it takes up very little room.
Feeding and the aphid problem
Brassicas are heavy nitrogen feeders while they are building leaf. Run EC around 1.8–2.2 through the vegetative stage, then drop to 1.6 once heads begin to form, because excess nitrogen at that point produces loose, leafy heads instead of tight ones.
Aphids are the defining pest. They arrive in autumn, colonise the newest growth, and multiply faster than you can pick them off. In a hydro system you cannot use most soil-acting controls, so the practical answers are: inspect the leaf undersides weekly, remove affected leaves immediately, keep airflow high, and accept that a heavy infestation usually means pulling the plant.
Flea beetles are the other one — small holes shot through young leaves, appearing overnight in spring.
Saving seed: the biennial problem
This is the part that separates brassicas from everything else in the catalogue. Oleracea types need two seasons. You grow the plant, it overwinters as a stump, and in spring of the following year it sends up a two-metre flower spike covered in yellow flowers.
Overwintering is the hard bit. The plant needs to survive cold without being killed by it — most kale and cabbage will take −8 °C (46 °F), but not a hard freeze in waterlogged ground. Lift the plant, trim the roots, and heel it into a sheltered bed or a large pot in a cold but frost-free place.
Rapa types are much easier: many will flower and set seed in the same season if sown early, and they bolt so willingly that getting seed is rarely the problem.
Isolation, honestly
The textbook figure is 800 metres to a mile between flowering brassicas of the same species. That is not achievable in a garden, and pretending otherwise wastes people's time.
The practical answer is time, not distance. Brassica flowers are insect-pollinated and stay receptive for about three weeks. If you stagger two varieties so their flowering periods do not overlap, distance stops mattering. This is how most small-scale seed savers manage the family.
Bagging works too: enclose the flowering head in a fine mesh or a paper bag and hand-pollinate with a brush, or introduce a few flies under the bag. Fiddly, but it fits a small garden.
What goes wrong
Bolting before a head forms. Heat or a check to growth — usually a dry spell or a root disturbance. Rapa types do this most readily. For seed that is convenient; for eating it is a loss.
Clubroot. Swollen, distorted roots and wilting plants that recover at night. It is a soil-borne organism and almost impossible to eradicate once present. In a hydro system it is rare, because there is no soil — one of the genuine advantages of growing this way.
Cabbage white caterpillars. Check the leaf undersides for yellow eggs and rub them off; a mesh cover over the net pot stops the butterflies reaching the plant at all.
Loose, leafy heads instead of firm ones. Too much nitrogen late, or too warm a reservoir. Cool the water and drop the EC.