How to grow alliums from seed
Although many alliums can be easily grown from bulbs which are readily available, most of them can also be grown from seeds. It takes a little bit more work and you will have to wait longer to see mature plants, but there are advantages, too. When you propagate plants from bulbs or divisions, you are making exact copies of the parent plants, whereas every seed produces a plant that is a little bit different. You could find a particularly strong, vigorous plant among your seedlings, or you might discover offspring with different coloured flowers from the parents.
Propagating from seed also avoids the spread of some diseases which are passed on through cuttings, divisions, etc. but not through seed propagation.
Most alliums that produce viable seeds will self-seed naturally, or you can collect seeds and sow them directly in the ground in early spring and they will grow where you want them. Germination can be erratic however, and the tiny plants that appear take a long time to get any bigger and can be easily swamped by weeds. Growing in trays or pots gives more control over the growing process and lets you nurture the young plants until they are big enough to compete with other plants in the garden.
Sow in an unheated frame or greenhouse from January-March, 0.5cm deep in pots or trays of seed compost. Keep moist. Cold periods help activate the seeds and germination should occur when the temperature range is regularly 12-15C.
At other times of year, seeds can be encouraged to sprout by providing a cold period artificially. There are different ways of doing this and not all allium seeds need it, but they will usually germinate much better after it. Try putting seeds in a plastic bag with damp sand or compost, keeping it at room temperature for 2-4 weeks, then in the fridge for 5-6 weeks. After that, spread the mixture on a tray of compost and the seeds should start germinating.
Germination can be erratic, from two weeks to two months so prick out seedlings individually as they develop . (if germination seems to be stalling and you think there are still a lot of seeds which haven’t popped, you can try putting the seed tray outside for a week or two then bringing it back into the greenhouse. The drop and rise in temperatures can often prompt further germination).
Pot up seedlings individually, or three or four to a pot. Keep outside from May and transplant to final positions 20-30cm apart in autumn.