I left some leeks in the ground from last year, and they shot up to form a bulb in December. They flowered shortly afterwards at a height of about 6 feet. I expected the seeds to be ready for planting this year but it's already into autumn and the seeds don't look ready. I've examined a few and they are still green.
Does anyone know how long I need to wait for the seeds to go black? And can I cut the flowers off and let the seeds harden off the plant, or do I need to leave them on?
Did the flowers get pollinated for seeds to form? If it's the seeds casing that you say are still green, I would take one or two flower heads and put them in a paper (not plastic) bag, hanging in a warm, dry dark room. The seeds should mature, the casings split and the seeds drop into the bag, for sowing when your season comes around. The outer seed pod would remain brown, containing the very small black flaky seeds. The rest I would leave on the plants, they may get eaten by birds, or simply mature on the plant, for harvesting later. If nothing else they are a very architectural structure in any garden or veg plot. I would have thought that from December until March that is long enough time to mature. For a basic estimate, If each flower head yields approximately 150 seeds of which 75% being viable and you take 3 flower heads, you can get approximately 330 seeds. That would be enough for most unless you're growing commercial quantities. Onion seed remains viable for 2 to 3 years, so the seed from one or two seed heads would be sufficient. Once the flower stalk starts to become dry the seeds will ripen and mature. If left on will naturally disperse. The time taken will depend on temperature and humidity, so very location dependant. But would be measured in weeks not months. The seed casings would be designed by nature to open when the conditions for natural sowing would be right, in the spring, warm, dry and a light breeze. You can mimic that by the collection of the seed heads as above.
It takes approximately 20 months to go from seed to seed. And you should have about 15 plants to save the seeds from to ensure adequate pollination. Each of the 100s of flowers will produce 2-3 seeds so that each plant will have hundreds of seeds. If your leek has a second flower head near the base of the plant, remove and discard it.
The seeds are covered in a thin green seed coat and this changes color as the seeds mature. When trying to determine when to take the flower head from the stem, sample a few of the seed pods from around the flower head to see if there are any black seeds present. If they are present, you can remove the whole flower head. As the seed pods age, they become more papery white and you can see the black seeds inside. But if you leave them too long on the stem, you can lose them to the weather as they open and fall to the ground.
When you remove the flower head, place it upside down in a cardboard box, allowing some space for ventilation. Leave the seeds to dry and mature over winter in a warm room, eg. 15 deg C, and the seed pods will open up dropping the seeds into your box ready for spring planting.
https://www.youtube.com/watch?v=VTRmFo_V5kM
To plant from seed, sow indoors 8-10 weeks before the last expected spring frost. Plant seeds 1/2 inch deep in pots at least 6 inches deep, thinning to 1 inch apart when the seedlings are 3 inches tall. Transplant when seedlings are hardened and the ground can be worked.