Onions (Allium cepa)
Family: Alliaceae
Onions and leeks share many characteristics. The main difference is that onion leaves are hollow tubes, while leek leaves are flat straps with a keel up the middle. Both are biennial, but the onion leaves die back to leave the dormant bulb while leeks are evergreen and winter hardy. This section deals only with those onions and leeks that reproduce mainly by flowering and setting seeds. Other sorts (garlic, elephant garlic, shallots, potato onions, rocambole etc) may flower occasionally but are usually propagated vegetatively. These are dealt with in a separate sheet, under Multiplier Onions & Shallots (Seed Saving Guideline No.15). Allium flowers are perfect but cannot usually fertilise themselves. This is because they are protandrous; the male anthers open first and shed their pollen before the female stigma of that flower is receptive. Individual flowers in ball-like heads open over a period of about four weeks, peaking in the second week. |
Growing & Roguing
· Leeks and onions are biennials, so in the first year follow your normal practice for crop
production
· Onion flower stalks can reach around 150cm in height, leeks up to 180cm, so remember to have
sufficient space and position stakes to support them later.
· In the second year plant the best of your onions in the spring and they will soon put out fresh
leaves and a flower stalk.
· Leeks are best left in their planting position to overwinter.
· Be ruthless about rouging any plant whose foliage is not up to scratch, or which flower in the
first year.
· Susceptibility to disease, e.g. leek rust, can vary within a variety, so select the most resistant
plants to save seed from.
· Only chose those onion bulbs that are characteristic of the variety.
· Avoid saving seed from ‘bull-necked’ (wide and rounded) or ‘bottle-necked’ (tapering up the
stem) onions.
· You can slice across an onion, about a third of the way down, to check the evenness of the rings
and the colouration, without any harm to the shoot inside. This allows you to check that it is
true to type.
· Leeks and onions are biennials, so in the first year follow your normal practice for crop
production
· Onion flower stalks can reach around 150cm in height, leeks up to 180cm, so remember to have
sufficient space and position stakes to support them later.
· In the second year plant the best of your onions in the spring and they will soon put out fresh
leaves and a flower stalk.
· Leeks are best left in their planting position to overwinter.
· Be ruthless about rouging any plant whose foliage is not up to scratch, or which flower in the
first year.
· Susceptibility to disease, e.g. leek rust, can vary within a variety, so select the most resistant
plants to save seed from.
· Only chose those onion bulbs that are characteristic of the variety.
· Avoid saving seed from ‘bull-necked’ (wide and rounded) or ‘bottle-necked’ (tapering up the
stem) onions.
· You can slice across an onion, about a third of the way down, to check the evenness of the rings
and the colouration, without any harm to the shoot inside. This allows you to check that it is
true to type.
Pollination & Isolation
Bees and flies are the main pollinators, although Alliums attract many other insects. Onions and
leeks will not cross with one another, although varieties within species cross readily. The easiest
way to maintain varietal purity is to grow just one variety of each species for seed each year,
allowing insects to do the work. They should be isolated from other varieties by at least 1,500m.
You can grow others to eat, but be vigilant about removing flowers, especially from potato onions
and shallots (for onions) and elephant garlic (for leeks).
If you do want to grow two or more varieties for seed you must either isolate by 1,500m or use one
of the caging techniques. Unless you use alternate caging you will have to hand pollinate daily.
Between 9.00am and noon remove the cage from each variety and use a soft brush to transfer pollen
between the flowers. Go round the flowers two or three times to ensure a good mix, and either clean
the brush in alcohol between varieties or use a different brush for each variety. You may need an
assistant to keep insects away while you are busy pollinating.
Bees and flies are the main pollinators, although Alliums attract many other insects. Onions and
leeks will not cross with one another, although varieties within species cross readily. The easiest
way to maintain varietal purity is to grow just one variety of each species for seed each year,
allowing insects to do the work. They should be isolated from other varieties by at least 1,500m.
You can grow others to eat, but be vigilant about removing flowers, especially from potato onions
and shallots (for onions) and elephant garlic (for leeks).
If you do want to grow two or more varieties for seed you must either isolate by 1,500m or use one
of the caging techniques. Unless you use alternate caging you will have to hand pollinate daily.
Between 9.00am and noon remove the cage from each variety and use a soft brush to transfer pollen
between the flowers. Go round the flowers two or three times to ensure a good mix, and either clean
the brush in alcohol between varieties or use a different brush for each variety. You may need an
assistant to keep insects away while you are busy pollinating.
Harvesting
Allium flowers need to be watched carefully as the seeds mature because the ripe seed pods shatter
easily, splitting and releasing the seeds. As soon as you can see the black seeds within the drying
flowers you should cut the whole head and place it in a paper bag to finish drying.
Please note: Leek seed tends to take a lot longer to ripen than onion seed.
Allium flowers need to be watched carefully as the seeds mature because the ripe seed pods shatter
easily, splitting and releasing the seeds. As soon as you can see the black seeds within the drying
flowers you should cut the whole head and place it in a paper bag to finish drying.
Please note: Leek seed tends to take a lot longer to ripen than onion seed.
Cleaning
Most of the ripe seeds will fall from the dry flower
head quite easily, encouraged by gently shaking. The
rest can be removed by rubbing the flowers between
your palms, or across a sieve that allows the seed to
pass through. The best way to clean seed is by sieving
and winnowing in a light breeze
Most of the ripe seeds will fall from the dry flower
head quite easily, encouraged by gently shaking. The
rest can be removed by rubbing the flowers between
your palms, or across a sieve that allows the seed to
pass through. The best way to clean seed is by sieving
and winnowing in a light breeze