Seed Saving Guideline No.
10
Onions & Leeks
Leek - Allium porrum
Onion - 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 guideline 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 Seed Saving
Guideline No.15 Multiplier Onions and Shallots.
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. Grow at least 30 plants to ensure at least 20 can be grown, after selection, in the
second year.
• 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 roguing out 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 1500m.
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 1500m 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.
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.
Storage Clean onion seed by rubbing flowers across a seive
Handle onion bulbs carefully, to avoid bruising, and
cure them in the sun for a couple of weeks before storing. Onions store best at low temperatures
and low humidity (0-7°C/32-45°F, <40% R.H.) or high temperatures and high humidity (25-
35°C/77-95°F, 60-70% R.H.). Room temperature (16-21°C/60-70°F) is the worst possible for
storing onions. Storage gives another chance to reject defective bulbs, such as any that do not last
for long, before replanting in spring.
Ripe onion and leek seed will store in a cool, dark place for at least two years.
Returning Seed to HSL
It is vital that seed returned to HSL is not cross-pollinated. So not send seed to us 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 for seed to get to us in the post so pack seed
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.
No. 10 Onions & Leeks 2016 ©Garden Organic
Garden Organic is the working name of the Henry Doubleday Research Association, Registered Charity No. 298104
