Onion : sowing, planting, cultivation, harvest

Description and Botany

Onion is a vegetable and condiment native to Central Asia where its closest wild relative, Allium vavilovii, can still be found today, between Iran and Turkmenistan. However, it is another species, Allium pskemense, that was gathered in its wild form or taken from nature to be replanted in gardens. Over the centuries, onion was domesticated and adapted to various regions through migrations (India, Mesopotamia).Its presence is confirmed in Egypt, where it has been found in the tombs of pharaohs. The Romans later introduced onion throughout Western Europe. It was during the Middle Ages that onion made its way to Northern France. Christopher Columbus introduced it to the Americas on his second voyage. Numerous new varieties were introduced, adapted, and selected in France during the 19th and 20th centuries. Today, more than 1000 varieties are listed in the European catalogue.

Onion, Allium cepa var. cepa, belongs to the Liliaceae family according to Cronquist’s classification or to the Amaryllaceae according to APG III classification. It is a herbaceous perennial cultivated as an annual or biennial for its bulb and leaves, harvested as spring onion. These are cylindrical and hollow, green in colour. The bulb is spherical, more or less flattened, sometimes horn-shaped. It consists of a tangle of thickened leaves, called tunics, which are filled with nutrient reserves. At the centre is the terminal bud. The epidermis or outer tunic varies in colour; depending on the variety, it can be yellow (e.g. ‘Paille des Vertus’), pink (Rosé de Roscoff), red (De Brunswick), or white (Printanier Parisien). Its flesh ranges from white, pink, red to purple, with a flavour that varies from sweet to strong. In the second year of cultivation, the onion flowers, with a flowering stem emerging from the terminal bud. The flowers are small, white, grouped in large numbers on spherical umbels. They produce black seeds, with a germination duration of about 2 years.

Cross-section of onion bulb – Onion in leaf, first year of cultivation – Onion flowers in the second year[/caption>

Onion varieties

Spring Onions

Storage Onions

Red Onions

Other Species and Original Varieties

White Onion Printanier Parisien - Vilmorin Seeds

Spring Parisian white onion is a variety producing white bulbs that are harvested from spring to summer. It is particularly hardy and is sown from February to April or at the end of summer in August and September.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

Spring Onion Cipollotto White Lisbon - Allium cepa

Lisbon white spring onion is an early variety producing elongated white bulbs with silvery skin and dark green foliage, firm flesh, and a sweet flavour. It is sown at the end of summer or in spring for harvesting from April to July.

• Height at maturity 30 cm

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White Onion Tonda Musona

Tonda Musona white onion is an early variety producing medium-sized white bulbs with sweet flesh. This hardy variety is recommended for early crops, so it is sown at the end of summer for an early harvest the following spring.

• Height at maturity 45 cm

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Onion Long Red Florence Simiane - Vilmorin Seeds

Florence red onion Simiane variety is ideal for consuming green in salads and raw dishes. Its elongated bulbs are a beautiful carmine red with a sweet flavour. It is sown in spring or at the end of summer for a harvest ranging from April to September.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

Untreated Ailsa Craig Onion - Ferme de Sainte Marthe seeds

Ailsa Craig is a yellow onion variety that produces large bulbs with a coppery yellow tunic and sweet, soft flesh, suitable for eating raw or cooked. It is sown from February to April for a summer harvest.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

Onion Stuttgarter Rieser - Ferme de Sainte Marthe Seeds

Stuttgart onion or Stuttgarter Rieser is a variety that produces large, slightly flattened yellow onions with good yield and winter storage qualities. It is sown in spring for a harvest in July and August.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

Onion Red Creole

Creole red onion is a variety suited to warm regions and tropical climates. It produces beautiful, slightly flattened bulbs with pale red tunics and white and red flesh, with a sweet yet slightly pungent flavour. It is sown in spring for a harvest at the end of summer.

• Height at maturity 40 cm

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Onion Red Brunswick - Vilmorin Seeds

Brunswick red onion is a hardy and late variety, harvested at the end of summer as a storage onion. It produces large, thick red bulbs with firm, thick flesh, suitable for eating raw or cooked.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

Keravel pink Onion - Allium cepa

Roscoff pink onion is the onion of the Johnnies, a variety benefiting from an AOP in Léon (Finistère). Its bulb with pink tunic and flesh is of excellent taste quality. It is planted in spring for a summer harvest.

• Height at maturity 30 cm

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Tree onion - Allium proliferum

Rocambole onion has the specificity of producing aerial bulbils. It is a very hardy perennial plant that can remain in place in the vegetable garden for several years. The leaves are used like spring onions, and the bulb is also consumed for its small onions. It is planted in spring or autumn and harvested for a large part of the year.

• Height at maturity 80 cm

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Scallion Ishikura Long White - Ferme de Sainte Marthe Seeds

Ishikura Long White onion is a variety of perennial spring onion close to the stem onion. Like leek, this onion produces very long white boles that can be enjoyed raw, finely chopped, or cooked like onion. Sowing is done from March to July for a harvest from late May to late October.

• Flowering time July to October
• Height at maturity 30 cm

See the product sheet

 

Sowing onions

When to Sow Onions?

Sowing onions takes place, depending on the varieties, in spring or autumn. Spring sowing concerns coloured onions (yellow, pink, and red) and some early varieties of white onions that are sown from February to April. This type of sowing is done directly in place.

Autumn sowing concerns white onions, which are sown at the end of summer (August to September) in trays or pots. They will be transplanted into the ground either one to two months later in regions with a mild climate or at the end of winter around February (for all climates).

Where and How to Sow Onions?

Direct sowing in the ground requires attention during the germination period, which takes an average of 18 days. Until germination, it is essential to ensure that the soil remains moist on the surface and to carefully weed the young seedlings of adventive plants.

To sow onions directly in the ground:

1. Mark the sowing rows by stretching a string, spacing the rows 20 cm apart.
2. Dig a furrow 2 cm deep and then sow sparsely, placing one seed approximately every 2 cm.
3. Close the furrow and lightly firm it with a rake.
4. Water with a fine spray and keep the soil moist until germination.
5. When the plants reach 5 to 10 cm (about 2 months after sowing), thin out by keeping one plant every 10 cm. The best plants removed can be transplanted to another growing bed.

More conveniently, sowing in a nursery, tray, or pot allows for selecting the best onions for transplanting.

For this:

1. At the end of winter, under a frame, sow sparsely in a line or in a tray (or pot), sowing sparsely by broadcasting.
2. Cover with one centimetre of soil or potting mix and firm down.
3. Water gently, using a hand sprayer or the fine rose of a watering can.

Transplanting Onions

When the young plants from the nursery or tray sowing measure at least 15 cm in height, it is time to transplant them to their final position.

Transplanting onions is done as follows:

1. “Dress” the plants if necessary: this involves keeping about 2 cm of roots and cutting a small piece of leaf.
2. Plant the onion shallowly, just 2 to 3 cm deep, then firm it in! Firming means packing the soil around the plant well; to do this, use a dibber to penetrate the soil around the young plant, then bring the soil towards the roots to ensure it is well packed. It is also possible to coat (soak in a mixture of soil and water) the roots before transplanting.
3. Water gently after transplanting.

Onion Seeds – Photo A. Gourmelen

Planting onions

The planting of onions concerns bulblets. Onions obtained by this method are generally bigger but do not store as well as those from direct sowing. The choice of variety is also more limited! However, they offer the advantage of being able to be harvested earlier in the year.

When to plant bulblets of onions?

Bulblets of onions are planted from late February to early April, depending on your climate.

How to plant bulblets of onions?

Planting bulblets requires planning for slightly wider planting distances than for direct sowing:

• in the row, allow 12 to 15 cm between each bulblet, depending on the expected development of the variety
• space the rows 25 cm apart

To plant your bulblets of onions in the vegetable garden:

1. Prepare your bed as you would for a traditional sowing
2. Mark your rows using a string line (a straight row is always easier to weed)
3. Plant each bulblet 2 to 3 cm deep, so that the tip is level with the surface.
4. Water if necessary.

→ Also read our tips in: How to plant onions in the vegetable garden?

Cultivation, maintenance and companion planting

Onions grow in sunny exposure; they thrive in all types of soil but prefer well-drained and light soils, as they do not like stagnant moisture. For example, the geographical area of Roscoff Onion features loamy, loose, deep, and fertile soil with good water retention capacity (due to this specificity and the climate, it does not require irrigation). The Sweet Onion of the Cévennes, grown on the famous terraces, flourishes in light, well-draining soil based on acidic rocks.

Onion ideally grows in fertile soil but the fertiliser should not be too fresh! Mature compost applied in autumn for sowing or planting in spring will be ideal. Onions are sensitive to excess nitrogen, so we advise you to avoid any manure application before sowing or to plant them after green manures, as this would increase the risk of rot and poor storage.

To limit the risk of diseases, carry out, as much as possible, a rotation of about 5 years before growing onions or vegetables from the same family (garlic, shallots, leeks).

Onion cultivation

In companion planting, alternate rows of onions and carrots to mutually deter the carrot fly and onion fly. Furthermore, onion enjoys the company of beetroots and lettuces. Strawberries will also appreciate its presence.

Maintenance tasks involve regularly weeding and hoeing, especially at the beginning of the growing season. Onions are sensitive to moisture: watering should be kept to a minimum and will mainly occur during sowing and transplanting.

You can certainly mulch your rows of onions. However, remember that they are sensitive to both excess nitrogen and moisture: never use fresh clippings but mulch with dry materials such as straw, previously dried clippings, or dry adventive plants.

Onion diseases and pests

The onion fly

If you notice a few individuals in your onion patch that are yellowing and drying out prematurely, there is a good chance they are affected by the onion fly. Uproot the struggling plants and check if they are being devoured by the larvae of this fly at the base of the bulb!

The onion fly, Delia antiqua, resembles a housefly; it lays its eggs in the soil at the neck of the onion. After hatching, the maggots feed on the onion bulb. They thus devour the plant at the collar and cut off the flow of sap, causing the leaves to yellow and eventually die. If the attack remains light, it also provides an entry point for moulds that can take over and kill the plants. Once the larvae are well developed (they measure 8 mm at the end of the larval stage), they easily move from one plant to another. This is why it is important to remove affected plants as soon as they are detected.

How to prevent an attack from this fly?

• Always avoid spraying nettle manure on onions and plants from the same family. Not only do they dislike excess nitrogen, but the smell of this manure attracts the onion fly!
• The onion fly dislikes the strong smell of carrots, alternate your rows of onions with rows of carrots.
• Insect netting, with fine mesh, is a good solution. Since its cost is generally high, use it if this is a recurring problem in your vegetable garden.

Shallot plants in poor condition (Similar symptoms on onion), by uprooting the bulbs one can observe the maggots at work. Photographs A. Gourmelen

Onion downy mildew

Onion downy mildew is caused by a fungus, Peronospora destructor, whose species name is very telling! The symptoms caused by this fungus are brown spots and a greyish-purple fluff on the leaves; it spreads rapidly and leads to complete drying of the foliage. Damage can be significant if conditions remain favourable for the development of the fungus. The conditions for the development of P. destructor are high humidity and mild temperatures, with 15 to 17°C being its optimum for development. These conditions are especially met in spring. Once temperatures exceed 25°C, the fungus dies.

→ Learn more about the diseases and pests of onion in our advice sheet.

Harvest and storage

Spring onions (white, some red) are harvested in spring when the foliage is still green. Storage onions (yellow, red, and pink) are harvested in summer when the stems are completely dried and bent over to the ground. Gently pull them up and leave them to dry for two to three days on the ground in the sun. Remove the excess dried soil by lightly rubbing them.

Storage onions can be kept for several months under good conditions. If you have kept the stems, you can braid them and then hang them up. Otherwise, cut the leaves and store the onions in a dark, cool, dry, and ventilated place. Check beforehand that they have not been bruised to avoid rotting that could contaminate the entire harvest. Over the months, inspect and remove any mouldy onions; usually, it is possible to notice them by their smell.

Braided and hung onions for storage

Uses and benefits

White onion can be consumed raw or cooked; it is often more digestible when cooked and takes on a sweeter flavour. Storage onion is mainly consumed cooked. To avoid crying while chopping an onion, you can peel it under running water or in a basin of water.

Onion is rich in sulphur compounds, fibre, iron and selenium, vitamin C, and finally antioxidants (especially for coloured onions rich in anthocyanins).

The sulphur compounds are responsible for its somewhat indigestible effect and provide the pungent flavour (the same one that makes you cry), but they promote blood circulation and help to lower cholesterol levels in the blood. Selenium is a very powerful antioxidant trace element. Onion boosts immunity and protects against microbes. Onion works in synergy with foods rich in iron, thus allowing for better absorption of this trace element.