Onions and shallots are the pillars of French cuisine. Impossible to imagine onion soup, béarnaise sauce, or a shallot confit without these flavorful alliums. The good news is that these vegetables are among the easiest to grow in the garden. They need little care, take up minimal space, store for a very long time, and even repel certain pests from neighboring crops. In this guide, we explain everything you need to know to succeed with onions and shallots, from planting to storage, including care, diseases, and choosing varieties.

Onions and shallots: the basics

Differences between onions and shallots

The onion (Allium cepa) and the shallot (Allium cepa var. aggregatum or Allium oschaninii) belong to the same family but have notable differences:

Both are grown in very similar ways, which is why we cover them together in this guide. The few differences will be noted as we go.

Planting from sets or from seed?

There are two methods for growing onions:

Planting sets (small bulbs): this is the simplest and most common method for home gardeners. You buy small calibrated bulbs (sets) from a garden center or by mail order and plant them directly in the ground. Advantages: ease, time savings (3-4 months of growing instead of 6-7), near-guaranteed success. Disadvantages: more limited variety choice, higher cost than seed.

Sowing from seed: more economical and offering a much wider variety choice, sowing requires more patience and know-how. Seeds are sown in a nursery bed from February to April, and young plants are transplanted outdoors when they are pencil-thick. The complete cycle is longer (6 to 7 months). Onions grown from seed generally store better than those from sets.

For shallots, planting bulbs is almost the exclusive method. Growing shallots from seed is rare and reserved for professionals.

Preparing the soil

Onions and shallots share the same soil requirements:

The ideal soil

Plot preparation

  1. The previous autumn: dig or use a broadfork to 20-25 cm depth. Incorporate well-rotted compost if the soil is poor.
  2. Before planting: rake the soil smooth. Remove large clods and debris. The soil should be loose but firm (not freshly dug and too airy).
  3. In heavy soil: form mounds 10 to 15 cm high to ensure perfect drainage.

Crop rotation

Don't grow alliums (onions, shallots, garlic, leeks) in the same spot two years running. Wait 3 to 4 years before returning to the same plot. Fungal diseases (fusarium, botrytis, downy mildew) persist in the soil and worsen year after year without rotation.

Planting onion sets

Planting sets: calendar and technique

When to plant?

There are two planting periods depending on the type of onion:

Autumn planting (October-November): for white onions and gray shallots. These bulbs will root before winter, remain dormant during cold months, then resume growth from February-March for an early harvest in June. Advantages: earlier harvest, larger bulbs. Risk: rot during winter if the soil is too wet or if winter is very mild and rainy.

Spring planting (February-April): for yellow onions, red onions, and pink shallots (Jersey type). Plant as soon as the soil has dried out and is workable, ideally in February-March in the south and March-April in the north. Harvest in July-August.

Planting technique

  1. Spacing: plant sets 10-15 cm apart in the row, with 25-30 cm between rows. For shallots, increase to 15-20 cm in the row as each bulb will divide into several cloves.
  2. Depth: push the set into the soil so that the tip just breaks the surface. Don't bury it completely. The rule is simple: one-third in, two-thirds out.
  3. Orientation: place the set with the pointed end up (where the leaves will emerge) and the roots down.
  4. Firm the soil gently around the set to ensure good contact with the ground.

Sowing onions in a nursery bed

If you prefer sowing, here is the method:

  1. In February-March, under cover or on a windowsill, sow onion seeds in trays or cell trays. Cover with 1 cm of fine potting soil. Ideal temperature: 15-20°C (59-68°F).
  2. Emergence takes 10 to 15 days. The seedlings look like thin grass blades.
  3. When the plants are pencil-thick (about 15 cm tall, 8-10 weeks after sowing), transplant them outdoors at 10-12 cm intervals.
  4. Trim the leaves and roots by one-third at transplanting time to help establishment.

The best varieties

Yellow onions (the most common)

Red onions

White onions

Shallots

Care during the growing season

Watering

Onions and shallots are undemanding when it comes to water. In fact, excess water is fatal to them. Here are the rules to follow:

Weeding

Onions and shallots are poor competitors against weeds. Their upright, thin foliage provides no shade, and weeds quickly overwhelm them.

Fertilizing

Onions are light feeders. If your soil was properly amended for the previous crop, no additional feeding is needed. Excess nitrogen is actually counterproductive: it promotes foliage at the expense of the bulb and reduces storage life. A surface application of wood ash (rich in potassium) in spring is the only potentially useful supplement.

Diseases and pests

Onion downy mildew (Peronospora destructor)

This is the most dreaded disease. It shows as yellowing and wilting of the leaves, which become covered in a grey-violet fuzzy growth. It is encouraged by humid weather and cool temperatures (10-15°C / 50-59°F). Damage can be very significant.

Neck rot (Botrytis allii)

This disease often appears after harvest, during drying or storage. The neck of the onion softens and becomes covered in grey mold. The rot then spreads to the entire bulb.

Onion fly (Delia antiqua)

Its white larvae tunnel through the bulb, causing yellowing and wilting of the foliage. The bulb rots from the inside.

Thrips

Tiny insects that cause silvery streaks on the leaves. In severe infestations, the foliage turns white and dries out. Treat with liquid soap or neem oil under heavy pressure.

The carrot-onion partnership

This is the classic companion planting combination in the vegetable garden, validated by generations of gardeners' experience and confirmed by scientific studies. The scent of onions repels the carrot fly, while the scent of carrots repels the onion fly. Alternate a row of carrots and a row of onions for free, natural mutual protection.

Onions drying in the sun

Harvesting

When to harvest?

Onions and shallots are ready to harvest when the foliage yellows, falls over naturally, and begins to dry. This is the sign that the bulb has reached maturity and the plant is entering dormancy. Depending on varieties and planting date:

Never force maturity by bending the foliage yourself (despite a persistent myth). Wait for the plant to do it naturally. Bending the foliage prematurely interrupts maturation and harms storage life.

Harvesting technique

  1. Choose a dry, sunny day for harvesting. Ideally after several days without rain.
  2. Lift the bulbs with a garden fork, taking care not to damage them. Wounds are entry points for storage diseases.
  3. Leave the onions on the ground for 2 to 3 days to begin drying in the sun. Turn them once for even drying. If rain threatens, bring them under cover.

Curing and storage

Curing

Curing is the crucial step that determines storage life. A poorly cured onion will rot within weeks.

  1. After the 2-3 days of field drying, move the onions to a dry, airy, rain-sheltered place: barn, garage, covered porch, attic.
  2. Spread them in a single layer on slatted shelves, wire racks, or open crates. Air must circulate freely around each bulb.
  3. Let them cure for a minimum of 2 to 3 weeks. The outer skins should become dry, papery, and deeply colored. The neck must be completely dry and sealed.
  4. Cut the dried foliage to 2-3 cm from the bulb, or braid the onions into strings if you wish to hang them (a traditional, attractive, and practical method).

Storage conditions

Storage duration by variety

Companion planting

Onions and shallots are excellent companions for many vegetables:

Avoid planting onions and shallots near legumes (beans, peas, broad beans) as alliums inhibit nitrogen fixation by the legumes' symbiotic bacteria. Also avoid proximity to other alliums (garlic, leeks, chives) for sanitary reasons (same diseases spread).

Summary calendar

October - November

Plant white onion sets and gray shallots in well-drained soil. Prepare plots for spring planting by incorporating well-rotted compost.

February - March

Plant yellow, red onion sets and Jersey shallots as soon as the soil is workable. Sow onion seeds in a nursery under cover if you choose the seed method.

April

Transplant nursery-raised onion seedlings. Last plantings of sets. Begin regular hoeing.

May - June

Hoe and weed regularly. Watch for downy mildew in humid weather. Harvest autumn white onions and the first gray shallots. Stop watering one month before the expected harvest.

July - August

Harvest yellow and red onions and spring shallots when the foliage has fallen over and dried. Sun-dry then cure under cover for 2-3 weeks.

September

Store well-cured onions and shallots in a cool, dry, ventilated place. Check regularly and remove bulbs showing signs of rot.

"A vegetable garden without onions is like a kitchen without salt. These humble bulbs hidden underground are the silent foundations of our gastronomy. Growing them means ensuring a self-sufficiency that is the gardener's pride."

Onions and shallots are among the most rewarding vegetables in the garden. For a minimal investment of time and space, they offer abundant harvests that keep for nearly the entire year. Start by planting a few sets this spring, and you'll see: once you've tasted a garden onion, slowly caramelized in the pan, you'll never want to go back to supermarket onions.