Onion

Light and exposure

The onion wants full sun, no compromise. Choose the most open and sunniest bed in the garden: in shade, the bulb swells poorly and stays small. Count on at least six to seven hours of direct sun to form fine onions.

A well-aired spot also helps keep the foliage dry, which limits disease in a damp climate. Avoid low corners where the air stagnates and the dew lingers late into the morning.

Watering

The onion needs little water. Water moderately after planting to help it root, then only in the event of a genuine prolonged drought. An excess of water rots the bulb and encourages downy mildew.

The important move comes at the end of the season: as soon as the foliage begins to yellow and fall over, stop watering completely. This is the signal that the bulb is finishing its ripening, and leaving it dry at this stage clearly improves storage.

Soil and repotting

The onion likes a loose, light, well-drained soil with no excess nitrogen. Work the soil deeply before planting, but above all do not add fresh manure: it rots the bulbs and attracts diseases. If you want to enrich it, do so the previous autumn with well-rotted compost, never just before planting.

Plant shallow: the set barely goes in, the tip just at the surface, the neck level with the ground. Space the plants ten to fifteen centimetres apart in rows twenty-five centimetres apart. In heavy clay soil, a raised bed or an addition of sand improves drainage.

Temperature and humidity

The onion is perfectly hardy. Sets are planted from the end of winter, as soon as the soil has drained and warmed, and cope without any trouble with the cool spells of early spring. Moderate cold does not worry it, it is stagnant moisture that harms it.

The critical point is often not frost but water. A damp summer and a rainy end to the season are the main enemy: a bulb that ripens in a waterlogged soil stores badly and rots quickly. The whole strategy then consists of harvesting and drying during a dry window before the rains arrive.

Pruning and upkeep

There is no pruning as such. Routine care comes down to keeping the bed clean: hoe and weed regularly, because the onion has few leaves and copes badly with competition from weeds.

Watch for bolting. If a plant sends up a flower stalk, cut it off or harvest that bulb first, because it will not keep. Break or bend the foliage over only if it does not fall on its own at the end of the season, never before maturity has arrived.

Pests and diseases

Onion downy mildew, favoured by a mild, damp summer, is the most common disease wherever the air stays laden with moisture. It shows up as pale spots and then a greyish-purple felting on the leaves, which yellow and fall over before their time. Well-drained soil, spaced-out plants and stopping watering clearly limit the damage.

The onion fly is the other plague: it lays at the neck and its maggots tunnel into the bulb, which rots. The plants wilt and yellow all at once. Rotation is essential, never replant onion, nor any other allium such as garlic or shallot, in the same spot for two to three years.

Propagation

The simplest and safest route remains planting sets. Buy these small calibrated onions in spring, push them in tip upwards as soon as the soil drains at the end of winter, and let them get on with it. The harvest follows in summer, with no worry about temperamental germination.

Sowing from seed is possible for anyone who wants more varieties or larger bulbs. Sow under cover eight to ten weeks before your last expected frost, or directly in place once the soil has drained and risen above ten degrees, in clear rows. It is longer and more demanding in terms of weeding, but economical. Some winter varieties, notably coloured onions, are planted in autumn for an early harvest the following year.

Toxicity

Harmless to humans, it is a vegetable. Onion is toxic to dogs and cats, however, raw or cooked: it destroys their red blood cells and causes anaemia. Keep the peelings and plants out of their reach.

Month-by-month care calendar

Green Hub tips

• Plant sets rather than sowing seed: it is the simplest and most reliable route to successful onions.
• Never fresh manure before planting. It rots the bulbs and invites diseases. Enrich the previous autumn with rotted compost.
• Stop watering as soon as the foliage yellows and falls over. A bulb that ripens dry keeps all winter.
• Harvest in dry weather and dry the onions in the sun before the autumn rains. An onion brought in damp rots in storage.
• Respect the rotation: two to three years without onion or any other allium in the same spot, to avoid the fly and soil-borne diseases.