When To Sow: Jan-February (Indoors); March-April (Direct) Harvest: August-September
Germination: 7-14 days Full Sun
Starting Indoors: Onion seed can be notoriously tricky to germinate and, as a result, many growers choose to start their onions inside in flats. Use light, well-draining compost or seed starter, sowing seeds at a depth of 1cm (½"). Cover lightly with soil and moisten. Onion seeds germinate best at a soil temperature of about 70F. Also, it’s best to use fresh seed, as the germination rate drops sharply with each year the seed has been stored. To aid in maintaining proper humidity, cover flats with a piece of plastic wrap until seeds have germinated.
Once germinated, remove onions to a cooler growing space and make sure they have adequate light. When the stems are pencil-thick, thin to about 25mm (1”) between plants. Moisten the soil and lift the seedlings carefully, handling the roots as little as possible. Make sure to remove all thinnings to help deter the onion fly. (Thinnings are great as spring onions.)
Once all threat of frost has passed, harden off gradually and plant outdoors 10cm (4") apart with 23 cm (9”) between rows. Water the plants thoroughly after transplanting.
Direct Seed: Plant in drills 1.5cm (½”) deep, in rows 30cm (12") apart in full sun. Once seedlings reach pencil thickness thin to a spacing of 10cm (4”) between plants.
Maintenance
Water during dry spells only. Feed occasionally, either by side-dressing or by applying fertilizer, particularly once the bulbs begin to swell (if you’re going for storage onions).
Keep weed-free as dense weed growth can seriously affect your onion yield. Mulch for deterring weeds and regulating water, but pull back once the bulbs have grown, as the bulbs will dry better if their tops are exposed to the air (keep the roots and lower halves of the bulbs under the soil, however).
Break off any flower stems that appear. As the onions begin to mature, ease up on any feeding or watering so as to encourage them to go into dormancy.
Harvesting
Onions are ready about a week or two after the leaves have begun to yellow and/or turn brown. Choose a dry day and gently lift the onions from the soil. Leave them on the surface of the soil for a day or two to cure in the sun before hanging them up to dry. Once dry, remove top foliage and store it in a well-ventilated, dry location.
Companion Planting
Onions grow particularly well with carrots and lettuce. They also help deter pests from any plants in the Brassicae family (cabbage, kale, radish, etc.). Also do well with beets, chard, and strawberries. Do not plant with peas or beans.