When To Sow: February-June Harvest: May-October
Germination: 7-14 days Full Sun
Spring onions can be started as early as February indoors, or from March to June direct seeded. Sow in the autumn for an early spring crop. They grow well in any well-drained soil with decent fertility.
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. Once all threat of frost has passed, harden off and plant outdoors 10cm (4") apart for bulbing/onions. For spring onions, feel free to plant them much closer together. Water the plants thoroughly after planting.
Direct Seed: Spring onions are easier to direct see as no thinning is needed after sowing. Simply sow thinly in any well-drained, moderately fertile soil in a sunny location. Plant in drills 1.5cm (½”) deep, in rows 23cm (9”) apart in growing position. If you’d like your Northern Holland onions to bulb out, plant 5cm (2in) for smaller bulbs or 10cm (4”) for larger bulbs.
Sow at 2 to 3-week intervals for a continuous supply.
Maintenance
Water during dry spells only. Feed occasionally, either by side-dressing or by applying a 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
North Holland onions are ready May through October as salad onions, or from late July as bulbs. For salad onions, harvest as you need and eat fresh. For bulbs, wait until 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 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.