When To Sow: March-June Harvest: May-October v
Germination: 10-14 days Full Sun
Welsh onions are perennials and as such need a fairly “permanent” place to grow. Choose a sunny location and amend the soil with plenty of organic matter before planting. (Never sow in freshly manured soil.) Onions do best in rich, moist loose soil, but Welsh onions will do well in pretty much any decent garden soil.
It’s fine to start Welsh onions indoors, but most gardeners leave that labor-intensive process to the bulbing onions.
Direct Seed: Once the soil has warmed in the spring, sow Welsh onions into finely cultivated beds. 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. Carefully pull any weeds that come up, so that the seedlings can get established.
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. Water the plants thoroughly after planting.
Maintenance
Keep young sprouts well-weeded. Water during dry spells only.
As these are perennial onions, spread compost on the bed yearly and weed well before sprouts emerge in spring. Dense weed growth can seriously affect your onion yield, so stay on top of it.
Mulch will help deter the weeds and regulate watering, but pull it back once the bulbs have grown if you’ll be harvesting the entire plant. (Ignore if you’re going for tops only.) The bulbs will dry better if their tops are exposed to the air. (Don’t pull the soil off the bulbs, just the mulch.)
Harvesting
Welsh 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.
The entire plant may be pulled and eaten like a green onion as early as when 8 to 10cm (3 to 4in) high, or leaf portions may be snipped off as needed for flavouring. If pulled as a green onion, 4 to 5 months are required from seeding to harvesting.
A cool thing about Welsh onions is that you replant the bottom half of any bulb you harvest. Simply save the rooted bottoms (choose the ones with the best rooting systems) and replant in damp soil.
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.