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Onions can be grown for their green immature stems or they can be grown for their mature bulbs. Onions grown for their green stems are called Spring/bunching onions. They are also called green onions and scallions. The terms are often used interchangeably.
The stems of most onions that produce bulbs can be harvested early as green onions. But not all bunching onions will grow bulbs if allowed to grow to maturity. When you select an onion to grow in your garden, make sure you get the type of onion you want—bulbing or bunching.
Bulb onion varieties differ according to the amount of daylight needed for bulb formation. Some varieties require 12 hours of light each day to form bulbs (called short-day); some require 13 to 16 hours of daylight to form bulbs (called long-day). If you live in the South of Australia where summer days are long, grow a long-day variety. If you live in the North, where daylight hours do not vary by much year-round, grow a short-day variety. Onions are not sensitive to mild frost in either spring or autumn. It may be worth noting, long-day onions tend to be round, and globe-shaped and short-day onions tend to be flatter in shape.
Where to plant onions?
- Grow bulb onions in full sun. Spring onions can be grown in a partially shady spot.
- Onions prefer loose, well-worked, well-drained soil rich in organic matter.
- Loosen the soil to 15cm deeps and remove all lumps, stones, and roots.
- Add well-aged compost or a commercial organic planting mix to the planting bed before planting. Turn the soil under to 12 inches (30cm) deep.
- Sandy loam is good soil for growing sets. Loam is good soil for seeds and seedlings. Heavy clay soil can impede the development of bulbs.
- If the soil is not well-drained, grow onions in raised or mounded beds.
- A soil pH of 6.0 to 6.5 is recommended for growing onions.
Onions are temperature sensitive: they require cool weather to produce their tops (early stages of growth) and warm weather to produce their bulbs (late stages of growth). Onions grow best in air temperatures of 13-24°C. Temperatures greater than 29°C can cause soft, gray, watery bulbs.
Planting seeds: Seeds can be started indoors 4 to 6 weeks before you plan to set seedlings out or you can direct sow seed in the garden when the soil temperature is at least 4.4°C. Sow seed 12mm deep. The seed will germinate in 7 to 10 days at 21°C, longer in cooler soil. Thin seedlings from 2.5-5cm apart in rows 30-45cm apart; thin again for bulb onions from 10-15cm apart. The final size of the onion will depend on how much growing space it has.
Planting seedlings: Seedlings are onions that have begun growing. You can start seedlings indoors from seed or you can purchase onion seedlings from Dave’s Seedlings. Place transplants in the garden just slightly higher than the surrounding soil and they will settle into place. Space seedling transplants 5-7cm apart in rows 30-45cm apart. Thin again to 10-15cm or more apart allowing for bulb development.