Growing Onions

The onion plant has been cultivated for 7,000 years. It is a biennial plant that is usually grown as an annual and the flower emerges in the second year.

Growth habits and planting needs

Select your onion type according to your needs from the many varieties available. Onions Australia has a guide to the different varieties that you can access at this link.

Select a sunny position that is weed free and well drained. Raised beds are ideal for providing the drainage that onions require.

Onions need a very fertile soil  for abundant yields and thorough bed preparation due to the rooting habit of the seedlings. The roots have difficulty penetrating heavy soils unless there is enough humus present to lighten them. However, a soil that is too light and open and not water retentive enough is unsuitable too, as the roots must grow in a continually moist soil and good supply of water is especially necessary during early stages of growth.

Onion seedlings
Root development of onion seedlings the same age: one on left in compact soil, on the right is in loose soil.                              Source: Soil and Health Library.

An ideal soil will be retentive enough to keep a constant supply of moisture around the roots but fine and friable enough to allow growth of the bulbs and easy root penetration.

The soil should have a neutral pH of 6.2 to 6.8 and be high in organic matter.  Onions, like garlic, should follow a fruiting crop in the crop rotation system as they need potassium but not so much nitrogen.

Dig the bed over well braking up any clods and removing any stones so that the end result is a loose soil with a good tilth that will make it easy for the roots to grow.Dig lots of well rotted compost and aged cow manure to prepare the bed at least 2 weeks before planting.

Sow seeds thinly, 5mm deep, 12mm apart, in rows 30cm apart in the garden beds.

To start seedlings inside, plant 4 or 5 seeds in each punnet cell, or sow seeds 5cm deep and 10 cm apart in trays. Then plant out after the last frost date.  Cover the roots and leave the seedling lying down and it will stand up when recovered from the transplantation.

Seedlings will emerge in 10 to 14 days. Thin to 10 cm spacing for large bulbs or 5cm spacings for smaller bulbs but higher yields. Take care when thining so the roots are disturbed as litle as possible.

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The main root zone of the onion plants is concentrated in a small area and so there is little competition between plants even though they are closely spaced. Weeds, however should be removed as they appear to prevent damage to the roots before they become too impacted.

This diagram from Root Development of Vegetable Crops by John E. Weaver and William E. Bruner shows the root development (grid in ft) of a mature onion plant grown under optimal conditions.

Mature onion root system
Root maturity of an onion plant.  Source: Soil and Health Library.

By spacing the onions well to allow normal growth and ensure the plants get enough water and nutrients to supply the top, the bulbs can become larger.

Onions have shallow root systems and need consistent moisture and good weed control. Water well if the weather is dry and mulch around them to conserve moisture.

When the bulbs grow, the onion bulbs should sit on the surface of the soil. Do not cover them with soil. They will take six to eight months more to mature.

Use diluted seaweed extract every couple of months from mid season onwards and side dress with blood and bone mid season.


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Harvested and cured onions stored by plaiting and binding together their stems.

The onions will be ready to harvest in 25-34 weeks when the bulb is papery and the tops start to dry, yellow and fall over. Leave the bulbs in the ground for a further 10 days.

Choose a sunny day to harvest. Harvesting is best done during an extended dry period so that the bulbs do not have too much misture which would lower their storage time.

Dig them out carefully lifting them with a fork and try not to damage or bruise the onions. Leave to dry out in a single layer in an open sunny spot for a couple of days until the skins are tight and the necks fully dried out.

Remove to another more protected warm, airy area to cure further and lay them out with plenty of space around the bulbs. Cover the onions with a cotton sheet to protect them and turn the bulbs from time to time. This should take about 3 weeks. The longer the onions cure in this way the longer they will keep.

When they are fully cured, papery and dry, store them in a cool, dry and  airy place. Use net bags or make a string by weaving the tops together.

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