How to Grow Onions

Botanical Name: Allium cepa
Plant Type: Hardy Biennial

The onion, Allium cepa, is one of the more traditional vegetables grown in this country. It is a member of the family Amaryllidaceae, which are generally biennial plants, producing large bulbs as an energy store over winter and flowering the following spring. Its closest relatives include garlic, leek, and chives. Its parentage is uncertain, because the wild ancestor no longer appears to exist, but it is thought to have its origins in the regions of Iran and Pakistan. The first evidence of its cultivation goes back to the time of the ancient Egyptians. Through the process of selection, it has been developed into a multitude of varieties and several distinctive types of vegetable, including the familiar onion, shallot, and potato onion. It is a close relative to the Welsh onion, A. fistulum, grown for its hollow stalks used in the kitchen as scallions.

Onions are similar to most other vegetables, in that there is a great deal of variation between varieties in terms of colour, flavour, degree of spiciness, and so on, which can have a strong influence on the selection of varieties by gardeners. Personal tastes and the end use of onions for cooking, pickling, or eating raw are major factors influencing variety selection. If intended for storage, a rough rule of thumb is that white onions generally do not store as well as brown onions.

Onions rejoice in rich, open soil and moist conditions. Long day/short night or late varieties are not suitable for the tropics but short day, early varieties may be grown anywhere in Australia.

Varieties

Of all the vegetables grown by Australian gardeners, the onion provides perhaps the most confusion for novices in terms of varietal selection. Onions are classified into short, intermediate, and long day-length varieties. The terms describe the minimum duration of daylight (photoperiod) required to induce bulb production. Short day-length varieties form bulbs when day length exceeds about 11 hours per day, while long day-length varieties can require 16 hours of daylight per day to develop bulbs. A handful of varieties are day-length neutral and can be grown anywhere. In reality, it is the length of darkness that exerts its influence on the onion, but by convention they are classified according to day length.

The dependence of onions on day length has a number of important consequences for gardeners. The first is that it places restrictions on the types of onions that can be grown at different latitudes. Australian gardeners living in the more southerly regions of the continent, roughly from the Riverina southwards, experience very long days during late spring and summer, where day length exceeds 15 hours, and consequently they can grow any variety of onion. Gardeners in the central temperate regions of the continent, north to about the latitude of southern Queensland, can grow intermediate and short day-length varieties, while those farther north are restricted to short day-length varieties. Northerners may have their delightful weather, but they haven’t got the freedom to grow the onion variety of their choosing like the southerners!

The second consequence is that in areas where all three types can be grown, each vary in the length of time they take to produce mature bulbs. If sown at the same time, traditionally late autumn to early spring in southern regions (winter daylight), short day-length varieties will mature first and long day- length varieties mature last (summer daylight). For this reason, short day-length varieties are sometimes called early varieties, and their long day-length counterparts, late varieties.

A final consequence of the effects of day length is that the longer the variety of onion takes to mature, the better its storage properties, that is long day-length varieties are generally the best to grow for stockpiling to feed a family and are capable of being stored for up to 12 months.

To promote the development of longer shanks some gardeners blanch leeks in a manner similar to that described for celery, which involves growing the plants in a trench that is progressively filled with soil. This practice entails a lot of additional work, plus it tends to get a lot of annoying grit in between the leaves, making preparation in the kitchen more troublesome. An alternative is to pile up hay around the shanks, which also acts as a mulch. In our experience the better varieties produce ample shanks without blanching.

Growth can be rapid in our mild climate, in some cases reaching full-size 90 days after transplant, so that leeks established in spring may be ready to harvest in summer. Staggered plantings from spring through summer can produce a continuous crop for much of the year. When harvesting leeks exercise some care, for some varieties produce miniature plants around the base, which are known as pearls. Treat these babies with care and they can be transplanted to provide the gardener with a bonus crop.

Early Barletta is a short day-length variety that is very much a gardening standard in this country. It produces flattened, white bulbs that are reasonably spicy. Creamgold, also known as Pukekhoe, is a variety of intermediate day length that has a strong flavour which is very spicy. It is an excellent storage variety. Mild Red Odourless is a red-brown variety, which, as its name suggests, is an excellent variety for people whose preference is for onions that have a flavour at the subtle end of the spectrum. It, too, is an intermediate day-length variety. For experienced gardeners wishing to try something different, there is Red Torpedo, which is an elongated onion, with a deep crimson skin and red flesh. It is an intermediate day-length variety that has a spicy flavour.

There are a number of variations of the common onion which are sometimes grown by gardeners. Potato or multiplier onions differ from the normal onion because they reproduce through the production of numerous small bulbs around a parent bulb. Tree, or Egyptian onions, produce miniature bulbs, not in the ground but in place of flowers on the end of a stalk! Both types are unusual variations of the humble onion which, perhaps, more gardeners should attempt to grow.

Cultivation

Ideally, onions should be grown in a light, well- drained soil that is rich in organic matter. However, some caution has to be exercised in preparing beds for onions, because the use of fresh manure can cause a few problems, such as rotting of the bulbs. Another problem is that excessive nitrogen levels can cause excessive stem growth at the expense of the bulb. Fertilisation strategies should be focused on providing phosphorus and potassium, and the use of well-decomposed manure or beds that have previously been used to grow a nitrogen-hungry vegetable, such as tomatoes, is recommended.

Beds should be established in full sun with good air circulation to discourage fungal diseases. Try to select areas with few weeds, because weed control can be a real headache with young seedlings. Onions have a low tolerance for soil acidity and beds should be limed if this is a problem. It is also important to prepare beds to some depth, to encourage root development and to produce healthy plants with large bulbs. It is critical to have a regular supply of water to the beds, in order to maintain continuous soil moisture, because onions have a limited root system, and any moisture stress will restrict bulb development.

Onions can be either direct sown or transplanted. Direct sowing has the disadvantage of problems of weed control and maintenance of moisture during germination, and they generally take longer to grow. Direct sowing is generally acknowledged, however, as producing healthier plants and bulbs. In view of their sensitivity to day length, sowing times are critical in order to get the best results, otherwise plants may mature and flower before a full-sized bulb is formed. Southerners should sow early varieties in late autumn, progressing through to early spring for late varieties. By sowing early-, mid- and late-season varieties in sequence, the gardener can achieve an extended season of harvest. Gardeners in central New South Wales should sow early varieties in early autumn and mid-season varieties during winter for best results.

Onion seed germinates over a wide range of temperatures, emergence taking four days at 25°C and a couple of weeks at 10°C.

Sun: Full Sun           

Pollination: Insect Pollination

Soil:  onions like excellent drainage. They like a soil that’s high in organic matter, but not overly rich Too much nitrogen will result in excessive leaf growth at the expense of sweet white swollen bulbs. pH is important- onions need something around 6.5. Add lime or dolomite if necessary to sweeten the soil and you’ll be guaranteed the best results.

Growing from Seed

Sow: sow direct or in punnets for transplanting as seedlings

Ease of Germination: medium

Germination Temperature: 10-35°C

Germination Time: 8-14 days

Spacing: 10cm

Harvest: 100-120 days

Yield: 10 bulbs/metre

Maintenance and Additional Care

The two big problems encountered are weed control and maintenance of soil moisture during germination. The traditional method is to mulch lightly when young. Once established, plants should be thinned to create a spacing of 3 cm to 5 cm between plants, with a second thinning several months later if large bulbs are desired. With care, plants thinned out can be transplanted into a new bed, though it is important that the tiny bulbs, known as sets, not be buried too deeply. As the plants begin to accelerate their growth in spring, it is important to use a light mulch around them, such as hay, to inhibit weed development.

Companion Planting

Good companions: Beetroot, Broccoli, Cabbage, Carrot, Chamomile, Leeks, Lettuce, Lovage, Marjoram, Parsley, Parsnip, Silverbeet, Strawberry, Summer Savory, Tomato

Bad companions: Asparagus, Beans, Gladioli, Peas

Pests and Diseases

Apart from weeds, the main problem encountered when growing onions are a number of fungal diseases, which can damage either the stems, bulbs, or roots. Downy mildew can be a problem in foggy or dewy conditions and, if severe, can kill the plants. It can be controlled with copper-based sprays. Several soil-borne fungi can kill plants; if they suddenly wilt and die, this indicates their presence. The best solution in this case is to rotate the bed and rest it for a few seasons.

Growing in Containers

Onions can be grown in pots, but you will need to grow quite a few plants to make it worth your while.

The way to grow onions in container gardens is much like growing onions in the ground. You need good soil, adequate drainage, good fertilizer, and plenty of light. Use a good quality potting mix and liquid feed regularly with an organic liquid fertiliser.

Harvesting

Onions can be harvested at any time for use as spring onions but are fully mature when the stems turn brown and collapse. Bulbs should be carefully dug up, to avoid injury, and dried for several weeks. Be careful with early varieties not to dry them in very hot conditions because they can sunburn and subsequently rot. Once dry, the stems are removed by twisting them off the bulb and the roots can be rubbed off. The bulbs can then be stored in netting bags in a cool, dry location that has good air movement to maintain dryness.

Seed Saving

Seed Longevity: 2 years

Isolation Distance: 1600 m

How to Save Seed

When onions go to seed, they form a seed stalk and a flower head that contains hundreds of tiny flowers. As the seeds form, the flower and plant begin to dry. The seeds are encased in tiny pods which shatter easily. Onion seeds should be harvested as soon as seeds mature, and the pods start to dry. The seed heads can be bent over into a sack and cut from the stalk to avoid losing any seed during harvest.

The seed heads should be placed in a protected area, away for direct sunlight, to complete drying.

Tips and Tricks for Success

• the key to growing big onions is early planting
• good onions need loose soil- dig a trench and fill with compost and fertilizer for giant onions
• once onions begin to swell, make sure that they can continue to grow above ground