Evidence suggests that onions have been part of the human diet for over 5 000 years. This means that it may have been one of the first cultivated crops. Knowing this, a quote by American cooking teacher, Julia Child, rings true: “It’s hard to imagine society without onions.”

The onion is a monocotyledonous flowering plant that forms a bulb beneath the ground which is harvested and consumed in a variety of ways.

The bulb is related to other vegetables in the Allium genus such as shallots, scallions, garlic, leeks, chives, spring onions, and Chinese onion. The onion itself has many varieties such as yellow onions, Bremuda or Vilidia sweet onions, white onions, red wing onions, and brown onions. These vegetables contain essential vitamins and minerals such as vitamin C and B6, as well as folate and potassium.

If you want to add this crop to your farm, there are a few requirements to consider.

Climatic and soil requirements

Onions are well suited to a climate that has cooler temperature in the early stage of its growth. As the plant matures, it’s able to tolerate heat much better. The ideal growing temperature for an onion is between 18 and 22°C. Higher temperatures such as 25 to 27°C encourage bulbing, but flowering or bolting is triggered by colder temperatures such as 8 to 13°C.

Onions can adapt to different soil types, but ideally, they prefer a loamy soil that is well-drained. They grow best in soils with a pH of 5,5 to 6,5.

(Source: Maruf Choudhury via Pexels)

By practicing crop rotation with crops that are not related to onions, soil fertility can be increased. Legumes such as beans and peas are a good choice.

Onions are very sensitive to light. The plants will start to form bulbs when they are exposed to more light than the minimum requirement, such as when daylight hours are longer in the summer months. Short-day onion cultivars are available.

Uses

Onions are mainly used as a food source. It can be consumed raw and some of the sweeter varieties are frequently used in salads. The vegetable can be roasted, cooked, pickled, juiced or sauteed. It can be prepared as soups, jams, side dishes like onion rings or as an ingredient in stews and other dishes.

Soil preparation

A fine textured soil is important for the seedbed. Make sure the beds are fairly deep and well-drained to a depth of about 120 cm.

Seedbeds should be 1 to 1,5 m wide. The paths between beds can be between 0,5 to 0,7 m wide. Soil can be removed from the path to build up the seedbed. The beds should be approximately 8 to 10 cm higher than the path.

Seedbeds should be 1 to 1,5 m wide. The paths between beds can be between 0,5 and 0,7 m wide. (Source: Pikist)

Planting

For beginner farmers, the most success may be achieved through transplanting seedlings, as sowing onions directly requires some experience.

The soil needs to be moist for the first five days to ensure plants overcome the shock of transplanting and can anchor their root systems.

Furrows should be spaced 15 cm apart. Seeds can be sown in rows, at a depth of 10 to 15 mm. Do not sow too densely, aim for between 1 500 and 2 500 seeds per 1 m2. To avoid the soil from drying out or too much water evaporating, a thin layer of mulch or grass cuttings can be applied. Once the seedlings start to emerge, the mulch can be removed.

If you are using seed trays, seedlings of about 8 to 9 mm in diameter and 12 to 20 cm high are ready for transplanting. They can be planted 7 to 10 cm apart. The distance between rows should be 20 to 25 cm.

(Source: Engin Akyurt via Pexels)

Fertilisation

100 grams of 2:3:2 (22) or 2:3:4 (30) per 1 m2 can be worked into the soil during preparation. Onions in particular need a lot of nitrogen (N) and potassium (K). Note that too much nitrogen in the later part of the season can cause too much leaf growth and too little bulb development.

The plants require phosphorus (P) and potassium throughout the growing season. 10 grams LAN per 1 m2 and 10 grams potassium chloride (KCl) per 1 m2 should be applied three and six weeks after planting. This is especially important for sandy soils. It can be achieved by working it into the topsoil with a fork, carefully avoid damaging the roots. Water immediately to ensure the fertiliser starts working.

Two red wing onions. The onion itself has many varieties such as yellow onions, Bermuda or Vilidia sweet onions, white onions, red wing onions, and brown onions. (Source: Klim Kin via pixabay.

Irrigation

The water requirements of onions are approximately 400 to 600 mm during growing season. Onion roots are generally concentrated in the top 30 mm of the soil and thus need to be kept moist.

Pest and disease control

There are a few pests that threaten the onion plant. These are as follows:

Thrips

Thrips are small insects that drink the plant sap. When a plant is attacked, the plant becomes silvery and flecked. Farmers should practice crop rotation and good management to combat this.

Dried onions in the sun. (Source: Alistair Mockett via Pexels)

Nematodes

Nematodes attack the roots but can be controlled with fungicides.

The most important diseases are:

Downy mildew

This appears as grey-purple mounds on the leaves. The leaves lighten in colour and then start to wilt. They turn pale green, then yellow and die off. Downy mildew is caused by the combination of low temperatures and high humidity.

Pink root

This disease is found on the roots of seedlings and older plants. As the name suggests, the roots become pink and starts to die. The best control method is to use pink root-resistant cultivars.

Purple leaf spot or alternaria blotch

This disease can be identified as large brown lesions on the leaves. It causes the leaves to die.

White bulb rot

It is identified as fluffy white fungal growths along the bottom of the bulb. The bulb eventually rots.

Basal rot

With basal rot, the tips of the leaves die. The bulb also shows brown rot in the middle.

Black mould

Black mould turns the bulbs black. The layer below the skin is black and the rest is rotten.

To prevent this, don’t store damaged onions and those showing disease symptoms. You should regularly inspect stored bulbs and remove infected ones.

The information provided in this article is credited to the Agricultural Research Council Vegetable and Ornamental Plant Institute (ARC-VOPI). For more information, contact +27(0)12-841-9611 or visit http://www.arc.agric.za.