onion

Vegetable production: Prepare to plant onions

Properly dried onions have a good storage life and the robust onion is easy to transport over long distances. The grower can hold his produce while he (or she) waits for a flat market to pick up.

SOIL AND CLIMATE

Tough and adaptable, onions grow in most soils. But for the best results, plant the crop in deep, well-drained, loamy soil, with a pH of between 5,5 and 6,5 (slightly acidic). For optimal growth the onion needs temperatures of between 18°C and 22°C. Temperatures between 25°C and 27°C speed up bulb formation and low temperatures between 8°C and 13°C trigger flowering.

A field of commercially grown onions in flower. Lower temperatures between 8°C and 13°C trigger flowering.
A field of commercially grown onions in flower. Lower temperatures between 8°C and 13°C trigger flowering.

In a tropical climate where the difference in seasonal day length is not that noticeable, short day onion varieties are more suitable. The higher up you are (increased altitude), the lower the growing temperatures, and the less likely it is that the onions will bulb prematurely.
The opposite is also true; the lower down you are (decreased altitude), the higher the temperatures, and the greater the chances of premature bulbing.

SOW RIGHT – REAP RIGHT

Direct sowing of onion seed is tricky, so beginners should grow out seedlings and then transplant them. Plant with the furrows at least 15cm apart and sow the seed 10mm to 15mm deep, at a rate of about 7g of seed per m(about 1 500 to 2 500 seeds). If you sow too much seed, you will get spindly plants.  It it’s hot sprinkle a thin layer of grass on top as a mulch. When the plants emerge at between seven (7) and 14 days after sowing take the mulch off. Plants get leggy if you leave the mulch on for too long.Once the seedlings are between 12cm and 20cm high and about as thick as a pencil, they are ready to transplant. Short-day onions should be ready for transplanting between 6 weeks and 8 weeks.

IN THE FIELD

To transplant, score furrows 2cm to 4cm deep in the ground and lay the white part of the seedling in the furrow. Don’t trim the leaves. Cover the roots with a rake and compact the soil around them with the back of the rake or with your hand. Don’t plant too deep because you may get elongated bulbs. Keep the soil moist for five (5) days, once the plants are in the field. It helps with the shock of transplanting and allows the root system time to settle.

It’s easier to transplant seedlings when they are about eight weeks old than to sow them directly into the land.
It’s easier to transplant seedlings when they are about eight weeks old than to sow them directly into the land.

The field beds should be 1m to 1,2m wide and the path between the beds, 0,5m to 0,7m wide. Use soil from the path to build the beds up 8cm to 10cm high.

Plant the seedlings 7cm to 10cm apart with a row spacing of between 20cm and 25cm (50 plants to 60 plants per m2).

FERTILISATION

During soil preparation, work in 100g of 2:3:2 or 2:3:4 per m2. Onions are heavy feeders and need nitrogen and potassium. To make a measure for granular fertiliser doses,  use a plastic cap, like those on aerosol spray containers, place the plastic container on a small kitchen scale and add fertiliser until it weighs the required amount of, for example in this case, 100 grams. Shake the cap to level the fertiliser and mark the level with a felt tip pen.

Too much nitrogen (N) late in the season may result in excessive leaf growth, delayed bulb development and thick necked plants. Phosphorus (P) and potassium (K) are required throughout the growing season. Use 10g LAN per 1m2, as well as 10g potassium chloride (KCl) per 1m2, three weeks after transplanting and again six weeks after planting, especially if the soil is sandy. Apply the fertiliser 5cm to 15cm from the plants and work it in with a fork without damaging the roots. Water immediately after applying the top dressing to get the fertiliser into the soil.

IRRIGATING

Onions need between 400mm and 600mm of water during the growing season. The soil must be kept moist because the roots are in the upper 30mm of the profile. Do not water onions three weeks before harvesting.

HARVESTING

In small gardens, harvest your onions when 100% of the leaves have lodged. Lodging means the plant has fallen over. For bigger areas and commercial plantings, harvest when 50% of the crop has lodged. Don’t leave the plants in the soil for too long.

Lift onions by loosening the soil with a fork or harvester. Collect the plants, make bundles and tie the leaves together. Hang them from the ceiling of a storeroom to dry. Once the neck of the bulb has dried completely, you can cut the leaves and store the bulbs.

Partially dry short-day onions to get to the market early and realise the best prices.

Yields as high as 40t/ha are possible if farmers manage effectively and the conditions are favourable. Top farmers can get as much as 60t/ha.

Once the onions have been lifted, they should not stay on the ground for too long.
Once the onions have been lifted, they should not stay on the ground for too long.

STORAGE

Once you have dried and cleaned the onions, store them in a cool, dry, well-ventilated space. Onions for storage should be mature, well dried and undamaged. Turn the bulbs often so that all sides of the bulb are exposed to air and light at regular intervals. Place them in layers no thicker than 10cm and take out any bulbs that show signs of rotting.

CROP MANAGEMENT

If you want optimal growth, never let the soil dry out, but remember that onions are sensitive to waterlogging. If your field slopes take care not to let the plants on the downside get waterlogged. They will turn yellow and remain stunted.

Weed control is critical because onion plants, especially seedlings, are easily outcompeted by weeds for nutrients and water. Weed carefully without disturbing the roots, and stop working between the rows when the foliage gets dense to avoid damaging the leaves.

CROP MANAGEMENT

if you want optimal growth, never let the soil dry out, but remember that onions are sensitive to waterlogging. If your field slopes take care not to let the plants on the downside get waterlogged. They will turn yellow and remain stunted.

Weed control is critical because onion plants, especially seedlings, are easily outcompeted by weeds for nutrients and water. Weed carefully without disturbing the roots, and stop working between the rows when the foliage gets dense to avoid damaging the leaves.

PESTS AND DISEASES

  • Thrips are small insects that feed on the leaves, sucking plant sap. Affected plants take on a silvery, flecked appearance. Rotate your crop if you want to avoid thrips.
  • Nematodes attack the roots, but you can control them.
  • Downy mildew shows up as grey to purplish mounds on the leaves that turn pale green and yellow before they die off. Low temperatures and high humidity provide a good environment for downy mildew.
  • Pink Root appears on the roots of seedlings and older plants. The root turns pink, shrivels and dies. Resistant cultivars are available.
  • Purple leaf spot or Alternaria blotch Large brown lesions show up on the leaves which eventually die.
  • White bulb rot A white fluffy, fungal growth appears on the bottom of the bulb, which rots.
  • Basal rot The leaves die from the tip and if you cut through the bulb you will see brown rot.

STORAGE DISEASES

Black mould The bulb shows blackening just below the skin and later rots.
Inspect your stored bulbs and remove infected onions straight away. Clean containers with a bleach solution.

Resource: Agricultural Research Centre (ARC)

share this