Growing Onions profitably

Introduction
Onions are consumed in almost every household in Zimbabwe. Not only are onions grown
for domestic consumption but are also grown for commercial purposes. Onions are one of the
few crops in Zimbabwe that will always have a ready market. This simple guide will equip
onion growers with recommended practices in the cultivation of onions.

Climatic requirements
Onions are a cool season crop. Germination of onions is slow at 6 to 7 oC. The optimum
germination temperature range is 10 to 35 oC, and the maximum temperature is 40 oC. Onion
is adapted to a growing season with air temperatures at 13 to 24 oC. Low temperatures early
in the season are desirable with higher temperatures after bulb formation. Onion is tolerant of
frost but seedlings are generally only tolerant down to -1 oC.
Onions are sensitive to photoperiod. Long days are favourable to onion production as this
enhances leaf development and which, in turn, is directly related to bulb size. Early varieties
require 13 hours for bulb initiation while late varieties require 16 hours for bulb initiation.
Onions begin to form bulbs when day length reaches the appropriate duration for the cultivar,
provided temperatures are high enough. Early seeding or transplanting is, therefore, essential.
Cool weather during early growth of the plant promotes formation of seed stalks (bolting).
Onion bulbs grow quickly in warm than cool temperatures. At 40 oC, bulb formation is
retarded. In onion, there is an interaction between day-length and temperature in the bulbing
process of an individual cultivar.

Soil requirements
Soils should be well-drained and well-supplied with organic matter. Sandy loams without
stones are optimal. Heavy clay soils should be avoided. Coarse sand soils may be used for
specialty onion production under irrigation.

Planting
Direct seeding will generally require 5 - 8 kg/ha. Use high quality seed that has been suitably
treated with fungicide dressing. For lands, when transplanting from seedbed, use a seed rate
of 3 kg/ha. Plant the seedbed at 10 g/sq m and often 300 sq m is adequate to plant out 1
hectare. The recommended planting period is from February - March for transplanting
6 - 8 weeks later when plants are 5-7 mm in diameter (Pencil thickness).

Irrigation
Onions have a shallow and limited root system which explores mainly the upper 30 cm of the
soil. This crop should be irrigated frequently throughout the growing season. Soil moisture
should not be allowed to fall below 50% of water holding capacity. Most soils should receive
2.5 cm of water per week from the combination of rainfall and irrigation. Soil moisture is
important in the growth of new roots; the soil moisture must reach the base of the bulb
periodically if the newly formed roots from the stem are to grow into the soil. New roots will
not grow into dry soil.
Crop Nutrition
Onions require a fertile but well balanced soil. Manure is not recommended as weeds are a
serious problem for this crop and also due to variable nitrogen content. Soils must have a pH
of 6.5 to 6.8 for satisfactory crops. On peat soils, pH of 5.5 is sufficient. The soil must
contain adequate calcium for crop growth. This means that calcium must be evenly
distributed and incorporated into the field. Crop failure is common on fields with inadequate
liming. Also, some cultivars may be more sensitive than others to low pH.
Nitrogen - Apply most of the nitrogen pre-plant incorporated (at least 2/3 of the required
amount). Side-dress the remainder in mid- to late June after the seeded onions are about 15
cm tall. Excessive nitrogen especially in July can cause delayed maturity (thick necks) and
soft bulbs.

Phosphorus - Should be banded if possible at the time of seeding. Otherwise relatively

heavy applications of phosphorus must be broadcast and pre-plant incorporated.

Potash - Potash should be broadcast and pre-plant incorporated. Application rates depend on
the level in the soil.
 Micronutrients
 Copper - deficiency occurs on acid or peat soils. Copper may be mixed with the fertilizer
and applied. On peat soils (as an initial application), 50 kg of copper sulphate per hectare is
recommended. Copper sulphate can be applied by spraying it onto the soil surface and
incorporating it into the soil (this material is extremely corrosive to metal).
 Manganese - At high soil pH's a deficiency may show up. Soil application of this element is
not suggested due to the large amounts required. Foliar applications of manganese sulphate
are recommended, starting when the plants are about 15 cm tall with 1.5 to 2.75 kg
manganese per hectare in 300 L of water and repeated in 4 to 5 sprays 10 days apart. Use the
low rate on small plants increasing the rate as the season progresses.

Fertilizer application
For seedbeds, apply Windmill’s basal fertiliser Compound S (7:21:7 9S 0.04B) at the rate of
45g/m2. After 2 to 3 weeks of emergence, apply Ammonium Nitrate (34.5% N) as top
dressing at the rate of 35 g/m2.
For lands, apply Windmill’s basal fertiliser of Compound C (5:15:12 11S.01B) fertiliser at
the rate of 600 kg/ hectare. Apply Ammonium Nitrate (34.5% N) at the rate of 100 kg per
hectare after 4 to 6 weeks after transplanting. For more information, visit Windmill’s 1
hectare sheet on the website (www.windmill.co.zw).

Weed control
Onions do not compete well with weeds. Good weed control requires integration of cultural
and chemical techniques. Herbicides will provide pre-emergence and post-emergence control
of annual weeds but repeat applications may be necessary. Cultivation and hand weeding are
usually required to supplement chemical control. Onions should be planted in soil where the
annual weed seed population has been reduced by cultural procedures such as crop rotation,
fallowing or stale-seedbed technique. Specialty onions can be successfully grown by
transplanting through black plastic mulch. This method provides excellent weed control and
crop growth. Care must be taken to avoid fields where residual herbicides from previous
years persist in the soil as crop injury may occur. For pre-emmergent control of annual weeds
and broad leaf weeds, apply Ronstar 25 EC or Linurex 50 WP. Use Agil 40 EC for post
emergent weed control.
Pest Control
Nematodes
Onions are often prone to nematode attack. It is recommended that one applies Basamid GR
against them in the seed bed.
Onion Maggot
Onion maggots overwinter in the pupal stage with adult flies emerging in the summer. Adults
resemble the common housefly but are slightly smaller (6 mm) and pale grey. The elongate
white eggs are laid in the soil at the base of onion plants. The creamy-white larvae emerge
within one week, and reach a length of 7 mm when fully grown. If onion maggots have been
a problem it would be possible to use onion setts along the margins of the field as a trap crop.
When injury is seen, or if onion maggots are known to be a problem in your area, apply a
fallow treatment. After harvest remove and dispose of any onions that are left in the field.
Diazinon 30 EC is a good remedy.

Onion Thrips
Onion thrips are minute insects that puncture the leaves or stems and suck up the exuding
sap. This causes the appearance of whitish blotches on the leaves. The insects may be found
in large numbers between the leaf sheaths. Thrips are slender, yellow, active insects, at most
1 mm long. They usually enter the field border areas first and become problems especially
under hot, dry weather conditions.
The threshold for thrips can be determined by counting their numbers. This is done by
generally looking at the newest leaves on the plant - the greatest number of thrips will be
found between the new leaves. Count the number of thrips per plant on 30 to 50 plants
throughout the field to calculate the average number of thrips per plant. You then divide by
the leaf number to give the average number of thrips per leaf. Apply insecticide treatments
when the number of thrips observed exceeds the threshold of 3 thrips per leaf for cooking,
onions, or 1 thrips per leaf for Spanish and green bunching onions. After the crop is
harvested, the tops should be raked together and burned. A chemical recommendation will be
Malathion 25 WP.

Cutworms
Upon detection of cutworms, apply Dursban/ Pyrinex 48 EC, Fenveralate 20 EC, or
Lamda-Cychlothrin 5 EC .
Diseases Control
Damping-off and Root rots
Damping-off occurs in seedlings which may topple over and die because of decay at the soil
surface. Surviving seedlings may be stunted because of a brownish rot on the roots and shoot.
Pink root is a type of root rot which results in reduced bulb size. Affected plants turn yellow
and wither and roots have a pink colour which eventually turns brown to black.
Control: Treat seed with Thiram 80 WP for damping-off. To control pink root-rot, practise a
rotation of several years.

Botrytis leaf blight

Botrytis leaf blight develops as white spots 1 to 5 mm in length with light green to silvery
halos. The centres of the spots become sunken and straw coloured. When spots become
numerous, the leaf tips die back down the entire length of the leaf.
A regular spray of Dithane M45 schedule throughout the summer is necessary to control
Botrytis leaf blight.

Downey mildew
Downy mildew first appears on leaves as elongated patches varying in size and slightly paler
than the rest of the foliage. Under moist conditions, these areas turn violet grey , which may
spread to surrounding tissue. Leaves fold over at the affected areas and the leaf tips wither.
Onions that are severely infected do not cure properly and are susceptible to storage rots.
A regular spray of Dithane M45 schedule throughout the summer is necessary to control
Downey mildew.

Purple blotch
Purple blotch appears on leaves as brown spots 1 to 3 cm in length with red-purple margins.
The brown areas may have alternating dark and light zones giving a target board effect.
Leaves weakened by purple blotch may fall over. Purple blotch frequently develops after
Botrytis leaf blight or downy mildew has appeared.
A regular spray of Dithane M45 schedule throughout the summer is necessary to control
Purple blotch. Practice a 2- year rotation and destroy infected crop debris after harvest and
destroy refuse heaps of onions culled from storage.

White rot
This is a very destructive disease of the onion family. The characteristic symptoms are a
white fluffy fungal growth and soft rot around the base of the bulbs. Masses of tiny black
sclerotia form in the fungal growth and in bulb tissues. These sclerotia allow the fungus to
survive in the soil for 4 to 5 years or longer.
The use of infected onion setts or transplants can introduce the disease to new areas. Do not
introduce the disease from infected areas, equipment, pallet boxes, etc. Follow a 4 to 5 year
rotation. For chemical control, use Quintozine 75 WP or Allisan 50 WP.

Bacterial diseases
There are at least three different diseases caused by bacteria: soft rot, slippery skin, and sour
skin. The bacteria causing slippery skin and sour skin enter the onion through wounds on
leaves and when heavy irrigation or rainfall results in water standing in leaf axils and the
neck. These bacteria can enter the bulb prior to harvest at windrowing or through injuries at
harvest. Soft rot often occurs when bulbs are damaged by onion maggot, bulb diseases, or
mechanical injury.

Harvesting and Handling

 You can always tell when onions have stopped growing. The leaves will lose their colour,
weaken at the top of the bulb and flop over. It's not good to leave the onions in the ground for
longer than two weeks after the tops die because they become open to organisms that can
cause rots in storage, or they might even start growing again. Pull your onions up on a sunny
day if you can, then let them sit in the sun for another day or so to dry (in hot climates this
usually takes just a few hours). This drying kills the root system at the bottom of each bulb.
The roots will be like little brittle wires when they're dry. Picking the right day to pull the
onions can determine how well the onions will keep. If you harvest them after some rainy
weather they'll have a lot more moisture in them and won't dry out well.
Storage and Conditioning
 Curing
Undercut or pull and windrow onions when at least 60% of tops have fallen down, and leave
in windrow until inside neck tissues are dry before topping and storing. If the season is too
damp for field curing, dry in storage at temperatures between 27 to 35 oC with relative
humidity at 70% to 80% for 2 weeks or cure with forced ventilation at 27 to 35 oC. Curing
with forced air at 35 oC can be complete in as little as 48 hours. Best skin colour develops
when onions are cured at 24 to 32 oC with 80% relative humidity. Properly cured onions will
have a tight neck and dry outer scales that rustle. Onions for setts are pulled and cured in late
summer or early fall while the tops are still green, and when the bulbs reach 2 cm in diameter.
Larger setts produce a high proportion of "bolters''.
 Storage
Following the curing period, onions should be cooled and held at 0 oC and 65% to 70%
relative humidity. With forced air circulation, it may be possible to allow humidity as high as
85%. Root growth and decay are stimulated at high humidity, and sprouting occurs at high
temperatures. Thick-necked onions will not keep and should be removed before storing if
possible. Onions are frequently stored in bulk storage, two or four meters deep, constructed
so that the bulbs can be cured by forcing air up through them. This method is considered
more efficient and economical than curing and storing in crates. Properly cooled onions can
be successfully stored at 0 oC for 6 to 8 months.