By Seed Co Technical Team
For best results from onions, cool climatic conditions are required during the first part of the growing season. Higher temperatures are preferable for plants to start bulbing and are particularly necessary during the harvesting period. Onion is sensitive to day length and temperatures, and hence each specific variety is suited to a specific region.
Day length influences the bulb formation and is governed by the degree of latitude and the time of year. Zimbabwe lies roughly between 16 – 32o south of the Equator, giving rise to comparatively short day length conditions. Therefore, cultivars able to form bulbs in a day-length between 10 to 12 hours must be selected in Zimbabwe.
Temperatures also exercise an important influence on bulb formation. Onions form bulbs faster at comparatively high temperatures. At low temperatures, bulbing takes place very slowly and if the temperature is too low, no bulbs are formed. Temperatures also influence the incidence of bolting. Fluctuating temperatures, e.g. mean temperatures varying from 7 – 12.5oC encourage bolting.
It must also be noted that onion seedlings older than 10 weeks are more susceptible to bolting due to the slowed growth and reduced vigour. At consistently low temperatures (i.e. below 7oC), few plants bolt as the inducing temperature is too low. Bolting reduces the storage period of onions and it is encouraged to harvest and sell bolted onions as soon as they are marketable.
Onions thrive on a great variety of soil types ranging from sands though to fine textured clays. They also do well on turf soils. The best results, however, are obtained from a fairly deep sandy loam soil. The soils should be well drained with an optimum pH of 5,8 (calcium chloride method), but onions can tolerate more alkaline conditions.
The fertilizer recommendations made here must be regarded as a general guide only. It is impossible to draw up a fertilizing programme which will give the best results under all conditions.
Onions react very favourable to organic manuring. Yields can be increased to a high degree by the use of kraal manure or compost applied at the rate of approximately 40 tonnes per hectare. This practice can, however, cause considerable weed problems. In addition late release of nitrogen may result in thick-necks ( bull necks ) and can also encourage bulb splitting therefore, preferably the manure should be applied to the previous crop.
Applied nutrient levels should be 100 – 150 N, 150 – 220 P2O5 and 120 – 160 K2O per hectare. Excessive applications of nitrogen must be avoided as this causes bull neck
Adequate applications of phosphate and potash are essential for good growth. Potash in the form of Potassium Sulphate improves storage qualities. A basic dressing of 1 000 – 1 300 kg/ha of Tobaccofert (6:18:15 or Vegefert (6:18:15) is suggested.
A top dressing of Ammonium Nitrate at the rate of 100 kg/ha should be applied 3 – 4 weeks after transplanting. A further similar top dressing may be necessary if the plants look yellowish.
It is claimed that onions can be grown for many years on the same land but the value of a rotation cannot be disregarded. Diseases such as root rot and pink root eventually build up and crop rotation is the only remedy.
Other methods such as fumigation have been found to work but can only kill particular strains of the disease and are costly.
A large variety of crops can be used for rotational purposes, e.g. legumes such as beans, peas and cowpeas, etc. and no specific recommendations need be made.
A minimum rotation of 3 years is essential for seedbed sites.
Three different methods may be used for onion production for the market:
- a) By transplanting seedlings from seedbed to land.
- b) By sowing direct into the land.
- c) By using onion sets.
In (a) and (c) the initial procedure is the same. The soil must be well prepared. The seedbeds must have a very good tilth and be as level as possible. The importance of level seedbeds cannot be over-emphasised. This factor often determines the success or failure of onion growing, even before the seedlings are transplanted. The plants on the high spots develop very slowly. They are, therefore, weak and the first to be attacked by thrips.
The seedlings in the hollows of seedbeds remain yellow and stunted, as onions are sensitive to waterlogging.
The seed may be sown in drills 1.2 cm deep or may be broadcast at a rate of about 200 g seed per bed. After sowing, the seed should be lightly covered to a depth not exceeding 1 cm.
The soil should be kept moist by two light irrigations per day until germination is complete. Thereafter, as soon as possible, reduce the watering to once every three days.
On some light soils it is advisable to apply a fine mulch to keep the surface of the soil moist. This should be thinned as soon as germination has commenced.
Late January to early June is the easiest period to raise seedlings. Growers are advised to obtain fresh seed every year in order to ensure a good stand.
The population of onion required per hectare ranges from 600 000 to 1million plants but for medium sized onions a population of 750 000 to 850 000 plant per ha is recommended
Seed Co Technical Team:
Silas Mutota is Seed Co Regional Sales Manager.
+263 773 473 948/ [email protected],com
John Basera is Seed Co Agronomy and Extension Services Manager.
+263 772 413 184/ [email protected] .
Kudakwashe Mashanda is Seed Co Product Development Manager.
+263 775605 057/[email protected] a) Transplanting