Optimum germination and emergence of onion seed occurs at temperatures of 55 - 83°F. While onions have the fastest growth rate of the allium genus, their relative growth rates are about half that of lettuce or spring cabbage.
In temperate and warm temperate regions, depending on cultivar and environment, the crop cycle can take 9-10 months and the crops are either direct sown or transplanted in the autumn for harvesting the following spring or summer. In the tropics, where they are more commonly grown in mountainous areas, they are generally harvested 90 - 150 days after transplanting in the spring. In some countries, e.g. UK, New Zealand and Israel, both autumn and spring sown crops are produced.
Bulb formation requires 12 - 15 hour-long days and high temperatures. High yields come in crops where early leaf cover is produced and then maintained for a long period prior to senescence. Planting crops at higher densities can increase light interception, but in thicker crops, bulb size will be lower. Thus, it is important to manipulate sowing density to provide yields of a desired bulb size.
Harvest index varies according to onion type. In salad onions and chives nearly 100% of the shoot may be harvested and in leeks 35-50%. Dry onions need a period of curing or drying in order to seal the neck, prevent invasion of diseases and rots and to create a bright crack-free skin.
In temperate regions, curing is usually carried out in store, but elsewhere, onions are lifted and left to dry in the field. In some countries, e.g. Argentina and South Africa they are covered with straw to protect from direct sun whilst drying.
Soils need to be well structured and fertile to maximize growth and produce high yields. Seedbed condition is critical, particularly if crops are being grown from seed. A fine, consolidated seed zone is required for maximum germination and good establishment.
Rooting is generally poor. Onions have sparse root systems with short lengths and few root hairs. Thus, it is important to maintain nutrient and soil moisture availability within the shallow rooting area.
Research suggests that water uptake is restricted to the top 10 inches of soil. While onions can survive long periods of drought stress, water availability is critical for growth and high yields of quality crops.
Under drought stress, onions are more likely to split or form double and multiple bulbs. Onions have a high water requirement, usually around 3 inches of water per week. However, late season irrigation can delay maturity and lead to skin cracking. When grown on particularly light soils, inter-row guard crops of barley or wheat, or the use of straw helps minimize erosion.
Clods and stones will hinder growth, herbicide efficiency and mechanical harvesting, so heavy or stony soils are usually avoided. Soil pH is normally in the range of 6-7, but on organic soils, onions can be grown down to pH’s of 4. When pH drops below 5.5, magnesium and molybdenum availability drops and above 6.5, zinc, manganese and iron become deficient.
Onions are very sensitive to salinity and so saline soils and salty irrigation water should be avoided.
In conditions where sodium chloride creates a soil solution with an osmotic pressure of 0.125MPa, growth is significantly reduced. Nitrate and sulfate forms of fertilizer are preferable; chlorides undesirable.
Onions are slow to emerge, or grow away following transplanting. Because of this, weed control is essential to reduce competition for moisture and nutrients. Their erect growth habit means that even when they are established they do not compete well with weeds. Pre and post emergence herbicide programs are commonly employed. Inter-row cultivating and hand weeding is also practiced in certain regions.
A range of nematodes attack the stem and roots, restricting moisture and nutrient uptake. Stem and bulb eelworm (Ditylenchus dipsaci) is particularly damaging, attacking stems and causing shoot twisting, distortion and young plant death. Seed needs to be nematode free, and soil sterilization and good rotational practice is needed to minimize nematode population build up.
Onion thrips are also a damaging pest worldwide, particularly in warmer regions. They congregate between the young leaf and the plant neck, extensively damaging leaves and slowing growth.
Onion fly or maggot is another major pest that can be controlled using insecticides. Onions are also very susceptible to foliar diseases and bulb rots. Bacterial diseases significantly reduce bulb quality, e.g. Pseudomonas alliicola, which causes slippery skin within the bulb and P. cepacia - slimy or ‘sour’ outer scales.
Neck rot (Botrytis allii), mildew (Peronospora destructor) rust (Puccinia porri) and leaf rot (Botrytis squamosa) are among key diseases that need fungicidal control.
Crops can require 15 fungicide sprays per season. Soil sterilization, deep cultivation to bury the source of inoculum, and wide rotations are important in helping minimize disease problems.
Onions should be mature and dry when harvested. Unless crops are to be cured in the field, harvesting of dry onions usually starts when 60-80% of the tops have fallen over.
Formation of chlorophyll - green scales - in the outer skin can occur when crops are left to cure for too long or when bulb shoulders are exposed to sunlight in the field.
Bulbs need to be carefully handled to minimize bruising and cuts that can allow disease to enter.
Controlled atmosphere storage is increasingly practiced to maintain quality in store over longer periods and extend the marketing period. Maintaining CO2 levels at 5% inhibits fungal sporulation and O2 at 3% reduces sprouting.
Temperatures should be around 34°F to help reduce respiration and relative humidity kept at 65-70% in order to inhibit fungal spread.