Onion Water Requirements and Irrigation Systems

Wikifarmer

Editorial team

4 min read
17/01/2025
Onion Water Requirements and Irrigation Systems

How much water does the onion need? 

Onion plants need sufficient soil moisture and water supply (rainfalls or irrigation) to establish on the field and produce bulbs of high quality. Based on scientific evidence and practical experience, it is clear that the crop can yield 25-50% more when the plants are irrigated. Depending on climate, soil type, onion variety, and plant density, onions need around 350-600 mm of water throughout their growth cycle. To meet these needs, commercial onion farmers generally irrigate every 2-4 days, applying around 1.5-2.5 cm (0.6-1 inch) of water per irrigation. This amount may vary based on soil texture, with sandy soils requiring higher volumes due to low water retention. 

At this point, it is important to mention that the water supply can affect some quality characteristics of the bulbs, such as spiciness and flavor. More specifically, onion bulbs produced under dry conditions tend to increase pungent flavors. A balanced water supply promotes uniform bulb growth and mildness, often desirable for fresh-market onions. It is essential to ensure that the drought-stresses will be minimal during the most water-critical developmental stages of the plants. These are the plant establishment and the bulb growing stage (bulb enlargement). During these periods and when the temperature rises, the weekly demand of the plants may be even 3.8-5 cm (1.5-2 inches) of water. Water scarcity during bulb growth may result in decreased size, splitting, or formation of double/multiple bulbs. Similarly, drought stress during onions' growth stages may lead to reduced rates of transpiration and photosynthesis, which, if the stress persists, can slow down the growth rate and decrease the final yield. The farmer should irrigate before and after transplanting the young onion seedlings to the field (1.5 in or 3.8 cm of water) to facilitate plant establishment and minimize the transplanting shock. Additional irrigation may be required after fertilizer applications if there is insufficient rainfall.

On the other hand, water applications should be decreased significantly and even stopped 15-25 days before harvesting, during the last maturation stages of the bulbs (when the tops of the plants fall over). This practice decreases the risk of bulb rot and facilitates harvesting. The plant's water needs are minimal during fall due to its small size and cool temperatures. In this case, most farmers irrigate only when rainfall is scarce and the soil looks dry. Excess humidity, especially on foliage, may favor fungal disease outbreaks. 

The most popular irrigation systems for Onions

Onion growers utilize various irrigation systems based on regional conditions, planting methods, and resource availability. Commonly used systems include furrow, sprinkler, and drip irrigation. Ensuring uniform water distribution across the field is vital since onions are sensitive to saline soils, which can impact growth. Farmers are encouraged to test the salinity of their irrigation water to prevent any detrimental effects on the crop.

1. Drip Irrigation

Drip irrigation is a popular choice for onions planted in rows on flat surfaces. This method allows for precise water delivery, helping to keep soil moisture levels steady while conserving water. When using drip systems, it’s important to moisten the soil to a depth of 30 cm (12 inches) each time. Drip irrigation also reduces the incidence of foliar diseases, as water is directed to the soil rather than the leaves.

2. Furrow Irrigation

When planting on raised beds, some producers prefer to irrigate their onions with furrows. This irrigation technique is widely applied in some regions since it is relatively easy and cheap. However, due to its low efficiency (water losses, uneven water distribution, lower final yields), it is not advised to be used. 

3. Sprinkler Irrigation

Most onion crops are sprinkler-irrigated with center pivots or traveling guns (more labor-intensive). Overhead irrigation may save up to 30% of water compared to furrow irrigation and can be used to cool the crop close to harvest. In some cases, farmers have seen that strategic use of light overhead irrigation has reduced the incidence of downy mildew and black mold (Aspergillus). However, if not applied correctly, it can dramatically increase the risk of foliage fungal disease outbreaks. 

Water Management Tips and Summary

Careful water management at key growth stages can further optimize yields and quality. For instance, reducing water supply during the early stages of bulb formation can help control growth rates, enabling growers to manage bulb size according to market demand. Additionally, mulching and soil moisture monitoring techniques can improve water retention, particularly in drier regions, reducing irrigation frequency and conserving resources. Advances in soil moisture sensors and automated irrigation systems allow farmers to adjust water application rates with greater precision based on real-time soil conditions.

References

https://www.fao.org/land-water/databases-and-software/crop-information/onion/en/

https://academics.hamilton.edu/foodforthought/our_research_files/allium.pdf

https://www.jica.go.jp/project/english/kenya/015/materials/c8h0vm0000f7o8cj-att/materials_20.pdf

https://extension.usu.edu/yardandgarden/research/onions-in-the-garden

https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0017/126305/Onion-growing-Agfact-H8.1.22.pdf

https://www.ndsu.edu/agriculture/extension/extension-topics/gardening-and-horticulture/vegetables/how-grow-big-onions

https://www.dalrrd.gov.za/Portals/0/Brochures%20and%20Production%20guidelines/Production%20Guidelines%20Onions.pdf

https://agsci.oregonstate.edu/system/files/sr1097.pdf

https://www.cambridge.org/core/journals/experimental-agriculture/article/abs/water-relations-and-irrigation-requirements-of-onion-allium-cepa-l-a-review-of-yield-and-quality-impacts/AC88DE1E0E45C91294253411037A54DD

https://dogr.icar.gov.in/index.php?option=com_content&view=article&id=118&Itemid=189&lang=en

Further information