Bulb onions irrigation: middle season and late season

Irrigation is critical for bulb onions in the middle part of the season, from bulb initiation to egg stage in early August. Towards the end of the growing season, more irrigation may increase bulb size but increases the risk of foliar diseases. 

This information was last updated in 2014.

Mid-season

From bulb initiation in early July to egg stage in early August is critical to build bulb crop biomass and promote bulb growth.

Irrigation little and often, if practical, can be more effective than the typical practice of larger doses of 20–­25mm every 7­–10 days (ie soil moisture deficit of 50% of available water content).

The approach

The ‘little and often’ approach should be used with care; it can increase the time that leaves are wet causing a greater risk of downy mildew (Peronospora destructor).

Late season

After the egg stage in early August, your aim should be to achieve controlled moisture stress to promote bulbing and mature the crop.

• The soil moisture deficit should be maintained at between 75% and 25% of available water content.
• Irrigation should stop when half the crop has fallen over.

Although more frequent irrigation after more than half the crop has fallen over may well increase bulb size and yield in most years, but it also increases the risk of crop damage from:

• foliar diseases, especially downy mildew (Peronospora destructor)
• delayed maturity
• poor bulb initiation
• reduced bulb dry matter, leading to bulb compression during storage

It can also endanger the harvest itself if the crop receives unexpected rainfall.

This means that only in particularly bright and dry late seasons, when bulbing and maturity are progressing naturally, should you consider more frequent irrigation.

In these conditions, careful irrigation may also be essential for efficient harvesting.