Bulb onions irrigation: crop establishment and early season

Generally, there is no need to irrigate at drilling time, but there are conditions when some irrigation will help establish the crop and make pre-emergence herbicides more effective. Likewise, during early season growing, it pays to water sparingly and carefully at the correct times.

This information was last updated in 2014.

Establishment stage

Seed beds are unlikely to need further irrigation just after drilling, but there are some conditions in which seeds might need extra moisture:

• if soil is very dry at drilling
• if seed has been drilled too shallow
• if weather conditions become exceptionally dry, warm or windy
• if heavy rain has capped the soil, making it hard for seedlings to emerge.

Limited irrigation may be needed to rewet (after soil capping) or irrigate to about 10-15mm.

This will also make pre-emergence herbicide more effective.

However, applications in this phase should be minimal to limit cool shock to delicate seedlings and reduce the danger of further soil capping.

Early season stage

The aim is to achieve a balance between over-watering and any severe water stress which would severely reduce crop potential.

Soil moisture deficit

There is little to be gained by maintaining a very small soil moisture deficit (SMD) during the early season unless the weather becomes particularly dry or warm.

Optimum irrigation should maintain the SMD at less than 50% of the available water content in the rooting zone.

Typically, this means 20–25mm applications every 7–14 days, adjusted to take account crop growth stage and weather.

Over-irrigation during this period can lead to excess canopy development too early in the season, which can lead to increased bolting if a vernalisation event (eg. a cold snap) occurs. Frequent irrigation early in the season can also promote greater weed development.

However, if onion thrip levels are high (eg during prolonged dry spells) more frequent irrigations through early and mid-season significantly reduce crop damage.