Saving crops after flooding and high rainfall

If your crops and cropping land have been affected by floods, you need to manage crop recovery and minimise the extent of the damage.

Waterlogged soils are low in oxygen, as the oxygen between soil particles is replaced by water. Oxygen is essential for healthy root growth and the lack of oxygen in soils over time results in root and eventually plant death.

Saving crops

The sooner you remove excess water and begin treating your tree or vegetable crops after flooding or high rainfall, the more chance they have of remaining healthy.

The less time that trees and crops are inundated, the more chance they have of remaining healthy.

Most trees will stay healthy if floodwater recedes within a few days. If the water drains within 24 hours, the impact on plant health is usually minimal. Some tree deaths can be expected if floodwater remains for extended periods (i.e. 3 or more days), especially on less well drained soils that remain waterlogged after floods have receded.

Vegetable crops are at particular risk following a flood or heavy rain, as they lack an extensive root system. Waterlogged plants may lack nutrients due to nutrient leaching from the soil and reduced uptake by the plant because of a damaged root system. Gradually replace these nutrients with fertiliser to help the crops recover.

Always seek professional crop advice before making decisions about removing or replacing crops damaged by waterlogging. Your crops may be salvageable.

Food safety

After floods or high rainfall, vegetable crops may become a risk to food safety, even if they look healthy.

On-farm run-off should not be a safety problem as long as the above-ground portions of the plants remain healthy.

Produce developed after water has subsided – such as tomatoes, capsicums, chillies, eggplants, sweet corn, squash, cucumbers and similar non-leafy vegetables – should be safe as long as the fruit is not cracked or soft.

Basic precautions:

  • do not use produce from plants that have yellowed
  • discard vegetable crops flooded with off-farm water that may have been contaminated, particularly leafy vegetable crops
  • wash vegetables thoroughly before packing.

Preventing contamination in flooded crops

  • Test soils before replanting or harvesting crops if you suspect chemical or biological contamination. Contaminants will reduce over time with follow-up rainfall and sunny weather.
  • Ensure wash water contains a postharvest treatment to reduce the risk of breakdown. Change water regularly, or as soon as it gets dirty.
  • Discard produce flooded with off-farm water that may have been contaminated.
  • Remove dying or dead branches that may become an entry point for disease organisms or insect pests.
  • Apply suitable disease control measures as soon as possible and monitor for pests.

Improve drainage

Good drainage is essential for maintaining crop health. Wet weather provides a good opportunity to improve the drainage of your crop land, as it allows you to identify and address any problem areas.

Assess poor drainage immediately after

After significant rain or flooding, inspect the crops when it is safe to do so and mark areas (e.g. with coloured pegs or GPS points) that are affected by poor drainage.

If possible, take immediate steps to improve the drainage of these areas so that the water can get away (e.g. by clearing debris or digging drains).

Manage irrigation to reduce waterlogging

To avoid recurrence of waterlogging, time irrigation by applying small amounts often until the crop's root system has recovered.

Identify future improvements

In the longer term, look for ways to improve the drainage of the affected areas, such as:

  • re-shaping the layout of the field
  • improving surface drainage
  • installing subsurface drainage
  • regularly maintaining drainage systems.

If the drainage can't be improved, consider using the area for some other purpose (e.g. for a silt trap).

Plant diseases and pests

It's important to identify and target diseases in horticultural crops affected by recent floods and wet weather. Grow Help Australia can test your horticultural crops for diseases and pests.

Replacing nutrients through fertiliser

Floods and heavy rains can remove essential nutrients from the soil. While accessing fields can be difficult if they remain saturated, replacing lost nutrients with fertiliser when you can get back on the field helps tree and vegetable crops to recover.

With the likelihood of continued heavy rain, fertiliser rates are best split into frequent applications of small amounts.

This means the amount potentially lost with each rainfall event will be lower and the levels will be topped up sooner with the next application.

Vegetable crops

Gradual replacement of fertilisers is critical for recovery of a healthy root system. Heavy applications may cause further root damage.

Foliar applications of soluble major and trace elements may help kickstart plants until their root systems re-establish.

Tree crops

Three of the important nutrients for tree crops – nitrogen, potassium and boron – are prone to leaching from the soil and levels are likely to be low after high rainfall.

Adjust their fertiliser applications to make up for expected shortfalls. Typically, rates are increased by up to 20% above normal. If you suspect damage to the root system after prolonged waterlogging, it's critical that you replace fertiliser gradually to recover a healthy root system. Heavy applications may cause further root damage.

Careful application of boron

Don't apply too much boron, especially on light sandy soils, since this element can easily reach toxic levels.

Advice for specific crops