How heavy rains and excess moisture can affect crops, what signs of damage to look for, and which factors influence plant survival after flooding.

 
When large rain events occur, producers, agronomists and media often ask: “What is the extent of crop damage from the heavy rains?”

Damage can vary by field, and field-by-field assessments over the coming days are needed to better understand the impacts.
 
How excess moisture causes crop damage
Germinating seeds and plant roots need oxygen in the soil for respiration. Without oxygen, these living tissues cannot perform critical life‑sustaining functions such as:
  • root and shoot development
  • nutrient uptake
Under flooding conditions, saturated soils become oxygen‑depleted. When soil oxygen levels drop too low, plant respiration shifts to a pathway similar to fermentation. While some life‑sustaining energy is produced during fermentation, energy production is reduced by up to 95%.
In short: low oxygen drastically reduces plant metabolism. This can reduce plant growth and, if prolonged, can cause partial or complete plant death.
 
What affects the extent of crop damage
The amount of damage from excess moisture depends on several interacting factors:
·       Stage of plant growth
·       Duration of saturated conditions
·       Temperature (water and air)
·       Soil characteristics
 
1.       Stage of plant growth
Seeded but not yet emerged crops
  • Sprouting seeds are vulnerable to flooding because they require oxygen for respiration.
  • The longer it takes for emergence after germination, the greater the risk of stand loss.
  • Saturated soil can delay or prevent germination when oxygen concentrations around the seed are too low.
Assessing ungerminated seed
  • Seeds can be cut in half to check for turgor pressure.
  • If the seed is extremely soft and does not hold its form, survival is unlikely.
 
Emerged crops
Young plants are often more susceptible to damage for two main reasons:
  1. For many crops, the growing point is at or below the soil surface and is directly exposed to oxygen‑depleted conditions.
  2. Young plants are actively establishing their root systems. Root injury or death caused by oxygen depletion is a major stress during this stage.
However, younger plants also have a lower overall oxygen demand, which may improve survival in some cases.
Checking plant survival
  • Examine the growing point three to five days after water drains from the field.
  • Healthy growing points are firm and yellowish‑white.
  • Mushy or discoloured growing points indicate damage.
  • Look for new leaf growth as a sign of recovery.
 
2.       Duration of saturated soil conditions
The longer flooding or water‑logged conditions last, the greater the risk of injury.
  • Many effects of low oxygen are reversible if saturation is brief.
  • Longer durations increase oxygen depletion and allow harmful chemicals to build up in the soil.
General tolerance guidelines
  • Under flooded conditions, 36 to 48 hours is often the upper tolerance limit.
  • Under water‑logged (not fully flooded) conditions, tolerance depends on crop species.
  • Most annual crops can tolerate 3 to 7 days of water stress.
Relative crop tolerance
  • Cereal crops: oats > wheat > barley
  • Pulse crops: fababeans > soybeans >>> field beans > peas
  • Oilseed crops: canola > sunflower > flax
Specific survival timelines are not widely reported because tolerance is strongly influenced by other factors such as soil type and soil and air temperature.
 
3.       Temperature (water and air)
Temperature affects the rate of plant respiration:
  • Warm water increases respiration, depletes oxygen faster and accelerates fermentation.
  • Cold water slows respiration and oxygen depletion.
Warm air temperatures (low to mid‑20s °C or higher) reduce survival chances for young plants. Cooler temperatures and cloudy conditions improve survival by:
  • slowing plant growth and respiration
  • allowing more oxygen to dissolve in cool water than in warm water
 
4.       Soil characteristics
Soil properties can intensify flooding impacts:
  • Soils with high clay content or restricted drainage stay wet longer and may remain waterlogged after surface water recedes.
Even when seeds or plants survive excess moisture, effects can persist after soils dry.
Additional concerns
  • Soil crusting:
    • Common after heavy rain followed by drying, especially on clay soils.
    • Crusting can physically block emerging seedlings and prevent emergence.
  • Root development:
    • Excess moisture during early vegetative stages can slow root growth.
    • Poorly developed root systems increase vulnerability during later dry conditions.
  • Disease risk:
    • Extended wet soil conditions favour the development of plant diseases.