Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
We have had some flooding in vegetable crops due to heavy rains already this year and soils in some areas have remained waterlogged for extended periods. The majority of watermelons and other fresh market vegetables have been planted, peas are being harvested, lima bean planting has started and significant acres of pickles, snap beans, and sweet corn are in the field. Growers may be concerned.
Of course, low lying areas of fields are most affected by excess rainfall. However, cropping practices can also increase water ponding. Field compaction will reduce water infiltration and increase ponding. In plasticulture, water can accumulate and persist between rows of plastic mulch. Because much of the rainfall runs off of the plastic, water pooling can be more of a problem in plastic mulched fields, especially where row middles have become compacted. Vining crops that fruit into the row middles can have vines and fruits sitting in water and this produces an ideal environment for diseases of wet conditions such as Phytophthora capsici to infect plants.
When water overflows the bed tops of plastic mulched crops, whole beds become saturated as water enters the planting holes. This often leads to plant losses as beds take a long time to dry once saturated in this way and oxygen is very limited in the root zone.
To avoid water accumulation between beds, tilling with a deep shank or a subsoiler in row middles can help improve drainage. Cut drainage channels at row ends to reduce blockage that can back up water. Where practical, sectioning fields to go into plastic beds and installing cross drains to remove extra water can reduce water damage potential. Growers may also choose not to plant lower areas in the field prone to water damage where plastic is laid.
In flooded soils, the oxygen concentration drops to near zero within 24 hours because water replaces most of air in the soil pore space. Oxygen diffuses much more slowly in water filled pores than in open pores. Roots need oxygen to respire and have normal cell activity. When any remaining oxygen is used up by the roots in flooded or waterlogged soils, they will cease to function normally. Therefore, mineral nutrient uptake and water uptake are reduced or stopped in flooded conditions (plants will often wilt in flooded conditions because roots have shut down). There is also a buildup of ethylene in flooded soils; an excess of this plant hormone can cause leaf drop and premature senescence.
In general, if flooding or waterlogging lasts for less than 48 hours, most vegetable crops can recover. Longer periods will lead to high amounts of root death and lower chances of recovery.
While there has not been much research on flooding effects on vegetables, the following are some physiological effects that have been documented:
- Oxygen starvation in root crops such as potatoes will lead to cell death in tubers and storage roots. This will appear as dark or discolored areas in the tubers or roots. In carrots and other crops where the tap root is harvested, the tap root will often die leading to the formation of unmarketable fibrous roots.
- Lack of root function and movement of water and calcium in the plant will lead to calcium related disorders in plants; most notably you will have a higher incidence of blossom end rot in tomatoes, peppers, watermelons, and several other susceptible crops.
- Leaching and denitrification losses of nitrogen and limited nitrogen uptake in flooded soils will lead to nitrogen deficiencies across most vegetable crops.
- In bean crops, flooding or waterlogging has shown to decrease flower production and increase flower and young fruit abscission or abortion.
- Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops.
Recovering from Flooding or Waterlogging
The most important thing that you can do to aid in vegetable crop recovery after floods or waterlogging is to open up the soil by cultivating (in crops that still small enough to be cultivated) as soon as you can get back into the field. This allows for oxygen to enter the soil more rapidly. To address nitrogen leaching, sidedress with 40-50 lbs of N where possible.
In fields that are still wet, consider foliar applications of nutrients. Since nitrogen is the key nutrient to supply, spraying with urea ammonium nitrate (28% N solution) alone can be helpful. These can be sprayed by aerial or ground application. Use 5 to 20 gallons of water per acre. The higher gallons per acre generally provide better coverage. As with all foliar applications, keep total salt concentrations to less than 3% solutions to avoid foliage burn. Research in on flooded vegetables in Florida showed the best response to foliar applications of potassium nitrate.