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Weed Management After a Flood

Richard Zollinger, Weed Specialist

Strategies for This Year and Next

Floods can affect weeds both the year they occur and in subsequent years.

The biggest impact in the flood year will be the reduced competitive ability of the stunted or injured crop. Weeds will readily germinate and thrive in areas where the crop has not developed a canopy and covered the soil.

New weed problems are likely the year after a flood as a result of floodwaters moving weeds and weed seed into field. Use both mechanical and chemical methods in the flood year and in  subsequent years to manage weeds.

Floodwaters can also move herbicide residues. A bioassay test, in which crop seeds are planted in flooded and non-flooded soil samples, can be helpful to determine if soils are safe for intended crops. To carry out a bioassay test:

  • Take several soil samples from the flooded field (1 quart per sample) and plant three or four seeds of the planned crop in each one.
  • Collect soil samples from a known herbicide-free site to use as a standard and likewise plant three or four seeds of the planned crop.
  • Grow the seedlings for two to four weeks.
  • If plants in the flooded soil are normal and appear to grow as well as those in the herbicide-free soil, indications are strong that it is safe to plant your crop.
  • If crop growth in the flooded soil is abnormal, have an agricultural professional determine if the symptoms are related to possible herbicide residues in the soil or to other causes, such as nutrient deficiencies or diseases.

If the Crop Recovers

Crop plants may recover and produce a yield if floodwater recedes quickly.

Help crops grow vigorously by applying nutrients through fertilizer.

Reduce the impact of weed competition by removing weeds that germinate after the soils dry. This may not be practical in portions of the field that remain wet. Note new weed species by making a field map of the weed locations and use it in planning next year's weed management program.

Chemical weed control may be the best option to prevent weed germination and kill emerged weeds, but consider whether herbicides can be safely applied. Most labels clearly specify the maximum growth stage of the crop at which the product can be used. Applications following a mid-season flood are very likely beyond this window of application timing. Most labels also caution against using herbicides if the crop is under any stress. Thus, the feasibility of herbicide use the same year as a flood occurs is limited. If herbicide use is feasible but conditions are wet, consider applying herbicides through aerial application.

When Crops are Damaged

Prolonged flooding usually kills or seriously injures the crop, providing little yield to harvest. In this case, prevent weeds from going to seed and becoming a worse problem the next year through the use of tillage or chemical application. Again, note new weed species, and make a field map of the weeds to plan next year's weed control program.

Mechanically tilling will destroy weeds. It also will help aerate the soil if wet. Applying non-selective, non-residual herbicides may be a good option if the soil is too wet to work mechanically. Repeat either tillage or chemical application to control successive flushes of weeds so no weed seed will be produced.

The Year After the Flood

Be alert for new weed problems the year after the flood. Some weeds may germinate after a weed assessment is made during the flood year. Others may remain dormant until the next season. The flood also may have deposited soil that is different in texture, pH and organic matter content. These factors may influence herbicide performance and crop safety. Take soil samples and base herbicide selection and rates on current soil characteristics.

A Closer Look at Herbicides

Herbicides decompose in the soil by microbial action. This breakdown is slowed under flooded (anaerobic) conditions. Soil temperatures also are cooler under flooded and wet soil conditions, slowing both microbial and chemical degradation. Thus, the potential for herbicide carryover that would injure the subsequent crop may increase after flooding.

Possible Effects of Flooding on Herbicide Degradation Under Product or Chemical Family: Anaerobic Conditions

  • Triazines (atrazine, metribuzin) -- slower
  • Thiocarbamates (Eradicane, Sutan+) -- slower
  • DNA (Treflan, Sonalan, Prowl) -- faster
  • Acetanailides (Dual, Outlook, Harness, Surpass) -- faster -- can degrade anaerobically
  • Roundup -- no difference -- is inactivated upon contact with soil
  • Sulfonylureas -- chemical degradation -- is affected more by soil pH than anaerobic conditions
  • ACCase Inhibitors -- no difference -- most do not leave a soil residue

Final Decisions

Should you allow even more time than product labels specify before planting rotation crops? Probably not, but knowing what herbicide residues are in the soil may be more imprtant. Consider whether floodwaters brought in untreated soil from other fields. Also consider whether runoff removed a significant part of the applied product. When in doubt, use the bioassay test described above or send a soil sample to a commercial lab for chemical analysis. In cases where long-residual herbicide residues may be suspected, allow an extra week or two beyond the normal plant-back interval and deep till the field to dilute any remaining residues.

Once the field has been planted, monitor for possible weed problems, and use the appropriate mechanical or chemical measures to control them.

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