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Weed Management After a Flood

Publication Number: IS1734
Updated: March 16, 2015
View as PDF: IS1734.pdf

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 crop. Weeds will take advantage of the stunted or killed crops and grow to maturity.

In the year after a flood, new weed problems will be likely. Some of the weeds carried into the field by floodwaters may not have germinated in time to be noticed during the previous growing season. Mechanical and chemical methods need to be considered in both the flood year and subsequent years to manage weeds. A bioassay test—in which seeds are planted in flooded and nonflooded soil samples—can be helpful to determine if soils are safe for intended crops.

If the Crop Recovers

If the crop recovers after the flood, make an effort to reduce the impact of weed competition. This may not be practical if fields are too wet to enter for mechanical or chemical weeding.

Check fields regularly to monitor crop and weed development. Take note of weed species. Are there any new species? This may happen if weed seeds were carried into the field by floodwater. Make a field map of these weed locations and use it to plan next year’s weed management program.

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 extremely wet, consider using a commercial sprayer equipped with flotational tires.

When Crops Are Damaged

Flooding usually kills the crop or at least injures it so severely that it will not be be worth harvesting. If this is the case, try to prevent weeds from going to seed through the use of mowing, tillage or chemical application.

As mentioned above, take note of any new weed species that are present. Make a field map of the weeds to plan next year’s weed control program.

Mowing will allow some weeds to survive but may hasten drying of the soil more than using herbicides. Mowing is also an option if the soil is too wet to be tilled. Mechanically tilling the soil, if it is dry enough, will destroy weeds. It will also aerate the soil more than either mowing or spraying. Applying non-selective, nonresidual herbicides may be a good option if the soil is too wet to work mechanically.

Repeat either mowing, tillage or chemical application if another generation of weeds emerges that will have time to produce seed.

Final Decisions

Should you allow even more time than product labels specify before planting rotation crops? Probably not if you have used DNAs (as noted in chart), but it’s difficult to say for other chemicals. 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 below or send a soil sample to a commercial lab for chemical analysis. In some cases it may be appropriate to allow an extra week or two beyond the normal plant-back interval and to deep-till the field to dilute any remaining residues.

Once the field has been planted, monitor it carefully for possible weed problems. If weed densities approach the economic threshold, use the appropriate mechanical or chemical measures to control them.

The Year after the Flood

Be alert for new weed problems the year after the flood. Some weeds may have germinated after you made an assessment of weeds during the flood year. Others may have remained dormant until this season. The flood may also 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.

The new soil may have herbicide residues from the previous season’s application. These levels are unlikely to affect this year’s crop, but it would be wise to do a simple bioassay test to determine if planned crops are feasible in the flood-deposited soil. To carry out a bioassay test:

  1. Take several one-quart soil samples from the flooded field and plant three or four seeds of the planned crop in each one.
  2. Collect soil samples from a known herbicide-free site to use as a standard and plant three or four seeds of the planned crop.
  3. Grow the seedlings for two to four weeks.
  4. If plants in the flooded soil are normal and appear to grow as well as those in the herbicidefree soil, indications are strong that it is safe to plant your crop.
  5. 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.

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. A summary of possible effects of flooding on herbicide breakdown is given in the table below.

Table 1. Degradation of pesticides under anaerobic conditions

Product or Chemical Family Degradation under Anaerobic Conditions
Triazines (atrazine, Bladex, Sencor) slower
Thicarbamates (Eradicane, Sutan+) slower
DNAs (Treflan, Prowl) faster
Acetanailides (Lasso, Dual, Frontier) can degrade anaerobically
Substituted ureas (Lorox) unknown
Roundup can degrade anaerobically
Accent and Beacon unknown
Hoelon much slower
Poast, Fusilade, Assure unknown

 


From The Disaster Handbook - 1998 National Edition, University of Florida/Institute of Food and Agricultural Sciences SP 2431.

Distributed in Mississippi by Clifford Hampton, Special Projects Coordinator, Extension/MAFES

Information Sheet 1734
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director (POD-09-04)

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