Weed Control Under Wet Conditions

The effect of flooding on weed control is unpredictable. Flood waters and other materials such as gravel for road reconstruction/repair and sand bagging materials are potential sources for the introduction of new weed species into areas affected by flooding. Monitor all flooded and repaired areas to determine the potential for impact on agricultural production. It may take several years for new species to become evident. Spring flooding will not significantly alter the growth of weed infestations already present in a field. Prolonged flooding (2 to 3 months) may destroy some perennial weeds. However, this effect is most noticeable during warm weather, when plant respiration is high. Under cold conditions, when plant activity is low, 1 to 2 months of flooding will not significantly reduce perennial weed problems. Weed seeds can remain viable under prolonged periods of flooding. Many weeds (e.g. red root pigweed, Canada thistle, field bindweed) will retain their ability to germinate after 2 or more years under water. Seeds of other weeds, such as quackgrass or barnyard grass, cannot survive prolonged immersion in water. However, established quackgrass plants will survive because of their rhizome system, while barnyard grass will grow under shallow flooding.

Water can spread many weed seeds. Some species (e.g. curled dock) have buoyant seeds, which are readily transported by water. Many other species (e.g. pigweed, barnyard grass and water smartweed) have been found in moving irrigation water. We can expect to see movement of weed seeds, especially buoyant-type seeds, in slow-moving floodwaters. Yellow nut sedge and purple loosestrife are known to spread along waterways and establish on land along rivers and streams. Flooded areas should be carefully inspected for introduced weed species.

Herbicide Efficacy* Flooding will reduce the efficacy of fall-applied herbicides if soil erosion occurs. Disruption or removal of the treated layer will reduce weed control. Because of the risk of physical soil movement, all growers should watch fields closely and be prepared to use a recommended post-emergent herbicide if required.

Leaching of some herbicides can occur if water moves downward in the soil profile. On heavy clay soils, this movement will be negligible and significant leaching of herbicides is not expected.

*Information on specific herbicides is based on information available from herbicide manufacturers.

If a herbicide is water-soluble, some loss through floodwaters may occur. Proper incorporation may reduce losses of this type.

Trifluralin/Edge Not recommended for use on flood-prone fields. The label states, "Do not apply to soils subjected to prolonged periods of flooding." Flooded soils produce anaerobic (absence of oxygen) conditions because water fills the air spaces, forcing oxygen out. Anaerobic conditions lead to chemical breakdown, reducing the efficacy of the herbicide. Therefore the effectiveness of fall-applied trifluralin/Edge may be reduced on fields subsequently subject to flooding.

Any trifluralin/Edge-treated fields should be planted with a trifluralin/Edge-tolerant crop. Growers should then watch the fields carefully and apply a recommended post-emergent herbicide if required.

Avadex BW Available information indicates that Avadex BW does not break down under anaerobic conditions. As long as physical soil movement does not occur, adequate weed control can be expected. Normal, shallow spring tillage is recommended.

Herbicide Residues Fields treated with residual herbicides will have restricted cropping choices. Assume that these cropping restrictions still apply, even though flooding has occurred. Refer to the most recent edition of the Guide to Field Crop Protection for more information.

Erosion may result in some movement of residual herbicides. Herbicide attached to soil particles will be moved with the eroded soil particles. Damage from the transported herbicide will be minimal unless the eroded soil is concentrated in the deposition zone.

Any grower who changes plans for a field should check to be sure the field was not treated previously with a herbicide that prevents growth of the intended crop.

Spring Application Of Soil-Applied Herbicides Use of spring-applied, soil-incorporated herbicides may be limited in wet soil conditions and late planting.

To allow for proper incorporation, soils must be sufficiently dry before application of herbicides. Incorporation in wet soils will result in poor weed control. Late seeding will limit the drying time available before seeding and will limit the time available for incorporation operations. If incorporation cannot be carried out as recommended, soil-incorporated herbicides should not be used. Other herbicides are available for post-emergent applications.

Weed Control In Water Logged Fields

1. Herbicides that are registered for aerial application include Achieve Liquid Gold, Avenge, Banvel, Basagran, Dyvel, Pardner, Puma, Reglone, Target, Thumper, etc. There are several other options too. Please refer to the Guide to Field Crop Protection for more details.
2. Crop seedlings growing under cold/wet conditions may be more sensitive to herbicide injury due to increased time of exposure. This is particularly the case with soil residual herbicides such Group 2, or Group 4 that are used as part of a preseed burnoff.
3. The efficacy of post emergent herbicides can be reduced when applied to weed seedlings that are stressed e.g. growing in cold/wet conditions.
4. In a situation where the field is infested with advanced winter annuals or other weeds, an application rate of up to 1 L/acre of the 360 g/L formulation of glyphosate would still be safe as a preseed burn-off option. Please refer to the Guide to Field Crop Protection for possible tank mix options for different crops that you plan to seed.
5. Heavy rain may leach herbicides (only those that are soil-applied are of concern) into the root zone of crop seedlings,causing crop injury and reduced weed control. Sencor/Lexone applied post emergence to lentils or chickpeas are possible examples.
6. The efficacy of Group 3 products may be reduced under anaerobic conditions, e.g. flooded fields. Available information indicates that Avadex is not affected by these conditions.
7. Almost all weed seeds will be unaffected by submersion in water for a short period of time.
8. When monitoring leaf development for herbicide application windows, note that different conditions favour the development of different species. Cool conditions favour C3 species such as wild oats while C4 species such as green foxtail and barnyard grass prefer warm conditions.

Monitor shifts in weed spectrum towards species that don't mind getting wet feet such as barnyardgrass, curled dock, smartweed spp. and field horsetail.