County Agent News

Dan Folske

June 14, 2010

Herbicide Applications and Temperatures 

After a couple of years of much cooler than normal temperatures we are finally getting at least normal temps this year. Current temperatures are nearly ideal for most herbicides but if it gets hot we need to be concerned and that can certainly happen by the time the late seeded crop is ready for spraying. 

All herbicides are influenced by environmental conditions.  Herbicides generally are most effective when applied to vigorously growing plants at 70 to 85 degrees Fahrenheit. Most herbicide labels caution against treatment of plants growing under extreme environmental conditions or stress, because treatment of stressed plants can result in increased crop injury and/or decreased weed control.  

Systemic, growth regulator herbicides such as 2,4-D, Banvel, Curtail and Stinger will provide greater broadleaf weed control with high temperatures, provided the weeds have not developed a thick wax layer on the leaf surface in response to the heat or moisture stress, but also have a greater risk of crop injury.  Avoid applying volatile herbicides such as 2,4-D ester, MCPA ester, and dicamba during hot weather, especially near susceptible broadleaf crops, shelterbelts, or farmsteads. 

The sulfonylureas herbicides or “SU” will also provide greater weed control with higher temperatures, provide there is sufficient moisture in the soil and the weeds are actively growing, while still maintaining a fairly good margin of crop safety.  The same can basically be said about products such as Pursuit and Raptor. 

Grassy weed control products such as Puma and Discover are more effective with cool or cold conditions. As such, in hot weather both weed control and risk of crop injury will be less.   However, optimum weed control may be achieved if weeds are actively growing.  More herbicide may be needed to get the same weed control as in normal or cool conditions.  If weed are moisture stressed in addition to heat stressed, grassy weed control will be significantly reduced.  The same basically holds true for other grassy weed control products such as Assure II, Poast, and Select.  Higher temperatures do not increase in risk of crop injury from these herbicides, however, the oil that is used with these herbicides may cause oil burn on the broadleaf crops.  

Application of systemic herbicides early in the morning after the plants have recovered from the heat of the previous day should provide better weed control than afternoon or evening application. Addition of adjuvants (spray additives), when recommended, also may improve weed control from these herbicides under adverse growing conditions.  

When it comes to the contact herbicides, most contact herbicides become more active as temperatures increase. Increased activity may provide improved weed control, but can also result in greater crop injury. Contact herbicides such as Basagran, Bronate, Buctril, and Results all need to be used with caution as temperatures increase to 85 degrees and above. Postponing application of these herbicides should be considered if temperatures exceed 90 degrees. However, good weed control with contact herbicides is dependent on timely application. The best control generally is achieved with thorough spray coverage and application to small seedling weeds. Although waiting until temperatures subside will lessen the risk of crop injury, the weeds may develop beyond the optimum treatment stage if application is delayed too long. In some cases, application of reduced herbicide rates may be better than delaying application, even when temperatures are over 90 degrees.  

Precautions can be taken to minimize the risk for injury with contact herbicides during hot weather. Use the lower labeled rates rather than higher rates, even if attempting to control larger or less susceptible weeds. The activity of contact herbicides is increased on all plants at higher temperatures, so the weeds also will be more susceptible to the treatment.  

The most critical time for crop injury following application of a contact herbicide is the first few hours after treatment. Therefore, injury can be minimized by applying the herbicide in the evening after the temperature has decreased. Evening application will allow several hours of moderate temperatures following treatment while morning application will be followed immediately by increasing temperatures.  

Many contact herbicides are labeled for use with various additives. However, most additives also increase the chance for crop injury. Omitting the adjuvant or using the lower labeled rate of the recommended adjuvant will reduce injury potential from a contact herbicide applied at high temperatures. 

Spraying Noxious Weeds

            This years warmer temperatures have hastened the development of many noxious weeds like leafy spurge, absinthe wormwood, Canada thistle, and common tansy. Most infestations of these weeds have reached the growth stage needed for optimum control with herbicides, especially if they were not treated last year. Leafy spurge should be at the true flower stage which is usually about one week after the yellow bracts appear. Absinthe wormwood should have at least 12 inches of growth. Canada thistle timing varies a little with the herbicide you are using but most need at least a large rosette before getting much control. Tordon and dicamba work best if the thistle is at least 12 inches tall. Common Tansy is best sprayed just prior to budding or very early bud stage. 

These optimum growth stages are for early summer spraying. Fall spraying is generally even more effective.