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How do Weeds Grow under Hot and Dry Conditions? (5/31/18)

Weather in eastern North Dakota and west central Minnesota has been unseasonably hot and dry.

How do Weeds Grow under Hot and Dry Conditions?

Weather in eastern North Dakota and west central Minnesota has been unseasonably hot and dry. Most crops are emerging or have emerged. Many growers and service providers have reported inconsistent stands in fields including sugarbeet fields. Believe it or not, hot and dry weather impacts the germination and emergence and growth and development of weeds, too. Even more importantly, dry weather can complicate weed control efforts.

Small seeded broadleaf weeds germinate from the soil surface to ½ inch deep in soil. That means, the same dry soil conditions plaguing our crop stands are affecting germination and emergence of weeds. Weeds that do emerge could be more difficult to control so control weeds when they are small. Weeds tend to be more sensitive to herbicides when they are actively growing under good moisture. Weeds grow more slowly and develop thicker cuticles on the leaf surfaces under extended dry conditions, which has the overall effect of reducing herbicide movement into and throughout the plant.

The following are suggestions for using systemic herbicides like glyphosate or contact herbicides like Liberty or Flexstar under dry conditions.

  • Use full rates of glyphosate products to help offset effects from thicker cuticles and lower translocation rates. Be sure you know the surfactant loading of the glyphosate product you choose and add extra non-ionic surfactant, as allowed per label directions to counter the effects of hot weather. I use 0.25% v/v non-ionic surfactant even when using PowerMax. This is especially important with thick-cuticle weeds like lambsquarters.
  • Use the full rate of ammonium sulfate (AMS), particularly when hard water is a concern. AMS conditions water and will aid in maximizing glyphosate uptake by target weeds.
  • Contact herbicides such as Liberty, Betamix or Flexstar become more active as temperatures increase. Increased activity may provide improved weed control but can also result in greater crop injury. Consider using these products and other contact herbicides in the afternoon or early evening on days when temperatures are forecast for 85 degrees and above. Postpone application of these herbicides if temperatures exceed 90 degrees to reduce risk of crop injury.
  • Target small weeds to ensure complete spray coverage with contact herbicides. Spray nozzles, water volume and droplets size are different than the ones used for systemic herbicides like glyphosate or dicamba.
  • Many contact herbicides are labeled for use with adjuvants like methylated seed oil (MSO), crop oil concentrate (COC), or non-ionic surfactant (NIS). 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 hot temperatures.
  • Oils from formulation from secondary tank-mix herbicides can increase the chance for crop injury from primary herbicide. Tank-mixes are an excellent resistance management strategy and broaden spectrum. However, formulated adjuvants may increase crop injury. Reduce or eliminate adjuvants under these scenarios.

Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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