How to Apply Herbicides in a Heat Wave

managing corn weeds in heat wave

Temperatures in the low 90s or above can hinder herbicide uptake. Wait a few days for things to cool off to protect your corn from weeds and added stress.

  • Avoid spraying herbicides, if possible, when temperatures surpass 90 degrees.
  • During a heat wave, weeds shut down and can’t uptake herbicides.
  • Using multiple sites of action is especially important during a heat wave.

“We want actively growing weeds whenever we apply herbicides.”

Mike Weber
Principal Scientist, Bayer

“If you're ready to get out and spray your herbicides, but are facing a heat wave, it's best to wait a few days for things to cool off,” says Bayer Principal Scientist Mike Weber.

Too much heat – temperatures in the low 90s or above – and weeds essentially shut down, which can hinder herbicide uptake.

Systemic herbicides are most effective on weeds that are actively growing and have open stomates – those tiny pores that offer a key entry point for the chemical. Once inside the plant, the herbicide moves throughout to kill it.

“We want actively growing weeds whenever we apply herbicides,” Weber said. “When the weeds are shut down, they're not able to uptake the herbicide at that point, and you won't get a complete kill out of the weeds.”

So what is the ideal temperature range for spraying herbicides? “Anywhere from the upper 70s to the low 80s,” Weber says.

Heat stress can increase risk of crop injury

The ideal temperature range for spraying herbicides is the upper 70s to the low 80s.

Another reason to wait? Like those weeds, your corn is under heat stress, too. Applying herbicides now can lead to increased risk of crop injury.

“Whenever our corn crop is under stress, we don't want to go out there and apply an herbicide at that point,” Weber said. “We should let our crop recover a couple of days from that stressful event.”

Bayer offers growers the unique benefit of its built-in, proprietary CSI™ Safener technology. Weber says these safeners are designed to help stressed corn plants metabolize DiFlexx® DUO herbicide, getting it out of the corn plant so it can recover. This, in turn, can lead to increased root growth and plant health.

“These safeners actually enhance and decrease the amount of crop injury that typically growers would see from other products on the market,” Weber said.

Using multiple SOA is critical in hot conditions

It's always important to use multiple sites of action that are effective against your targeted weed – but especially so during a heat wave. That's because certain herbicides are particularly susceptible to the effects of high temperatures. For example, Kansas State University researchers found that the HPPD-inhibitor (27) class of herbicides is less effective on Palmer amaranth as temperatures climb into the upper 80s.1

“Using multiple sites of action can mitigate this by providing the safety net of a secondary herbicide,” Weber says. DiFlexx DUO, for example, contains both an HPPD-inhibitor and a growth regulator herbicide (4) for strong control of tough broadleaf weeds, such as Palmer amaranth, waterhemp, kochia, marestail and giant ragweed.

“Whenever we have an environment of heat stress, it can affect our weed control, and it's very important at that point to have multiple sites of action that are effective against your targeted weed,” Weber said.

Using DiFlexx DUO corn herbicide

DiFlexx DUO (4,27) takes down tough weeds, like waterhemp and Palmer amaranth, with two powerful and effective sites of action.

  • Application rate: 24 to 40 fl oz/A
  • Timing: Corn up to 36 inches or up to V7 growth stage for broadcast application/up to V10 for directed application.
  • Recommended tankmix options: Atrazine (5) for corn under 12 inches tall, Roundup® Brand Agricultural Herbicide (9)

To learn more, contact a Bayer representative.

*Amongst growers – Source: Brand Health Check

Related Reading

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Work Cited

1 2015. "K-State Scientists Discover Temperature's Role in Palmer Amaranth Control." K-State Research and Extension.
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