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Temperatures are still warm enough to use herbicides, but options narrow as it gets colder.

November 16, 2020

3 Min Read
Close up of a field of dandelions
NOT A PRETTY SIGHT: The best time to control dandelions in a field is right now. In fallow fields, a combination of glyphosate plus 2,4-D ester is effective for control of most winter annual weeds and dandelions. Lara_Uhryn/Getty Images

As temperatures get colder and even a few snowflakes start to appear, some people wonder if it’s too late to control weeds, especially perennials. But with milder temperatures forecasted for the coming week, now would be a good time to consider spraying, if you need to.

Spray when green

In fall, foliar-applied herbicides can be effective if the plants are green and appear healthy. For best activity, apply herbicides when daytime temperatures are above 50 degrees F and nighttime temperatures are above 40 degrees for several days during application time. Don’t apply herbicides immediately after a frost.

Some research from Iowa State and Ohio State shows that many perennial and biennial weeds can still be effectively killed after a few hard frosts. Research with quackgrass and glyphosate actually found greater translocation of the herbicide after the first frost than before frost.

Plants that have a prostrate growth habit, such as the biennial musk or bull thistle, will be more tolerant of frost since they are protected somewhat by heat released from the soil. With most plants, it’s possible to determine whether the foliage has been severely affected by frosts, so scouting the field before application is important to ensure that active foliage is still present.

Regarding quackgrass and Canada thistle regrowth after harvest, if these weeds are more than 8 inches in height, then an application of glyphosate may provide good control of the above- and belowground plant parts. If temperatures drop below 28 degrees at night for more than four hours, then these plants may die, and herbicide applications may not be effective.

Quackgrass can handle colder temperatures. If warm temperatures — more than 65 degrees — return for several days and the plants appear to be growing, then herbicide treatment may still be effective.

Control dandelions and winter annuals now

Fall is the best time to control dandelions, while fall and early spring are good times to control winter annuals.

In fallow fields, a combination of glyphosate plus 2,4-D ester is effective for control of most winter annual weeds and dandelions. Application of 2,4-D alone can control many winter annual weeds, but 2,4-D will not control chickweed and is less effective on dandelions than when mixed with other herbicides.

Contact herbicides — such as Sharpen and Gramoxone — and systemic products — like glyphosate, 2,4-D and dicamba — are much less active at low temperatures. Furthermore, 2,4-D and dicamba are generally more active than glyphosate in cool weather — less than 40 degrees. Therefore, tank-mixing improves overall control.

Use residuals later

As we move into late November — since foliar herbicide effectiveness decreases — the inclusion of a residual herbicide may be desirable in corn or soybean rotations.

Research has shown that any chlorimuron-containing product — such as Canopy EX or Blend — is at the top of the list for fields going into soybeans, and simazine is a good product for fields going into corn next spring. Other products that have had some success include Valor for soybeans and Basis Blend for corn.

In general, 2,4-D should be tank-mixed with any residual product. Also, when applying systemic herbicides this late in the year, make sure to include adjuvant such as AMS or crop oil concentrate/methylated seed oil to ensure adequate uptake of the herbicide.

Another situation to consider is a pure stand cereal rye cover crop that has broadleaf winter annuals or perennial weeds. In this case, 2,4-D plus dicamba can be applied to control these weeds either now or in the early spring. However, if you are applying systemic herbicides with spray additives in a cereal rye cover, crop injury might occur.

Make sure it’s worth it

In general, fall herbicide treatments should be kept in the cost range of $4 to $12 an acre. Thus, glyphosate plus 2,4-D — or dicamba — can be an initial low-cost option to consider that provides control of a relatively broad spectrum of weeds.

Lingenfelter is a weed science educator with Penn State Cooperative Extension.

Source: Penn State Extension, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.

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