As temperatures get colder and even a few snowflakes start to appear, some farmers wonder if it is too late to control weeds, especially perennials. However, with milder temperature forecasted in Pennsylvania for the upcoming week, now would be a good time to consider spraying.

Fall Herbicide Applications

In the fall, foliar-applied herbicides can be effective as long as the plants are green and appear healthy. For best activity, apply herbicides when daytime temperatures are above 50°F and nighttime temperatures are above 40°F for several days during application time. Don’t apply herbicides immediately after a frost. Some research from Iowa State and Ohio State indicates 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 having 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 by the soil. With most plants, it is possible to determine whether the foliage has been severely affected by frosts; thus scouting the field prior to application is important to ensure that active foliage is still present.

Regarding quackgrass and Canada thistle regrowth after harvest, if these weeds are greater than 8 inches in height, then an application of glyphosate may provide good control of the above and below-ground plant parts. If temperatures drop below 28° at night for more than 4 hours, then these plants may die and a herbicide application may not be effective. Quackgrass can handle colder temperatures than Canada thistle. If warm temperatures (greater than 65°) return for several days and the plants appear to be growing, then a herbicide treatment may still be effective.

Fall is the best time to control dandelions, while both fall and early spring are the good times to control winter annuals. In fallow fields, a combination of glyphosate plus 2,4-D ester is fairly effective for control of most winter annual weeds and dandelion. Application of 2,4-D alone controls many winter annual weeds, but 2,4-D will not control chickweed and is less effective on dandelion than when in tank mixture with other herbicides. Contact herbicides (e.g., Sharpen, Gramoxone) and systemic products (glyphosate, 2,4-D, dicamba) are much less active at low temperatures. Relatively speaking, 2,4-D is slightly more active than glyphosate in cooler temperatures (<40°F), whereas dicamba tends to be more impacted by cold weather; therefore, tank mixing them improves overall control.

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. If you include a residual herbicide, research over the last several years has shown that any chlorimuron-containing product (Canopy EX, Blend, etc.) is at the top of the list if soybeans will be planted next spring and simazine is one of the better products for fields going into corn. Other products that have had some success include Valor for soybean 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 an adjuvant such as AMS and/or crop oil concentrate or methylated seed oil to insure adequate uptake of the herbicide. Fall herbicide treatments should be kept in the cost range of $5-15/acre, if possible. Thus, glyphosate + 2 4-D (or dicamba) can be an initial low-cost option to consider that provides control of a relatively broad spectrum of weeds.

Cereal Rye Cover Crop Issues

Despite delayed harvest in certain areas, some would still like to sow a fall cover crop such as cereal rye. Keep in mind that this late in the year timing for burndown and rye planting will be very tight, so if possible, you may need to forego a burndown program and immediately plant after crop harvest. Burndown herbicides for no-till small grains include dicamba, Gramoxone, glyphosate, Harmony Extra and Sharpen. Refer to the specific product label for more application information. The legitimate use of 2,4-D for burndown in wheat and other small grains is uncertain. None of the 2,4-D ester or amine labels specify application just prior to small grain seeding or emergence. Some research suggests a minimum delay of 7-10 days after application at rates up to 1 pint/A 2,4-D ester. Since 2,4-D burndown in small grains is ambiguous at best, if injury occurs liability rests with the consultant or applicator.

Another situation to consider — you were able to plant a pure stand of cereal rye and it's actively growing but has broadleaf winter annual and/or perennial weeds growing in it. In this case, 2,4-D ± 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.

Roughstalk Bluegrass in Wheat

We are getting more calls about roughstalk bluegrass in wheat. Most of the wheat is growing well, but the bluegrass has broken dormancy and is developing alongside it. Once it is out of the ground, products such Osprey, Axial and PowerFlex (and their related product lines) will provide control/suppression of it. Fall and early spring are the best times to apply effective herbicides. Osprey provides the most consistent control and generally works better on this weed in the fall, however, early spring applications have proven effective too. Keep in mind that these herbicides should be applied to roughstalk bluegrass during its 1-leaf to 2-tiller growth stage range. In the spray mixture, make sure to include all the required adjuvants that are specified on the herbicide label.  

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