By Bill Curran, Weed Scientist

We are routinely asked whether it is still possible to kill winter annual and perennial weeds with herbicides this late in the fall, as well as some cool-season forages like alfalfa. Weeds like johnsongrass, pokeweed and other more tender herbaceous perennials have already entered dormancy, however, there may still be opportunities for some of the cool season species.

Perennials

Here in the central part of the state our nighttime low temperatures have routinely been around 35 F or so. Usually fall can be an excellent time to target herbaceous plants for control, but we have moved beyond the optimum time for effective control. 

Research conducted in Nebraska back in the late 1990s showed that applications following the first frost of the season (temperature drops below 32 F, but leaf tissue is not damaged) actually provided a significant increase in control with several perennials. This research reported a 17-39% and 45-58% increase in control of Canada thistle and dandelion, respectively, if dicamba (Banvel, Clarity, etc.) is applied after the temperature dropped below 29 F compared to 5 days before the first frost (Wilson and Michiels 2003). 

This would probably be similar for other effective herbicides as well, but this elevated control is dramatically reduced after a hard freeze that kills foliage of Canada thistle. With most plants, it’s 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 green foliage is still present. 

In general, if temperatures drop below 28 F at night for more than 4 hours then these plants may die and an herbicide application may not be effective. The study we have often quoted in an article like this is some work conducted by Nathan Hartwig back in 1970s looking at the effect of glyphosate timing on quackgrass control. Dr. Hartwig applied a 1 pound-per-acre rate (1 quart of a 3-pound acid equivalent per gallon formulation) in late September, early October and November and rated the plots the next spring. The late September treatment provided greater than 90% control, the early October 80-90%, and the November treatment 50-60% control. Of course, more mild weather or an Indian summer would likely improve the November results.

In general, fall foliar-applied herbicide applications should be made when daytime air temperatures are at least in the 50s and preferably higher. If warm temperatures (greater than 60 F) continue, the time is now to apply. Applications when plants aren’t actively growing will limit herbicide uptake or movement, resulting in poor control the next year. Also, when applying systemic herbicides this late in the year, make sure to include adjuvant such as AMS and/or crop oil concentrate to insure adequate uptake of the herbicide.

Marestail

Marestail has been a growing concern the past several years. Here are a few helpful insights to consider from fall burndown research that has been conducted at neighboring universities.

• Keep cost of fall herbicide treatments in the range of $4-$12 per acre. Glyphosate and 2,4-D (or dicamba) can be an initial low-cost option to consider that provides control of a relatively broad spectrum of weeds.
• Products that contain chlorimuron (e.g., Canopy) tend to provide the most residual marestail control into spring. Keep in mind there are areas in Pennsylvania, Delmarva and in the Midwest that have ALS and glyphosate-resistant marestail, so chlorimuron or other ALS herbicides (group 2) would not provide control in these situations.
• Fall-only burndown/residual applications generally do not provide enough control of marestail into next season. Two-pass burndown programs (fall followed by spring applications) are better at obtaining season-long marestail control. If applying a fall burndown, make sure to select the correct product depending on what crop you intend to plant next spring. For example, don’t apply a Canopy product if you plan to plant corn.
• If the field is planted to a cereal rye cover crop, the use of 2,4-D or dicamba may help control marestail populations. Spring applications may be better. Also, Authority MTZ has a label for use in rye cover crops and can be applied once the rye is a few inches tall (2 or more leaves). This will provide residual control of marestail but does not have foliar activity. Unfortunately this label currently only applies to our surrounding states of Maryland, Delaware and Virginia. If there is interest for this label in Pennsylvania, please let us know and we can petition for it.

Common Chickweed

Common chickweed is another weed we’ve been hearing more about, not only in small grains but in forage crops as well. In wheat, barley and oats — unless it is ALS-resistant — Harmony Extra (or equivalent generic product) is one of the better options. (On a side note, if you have non-ALS resistant marestail in small grains, Harmony Extra usually provides adequate control; Huskie also has good activity.) 

If you are one of the unfortunate who have an ALS-resistant chickweed population in wheat, then programs that include metribuzin and Starane (fluroxypyr) appear to be providing the best control. If chickweed is a problem in alfalfa, then some options include Pursuit, metribuzin, Chateau and Gramoxone. Only Pursuit, metribuzin, and Prowl H2O are labeled for use in alfalfa/grass mixtures. Also, as we get later in the year, products like Pursuit tend not to be as effective due to colder conditions that affect weed growth. 

If you are struggling with chickweed in grass hay or pasture, a combination of dicamba plus 2,4-D will provide some suppression (70-80% control). This tank mixture works better on chickweed in the fall compared to spring applications, where it typically provides only about 50-60% control.