I am sure you confronted many stubborn weeds this year.

Here’s a review of some of the biggest problem weeds that caught my attention this season, based on seeing them in fields or hearing about them from producers. Some of these species were widespread across the region, while others were only problematic in certain areas.

And by no means is it exhaustive. It just highlights the weeds that I was asked about or encountered during the recent growing season.

Marestail, ryegrass at burndown

During spring burndown, marestail was a typical problem because effective control can be difficult once it gets taller than 6 inches.

Many problems with control occurred because of poor herbicide choices and application timing, improper adjuvant use, inadequate spray volume, and less-than-desirable weather.

Terminating annual ryegrass cover crops proved to be quite challenging, too. I received many calls this past spring about incomplete control from glyphosate, especially when it was cool, wet and cloudy.

Problem weeds in small grains

Common chickweed, speedwells and field pansy tended to dominate certain small-grain fields.

Chickweed had a good head start going into winter and never slowed down in early spring, causing many fields to be overtaken by it. Effective herbicides, especially in fall, are necessary to manage it. However, with more ALS-resistant populations around the region, products such as Harmony Extra are not effective on it.

Speedwell species and field pansy are always hard to control, especially in spring.

Annual ryegrass and roughstalk bluegrass were once again hard to control. It is best to start effective herbicide programs in fall. Once these make it through winter, herbicides tend to only suppress them. In some cases, both fall and spring herbicides will be needed to provide necessary control of these.

In spring oats, I received more inquiries about how to control weedy summer annual grasses such as foxtails. Unfortunately, there are no herbicides in our region that are labeled for use in oats that will control these types of weeds. However, we are conducting research on certain herbicides at Penn State to hopefully find some solutions.

‘Old-school’ weeds return

In corn and soybean fields, a theme of “what’s old is new again” was prevalent.

Old-school weeds such as cocklebur and common ragweed caused problems again. In some cases, it can be attributed to using less atrazine in corn and by enhanced degradation of that herbicide by hardy populations of soil microbes — which degrade it — that have built up over the years, thus shortening its effective residual activity.

Furthermore, some populations of ragweed can be resistant to triazines. However, selecting herbicide programs that include longer residual impacts and effective post herbicides are keys to adequate control of these species.

The same can be said for burcucumber, as well as Palmer amaranth, waterhemp and marestail, which continued to cause problems. Make sure to use two-pass programs with overlapping residual herbicides to control germinating weeds during the heart of the growing season. Our research at Penn State and at other universities shows that these species can be controlled if proper herbicide programs are utilized.

JAPANESE STILTGRASS: This weed persisted in many grass forages this season. It is an aggressive annual grass that thrives in moist, shaded areas and can spread into pastures from woodlands.

Also, having a thick crop canopy by midseason really helps to shade and reduce the growth of Palmer, waterhemp and marestail, especially in soybeans. Establishing the crop into a thick cover crop residue such as cereal rye or triticale tends to reduce the growth of small-seeded weeds like Palmer, waterhemp and marestail. But larger-seeded species such as burcucumber and cocklebur are not thwarted by cover crop residues.

Pokeweed continued to be a nuisance, too. In most cases, including glyphosate or dicamba in the program, and spraying it from mid-June to mid-July, has done well in our research. Other herbicides can be included in the mix, but always be sure to get good spray coverage of the entire weed.

Late-season escaped annual grasses such as foxtails, crabgrass and fall panicum were a problem, especially in corn. Most of the Group 15 herbicides (s-metolachlor, acetochlor, pyroxasulfone) applied at planting dissipate after several weeks — yet these weedy grasses can still germinate, especially if rainfall comes after a period of drought during the season.

A post herbicide is usually necessary to control them because they can become unruly by the later stages of the growing season. In some cases, additional residual herbicides during the season may be required.

Problems in pastures, hayfields

In grass pastures and hayfields, old stalwarts such as hemp dogbane, horsenettle, Canada thistle and smooth bedstraw continued to plague farmers, causing many to ask for management recommendations. These can be very difficult to control, but with routine mowing during the season and a well-timed fall herbicide application, their populations can slowly be reduced over a few growing seasons.

Japanese stiltgrass persisted in many grass forages. For control recommendations, refer to a previous article I wrote about this aggressive weed.

You likely have battled many of these weeds and others over the years. But if you stay vigilant and consistently use a combination of management tactics, their negative effects on crop yield can be lessened.