Time to Scout for Weeds

Kurt M. Vollmer, Postdoctoral Researcher – Weed Science, University of Delaware; kvollmer@udel.edu

With most of the corn and soybeans planted, now is the time to start scouting for weeds. Doing so will prevent major headaches later in the growing season. While scouting, be sure to note the weed species present, height, life-cycle, and severity of the weed infestation. When looking at fields this year, pay attention to those areas that were drowned out last summer. The weeds in many of those spots produced seed and now have very high seed banks. So while weed pressure in the rest of the field may not be too heavy, weeds present in these spots may be at unacceptable levels.

In particular, Palmer amaranth can quickly become unmanageable if not spotted early. Many herbicide labels suggest spraying this weed when it is less than 4 inches tall, but the UD Weed Science program recommends applying postemergence herbicides before its 3 inches tall. Our research with soybean shows that the best time for this second application is no later 28 days after applying a residual herbicide. Furthermore, Palmer amaranth can quickly exceed 4 inches, and research at the University of Maryland has shown that delaying the postemergence application to 32 days or longer can result in reduced levels of control. Remember, the earlier Palmer amaranth is spotted the better. Furthermore, keep in mind there could be several days between scouting and actually getting the sprayer into the field, allowing Palmer amaranth to reach heights that prevent complete control.

Last Year’s Drowned Out Corn, This Year’s Weed Problems

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

A number of fields last summer had poor stands of corn due to the wet soils. As a result, the weed control in those areas was very poor and the number of weed seeds in the soil is probably very high.

Be sure to scout those areas separately from the rest of the field to evaluate if additional weed control is needed or if the area needs to be treated the sooner to achieve good spray coverage.

  • Start clean, no weeds should be present at time of planting
  • Apply residual herbicides within 1 to 2 weeks of planting
  • Use full herbicide rates
  • Scout to be sure the residual herbicides were activated and evaluate if postemergence herbicides are needed
  • Treated emerged weeds while they are small and most susceptible (less than 4 inches tall)
  • Scout again to be sure treatments were effective and determine if a follow-up treatment is needed

New Mid-Atlantic Field Crop Weed Management Guide Available

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

There is an updated “Mid-Atlantic Field Crop Weed Management Guide” developed by weed specialists from Penn State, Univ. of Delaware, Univ. of Maryland, Virginia Tech, and West Virginia Univ. The 260-page guide covers corn, sorghum, soybean, small grains, and hay and pastures. The guide includes information on commonly used herbicides for these crops, including relative effectiveness for burndown, preemergence, and postemergence control of most of the common weeds in the region. There are tables on premixes and what is included in the premixes, and a section on management of problem weeds. The guide is available in the Delaware county offices for $15 or can be ordered on-line at https://extension.psu.edu/mid-atlantic-field-crop-weed-management-guide. Available on-line are the printed copies for $25; an enhanced pdf copy for use on computers and tablets for $15 or both a hard copy and pdf for $35.

What to Think About When Using Cover Crops for Weed Control

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Cover crops can provide many benefits, including preventing soil erosion, improving soil structure, alleviating compaction, nutrient management, and weed control. The most successful approach with cover crops is to identify your goals, and then select the best cover crop(s) and actively manage the cover to reach the goal(s). When considering cover crops to help with weed control, consider a cover crop species that will not only produce a lot of biomass, but also produce biomass that does not breakdown quickly. Local research has shown that weed density can be reduced by more than 50% with a cover crop and cover crops can outcompete weeds for light resulting in small weeds that are easier to control with other tactics, including herbicides. Use of cover crops has shown promise for resistance management by helping to reduce selection pressure when herbicides are used. Most cover crops in Delaware are planted in the fall and terminated in the spring, so they have the potential to affect both winter and summer annual weeds.

For fall planted cover crops, cereal rye or triticale are the best two species for weed control from our research (annual ryegrass also has many similar attributes, but it can be difficult to kill in the spring). They both shade the ground quickly in the fall because they emerge fast and grow late into the fall, and they grow more prostrate than other grass species to provide ground cover very soon after emergence. Legume cover crops are not as effective for winter annual weed control because they are slow to establish and often do not produce significant amount of biomass until the spring. In addition, legume biomass tend to breakdown much quicker than winter cereals, reducing their ability to suppress late emerging summer annual weeds. Winter sensitive cover crops, such as forage radish, produce a significant amount of biomass in the fall, and are a good option for control of winter annuals such as henbit or chickweed. However, the biomass breaks down quickly in the early spring and they provide very little suppress of summer annual weeds.

Both have cereal rye and triticale have a wide window for planting and quite adaptable to a range of soil conditions. Forage radish has a very short window for successful planting, while legumes have a wider window, though is not as wide as cereal species. Getting a uniform stand across the field is critical for maximum weed control or suppression. Where the cover crop stand is not uniform, the weeds are not subjected to the plant competition, the weeds are more numerous and larger, and can be harder to control than those plants growing amongst the cover crop. Achieving this uniform stand is best with a drill due to the improved soil to seed contact. Other seeding methods can sometimes provide good stands, but the results have not been consistent. Many farmers are compensating for the challenge of achieving a good stand by increasing their seeding rates compared with rates used when drilling.

Most of the local research has used 2 bu/A of cereal rye (or triticale) because that was the seeding rate for cost share programs, but experience tells us we can reduce that seeding rate without sacrificing weed control. We do not have enough local research to say how low we can plant rye and still get the same level of weed suppression. We are asking that question with ongoing research.


Herbicide carryover: what herbicides were used throughout the summer (and when)? Is there a risk that the herbicide can slow the growth or reduce the stand of the selected cover crop species? Forage radish and legumes tend to be more sensitive to herbicide carryover than the cereal species.

Cover crop termination (when): the maximum benefit of the cover crop for weed control occurs when the cover crop is terminated as late as possible. Terminating the cover crop at or close to flowering/heading time allows for the high levels of biomass, but also allows for the maximum production of lignified tissue (which is the tissue most resistant to breaking down). Terminating the cover crop more than 2 weeks before planting lessens the weed control benefits. Earlier termination of the cover crop results in less biomass production, less lignified tissue produced, and longer opportunity for the cover crop to breakdown (and less weed suppress after planting). UD Weed Science is examining the weed control benefits of planting green, and we have some promising results, but more work is needed.

Cover crop termination (how): ask yourself how you will terminate the cover crop in the spring. It is best to know how difficult it might be to kill the cover crop; what herbicides are needed to control the cover crop; and will the herbicides used to kill the cover crop interfere with planting your spring crop. If planting a cover crop mixture, consider how you will kill all the species. Do not plant something that you cannot successfully terminate in the spring. Annual ryegrass is one of the species that can be challenging to kill in the spring.

Is an herbicide needed at planting: Are weeds present when you are planting your cover crop, how big are they? If winter annual weeds have emerged before you plant your cover crop, you may see benefits from a burndown herbicide. Winter annual weeds emerging after planting are seldom a problem with a competitive stand of cereal rye or triticale, but weeds up before planting could compete with the cover crop and limit it from reaching its full biomass potential.

Fertility levels: some cover crop species’ growth can be reduced if there are low levels of soil nutrients (including micronutrients). Tillage radish is more sensitive to fertility levels than some of the cereal cover crop species.

Multi-species mixtures: using more than one species is a great strategy for some soil health objectives. However, as more species are added to the mixture (and the amount of any one species is reduced) the benefits for weed control could be reduced. Terminating a multi-species cover crop could be difficult.

In the early spring newsletter we will discuss related topics to using cover crops for weed control, including challenges:

  • Cover crop/cash crop competition (moisture/light/nutrients)
    ● Volunteer cover crops
    ● Pests (slugs)
    ● Additional expense / management
    ● Establishment a good crop stand

Fall Control of Perennial Weeds

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Fall is often the best time and the most convenient time to treat most perennial weeds because it is the time that plants are best able to translocate the herbicide to the roots where it will do the most good. When considering fall weed control the emphasis should be on what the patch of weeds will look like next spring or summer not the amount of dead stems this fall. Also, it is important to consider that a fall application will not eradicate a stand of perennial weeds; the fall application will reduce the stand size or the plant vigor, but applications in consecutive years are likely needed. Fall application of glyphosate is the most flexible treatment for most perennial weeds such as bermudagrass, Canada thistle, common milkweed, common pokeweed, yellow nutsege, horsenettle and johnsongrass. Rates of 1 to 1.25 lb acid per acre are consistently the most economical (or about 1.5X the normal use rate for annual weeds). Dicamba (Banvel) at 2 to 4 pints is also labeled for artichoke, bindweeds, dock, hemp dogbane, horsenettle, milkweeds, pokeweed or Canada thistle. Planting small grains must be delayed after dicamba application 20 days per pint of dicamba applied. Fall herbicide applications should be made to actively growing plants. It is best to spray prior to mowing the corn stalks and allowing plants to recover after harvest. Allow 10 to 14 days after treatment before disturbing the treated plants. If fall applications are delayed, remember weed species differ in their sensitivity to frost; some are easily killed by frost (i.e. horsenettle) others can withstand relatively heavy frosts. Check the weeds prior to application to be sure they are actively growing.

Tankmixing Reflex (fomesafen) with Glyphosate (reprinted)

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

There have been many situations where both fomesafen (active ingredient in Reflex) and glyphosate will complement each other for weed control. Syngenta has a premix of fomesafen plus glyphosate called Flexstar GT. Also, Reflex and glyphosate can be tankmixed, but there have been some situations of these two products may not mix well. The following is an article from Ken Smith from University of Arkansas entitled “Problem Solving Incompatible Tankmixes of Glyphosate and Reflex®”

Some growers have experienced cottage cheese spray mixtures when Reflex® and glyphosate were tankmixed in an effort to burn down existing weeds while applying Reflex® prior to cotton or soybean planting.

It seems that the potassium salts of glyphosate (WeatherMax, Touchdown, PowerMax etc.) are not very compatible with Reflex® . . .  Many of the generic glyphosate formulations are isopropyl or diammonium salts (not potassium salts) and will mix fine. A quick check of the label will give the salt used in the formulation. 

If . . . Reflex® and the potassium salt of glyphosate is mixed and found to be incompatible, it can likely be brought back into solution by adding household ammonia. Start with 1% ammonia and begin agitation. More ammonia may be added if needed.

Considerations for Controlling Weeds in Drowned Out Crops

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

An area of the field where the crop has drowned out gives weeds an opportunity to grow without crop competition, and potentially produce a tremendous amount of weed seeds. If a particularly troublesome species such as Texas panicum or Palmer amaranth is growing in these spots they could really cause problems for the next few years if they are allowed to produce seeds. So, what should you do? Some options to consider are whether you can reach these spots with equipment such as mower or sprayers; what crop is in the field; what you intend to plant in the field after harvest; and what will effectively control or kill the weeds?

Mowing is an option, but in all likelihood the areas will need to be mowed multiple times to prevent seed production.

If considering a herbicide, first assess the situation. If you are treating areas of a field, and will be harvesting the crop around the bare areas, you are limited to herbicide options for the crop planted in the field. Furthermore, you are limited to the same herbicide rates and herbicide application timings. Using herbicides with residual control is going to be important because you will not have a crop canopy present for later emerging weeds.

Also, consider what will be planted in the field next and check your rotational intervals. Will you have enough time between herbicide application and planting the next crop? This is a situation where you will have to assess each field individually, but these drowned out areas may need special attention.

Use Caution when Selecting Adjuvants

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

The weather patterns lately have resulted in situations where the risk of crop injury from postemergence herbicides is higher. Specifically, prolonged periods of overcast skies, cooler weather, and plenty of rain. If postemergence herbicide applications are made as the days turn hot and sunny, the risk of injury is greater. This is due to the wax layer on the leaves not having developed and the leave surface being “tender”. If spraying during these sensitive periods, switch to “softer” additives if the label allows it; for instance MSO increases the risk of injury over COC; and non-ionic surfactants (NIS or 80-20’s) reduces the risk further. Consider using the lower allowed rate of the surfactant or nitrogen. Be sure to read the label and see what is allowed by the manufacturer.

Controlling Perennial Weeds When They Emerge from Seed

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Perennials often produce seeds that are adapted to being moved by the wind (hemp dogbane, milkweed, or Canada thistle) or produce large seeds or berries that are eaten by birds and animals and spread around (pokeweed). We conducted a greenhouse study (funded by DE Soybean Board) to examine soil-applied herbicide control of some perennials when they originate from seeds. Perennial seedlings (emerged from seeds), can produce a perennial root system after only 3 to 4 weeks. Being able to select the correct herbicide when you know seeds are coming into your fields can help prevent headaches and frustration in years to come. If at all possible, keep the perennials mowed along ditches and field edges to reduce (or eliminate) seed production. Prevention is the best approach.

Johnsongrass, bermudagrass, Canada thistle, hemp dogbane, common milkweed, common pokeweed, and horsenettle were planted in the greenhouse and sprayed with common soil-applied herbicides (Dual, Prowl, Command, Lorox, metribuzin, atrazine, and Canopy). Next to each weed are the herbicides that provided the best level of control (over 90% control). This study was conducted a few years ago and neither Lumax, Valor, nor Authority were available at that time for testing.

Bermudagrass: Dual, Prowl, Command, and metribuzin

Johnsongrass: Command.

Canada thistle: Command, metribuzin, atrazine, and Canopy.

Hemp dogbane: Command, Canopy, metribuzin, and atrazine.

Common milkweed: metribuzin and Canopy.

Common pokeweed: Canopy, and metribuzin.

Horsenettle: metribuzin, atrazine, and Canopy.

Have You Purchased this Year’s Mid-Atlantic Field Crop Weed Management Guide?

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

The guide covers weed control in corn, sorghum, soybean, small grains, and hay and pastures. The guide includes information on commonly used herbicides for these crops, including relative effectiveness for burndown, preemergence, and postemergence control of most of the common weeds in the region. There are tables on premixes and what is included in the premixes, and a section on management of problem weeds. To find out more about the guide watch this short you-tube video at https://www.youtube.com/watch?v=wH5AyULjKL0

The guide can be ordered on-line at https://extension.psu.edu/mid-atlantic-field-crop-weed-management-guide. Available on-line are the printed copies for $25; an enhanced pdf copy for use on computers and tablets for $15 or both a hard copy and pdf for $35 + shipping. A free low resolution pdf is available at http://extension.udel.edu/ag/weed-science/weed-management-guides/. Note the low-resolution version is not “searchable”.