What’s new with herbicides?

What’s new with herbicides?

2020 brings new registrations of use to the vegetable producer. Weed control in vegetables usually requires a careful integration of tillage, herbicides, cultivation and mulches. No single herbicide will control all weeds for the entire season. The following summarizes guidelines for use on crops that extracted from product labels, the final authority. If you have further questions please contact me, or your local agricultural educator.

Pumpkin, Squash & Tomato. Reflex Herbicide (EPA Reg. No. 100-993). (Ohio Section 24 OH0993019AA0619 OH-200002; for use only in Ohio). Pumpkin, squash and tomato varieties may vary in their tolerance to Reflex herbicide. Be certain of crop tolerance before using Reflex on a new variety. Butternut squash is very sensitive to Reflex. Cold and wet soils during pumpkin and squash germination and maturity may result in delayed maturity and/ or yield. Heavy rain shortly after transplanting may also result in delayed maturity and/ or yield. Reflex persists in the soil for a considerable time providing weed control; however, this characteristic also means that several crops may be damaged if grown in close rotations following use of Reflex.

• Direct Seeded Pumpkin, Summer and Winter Squash (32 day PHI). Apply Reflex at ½ – 1 pint/A broadcast preemergence after seeding and before the crop emerges. If rainfall does not occur before crop emergence, an overhead irrigation will activate the herbicide and reduce the potential for crop injury from splashing soil onto crop leaves.
• Transplant Pumpkin, Summer and Winter Squash(32 day PHI). Apply Reflex at ½ – 1 pint/A as a pre-transplant, non-incorporated preemergence broadcast application up to 7 days before transplanting. Minimize further soil disturbance during transplanting or weed control may be compromised, and avoid any practice that may concentrate the herbicide around the transplant plug.
• Transplant Tomato on bare ground (70 day PHI). Apply Reflex at 1 pint/A as a pre-transplant, non-incorporated preemergence broadcast application up to 7 days before transplanting. Minimize further soil disturbance during transplanting or weed control may be compromised, and avoid any practice that may concentrate the herbicide around the transplant plug.
• Transplant Tomato in plastic mulch (70 day PHI). Apply Reflex at 1 pint/A as a pre-transplant, non-incorporated preemergence broadcast application after the final raised bed has been formed but before mulch has been laid. Do not disturb the soil after laying the mulch.

Onion (bulb) & Leeks (60 day PHI). Zidua Herbicide (EPA Supplemental Label, Registration No. 7969-338). Apply postemergence, after seeded onion emergence or after transplanting leeks but before broadleaf weeds have emerged. Zidua does not control emerged weeds. Apply 1.3 to 1.7 ozs/A to onion and leek in medium or fine textured soils. Apply 1.7 ozs/A to onion or leak in muck soils with 20% or more organic matter. Crop-stage should be 2-6 true leaves. Do not apply to coarse textured soils.

Asparagus & Rhubarb. QuinStar 4L Herbicide (EPA Reg’n No. 42750-169). For control of emerged field bindweed and hedge bindweed, apply Quinstar in the fall but before frost to bindweeds that are actively growing. Apply Quinstar at 12.6 ozs/ Acre mixed with crop oil concentrate at 2 pints/ Acre.

Okra (14 day PHI). Caparol 4L (EPA Reg’n No. 100-620). For control of broadleaf weeds apply Caparol 4L preemergence after seeding and before crop emergence at the rate of 3 pints/Acre. Alternatively, apply 1.5 pints/ Acre preemergence and a second directed-postemergence application of Caparol when the okra plants have 7-9 leaves and weeds are 2 inches tall or smaller.

Lettuce. Dual Magnum. (EPA Reg’n No. 100-816 EPA SLN No. OH-190001; for use in Ohio only). For control and suppression of annual grasses, yellow nutsedge and small-seeded broadleaf weeds.

• Leaf lettuce, seeded or transplant (25 day PHI). Apply Dual Magnum at 0.67-1.0 pints/ Acre preemergence to weeds either before crop emergence or transplanting. Alternatively, an over-the-top- or directed- postemergence application of Dual Magnum at 0.67 pints/ Acre can be made before weed emergence. If pre and post applications are used the total amount of Dual Magnum should not exceed 1.33 pints/ Acre per crop.

• Head Lettuce, seeded or transplant (20 day PHI). Apply Dual Magnum at 0.67-1.0 pints/ Acre preemergence to weeds either before crop emergence or transplanting. Alternatively, an over-the-top- or directed- postemergence application of Dual Magnum at 0.67-1.0 pints/ Acre can be made before weed emergence. If pre and post applications are used the total amount of Dual Magnum should not exceed 2.0 pints/ Acre per crop.

The research behind these new uses was paid for in part by the IR-4 Program, the Ohio Vegetable and Small Fruit Research and Development Program (OVSFRDP), and was only possible because we have a great network of OARDC research stations and personnel in key vegetable producing areas of the state. Thanks also to the various pest management supply companies that supported development of their products for use on specialty crops.

New Video Resource on Herbicide Drift and Sensitive Crops

Ohio grain farmers are using dicamba and 2,4-D more frequently against herbicide-resistant weeds. These herbicides are stronger and prone to off-target drift. A new video from Ohio State discusses the high cost of drift damage on specialty crops, ranging from lower yields to delayed or lost harvests and even the loss of long-term investments like grape vines or certification status.

2,4-D and dicamba are synthetic auxins, herbicides that affect multiple growth processes by mimicking natural plant hormones. Not all plants are highly sensitive to synthetic auxins, but most will react to exposure in characteristic ways (see photos). Damage is usually characterized by severe distortion of stems and leaves but can also cause delayed or uneven fruit ripening and plant loss.

Good Neighbors: Herbicide Drift Issues and Sensitive Crops
Running time 7:36 minutes

Make a Withdrawal from your Soil Weed Seed Bank: Stale Seedbank Technique

Ah spring!  The war against weeds begins anew. The first major skirmish of the growing season should happen before planting. The stale seed bed technique is an often over-looked practice that can be used before planting. It works by first encouraging weeds to sprout and then killing them when they are young and most vulnerable. For organic growers, a stale seed bed can replace the effects of a pre-emergence herbicide. And when used properly, it can contribute to both short-term and long-term weed management.

Weed control can be handled with short-term or long-term approaches. Short-term management focuses on controlling weeds during the first part of crop growth when weeds are more likely to affect crop yields. Long-term weed management, however, works all season-long to deplete weed seeds from the seedbank (the reservoir of viable weed seeds in the soil). Whichever approach you take, using a stale seed bed is a great cultural weed control technique.

To use the stale seed bed most effectively, start several weeks before planting. An initial cultivation kills any emerged weeds that have overwintered. It also brings weed seeds to the surface where exposure to light and oxygen stimulate germination. Depending on the weather and types of seeds present in the soil, weeds may sprout up overnight or over a few weeks. When weeds have germinated and are still small and young, they are easy to kill with a second light cultivation. This process is then repeated as needed and as time allows. As few as three cycles of light/ shallow tillage can reduce the number of subsequent weeds noticeably. For fields and gardens with very heavy weed infestations more cycles of repeated tillage over a few years will be needed. Using a stale seed bed may push back your planting date; but in the absence of weed competition, the crop will have more access to water and sunlight and be able to make up for lost time.

Keys to Success

  • Do not allow emerged seedlings to grow large. It is best to till lightly just as the first seedlings are emerging as this and the earlier ‘white thread’ stage are the most susceptible to desiccation. The more time new weeds have to develop roots, the harder they become to kill with a shallow cultivation.
  • Keep the cultivation shallow to avoid bringing new weed seeds to the surface. The implement used to stir the soil should not go deeper than 2 inches with most of the stirring in the top inch.
  • The technique is dependent upon having adequate soil moisture. Under drought conditions preparation of a stale seedbed may require irrigation to stimulate weed seed germination.
  • Deeper initial tillage can be used to bury an existing weed problem. Tillage, especially when done with a disc or a power tiller, distributes the previous year’s weed seeds throughout the top 6 inches or so of soil. In contrast, an inversion tillage that turns sod upside down will place last year’s seeds 6 inches or so under the surface. From there they are unlikely to emerge unless further discing or lighter tillage moves them closer to the surface. Used skillfully, a deep inversion plowing followed by stale seed bed can put a serious surface weed problem out-of-sight and out-of-mind, at least until the next time the field is plowed deeply.

Stale Seedbed is most effective when it is part of a zero weed threshold system.

The common short-term approach to managing weeds, (weed scientists usually call this the “critical period approach”) is to control weeds aggressively during the first 4-6 weeks after the crop is planted. This 4-6-week period is the critical period during which crops stands are established and yield is secured. Afterwards weeds are of less threat to production; therefore, many farmers scale back control efforts. However, weeds that grow before and after the critical period are still a problem. If allowed to flower and set seed, they will be planting a future crop of weed problems. A long-term approach to weed management, called zero weed seed threshold, requires constant diligence and removal of all weeds before they produce seeds–even after harvest. Research indicates that 3-4 years of using this approach will result in a field with relatively few weeds, provided weed seeds are not introduced from without the field (in seed, irrigation water, on equipment, etc.).

Both short-term and long-term approaches have benefits and drawbacks, many of which depend on a farmer’s individual goals, crops, and available resources. A new online tool from Ohio State allows farmers to think through various weed control approaches in the context of their own individual situations. For those looking to make changes to their weed management, the Organic Weed Decision Making Tool, shows pros and cons of various strategies over time and gives steps to implementing new tactics. Learn more at go.osu.edu/eco-weed-mngt.

Five-year soil balancing project results

Another wet spring, and many farmers postpone field work awaiting drier conditions. Could improved drainage be obtained through the application of common gypsum? This is one of the claims made by many consultants and farmers who use a practice called soil balancing.

Ohio State’s five-year study on soil balancing has been mentioned in previous VegNet articles. The project involved multiple long-term field tests, as well as interviews and surveys to better document practices and beliefs surrounding soil balancing. Despite a lack of past research proving soil balancing’s effectiveness, we found that the practice is used heavily by organic and conventional farmers in our region to reduce weeds, and improve soil quality, crop quality, and yields. While we were unable to demonstrate improvements in crop yields or quality, we did see limited effects on soil quality and weed populations in some of our test sites during the final year of the study.

Defining Soil Balancing

Traditionally, soil balancing strives to keep base cations calcium (Ca), magnesium (Mg), and potassium (K) at a recommended ideal ratio (typically 64:10:5). Although long practiced by farmers, soil balancing is not recommended by most researchers and Extension educators. Our study indicated around half of organic corn growers in the Midwest used a soil balancing approach, but more than 75% of the Extension researchers we surveyed felt soil balancing had no scientific merit.

It’s true that most soil balancing studies done in the past 20 years have reported the practice had no effect on production. However, our research reveals several potential gaps in these studies. Consultants and farmers we interviewed commonly reported that soil balancing improved overall soil quality and structure, which led to improved drainage and reduced weeds. While farmers also reported improved yields and profit, it was generally not the first improvement they mentioned. Interviewees noted that these improvement often happened gradually over several years. In short, past research may not have captured long range positive effects. Most recent studies were short-term, lasting one or two years; were conducted in a greenhouse rather than field; focused only on improved yields; and were conducted on limited types of soils. (Chaganti and Culman, 2017)

We also found that many farmers pair cation balancing with other soil improvement practices such as cover crops and biostimulants. The goal, according to the “balancers” we spoke with, is to improve the physical and biological properties of the soil.

Field Testing

Using both on-farm and Ohio State research station sites, we collected data on soils, weeds, and crops, while applying a variety of soil amendments to change Ca:Mg ratios. We measured crop quality using Brix, color, size, and other characteristics specific to individual crops. Vegetable crops included tomato, butternut squash, cabbage, popcorn, and edamame. Agronomic field crop trials were conducted as well.

We were unable to document any treatment effect on yield or crop quality. In the last year of testing, we did see effects on weed populations (either lower weed populations overall or lower populations of foxtail on “balanced” soils) and on soil root resistance (indicating improved soil structure with higher Ca saturation). These effects appeared only on some fields, but they do support our hypothesis that the positive results of soil balancing are related to improvements in soil structure and drainage. We hope to continue monitoring these fields to see if results become more consistent over time.


For now, we are unable to officially encourage or discourage the use of soil balancing. The following recommendations are based on field trials and on the experience and advice of our stakeholder advisory committee.

  • Soil test data is critical to making informed decisions about what to apply. Some Ohio soils may already have large concentrations of Ca due to Ohio’s limestone bedrock.
  • Watch your pH if using lime. Gypsum is a better choice to change the Ca level without affecting pH and it also provides sulfur.
  • Soils with a CEC below 10 may develop deficiencies. In soils with a low holding capacity for cations, excess Ca can lead quickly to deficiency levels of K, and possibly Mg. We did work in fields with Ca saturations well above 80% and observed K deficiencies in the soil and vegetables in these situations.
  • Consider economic factors. The higher your CEC, the more time and amendments will be needed to increase the Ca:Mg ratio. At some point—depending on the amount of change needed and the value of your crop—using soil balancing becomes an expensive practice.
  • Any time you try a new practice, monitor the results. If possible, try using the new practice on only part of your farm and compare it with a similarly managed area to see if the new technique is making a positive contribution over time.

With widespread use of the practice, soil balancing is a pertinent area for research and cooperative education. Our team hopes to continue studying the practices and long-term effects of soil balancing on a larger variety of soils. Drawing on experiment data and the experience of farmers and consultants, we will work toward guidelines and toward a mutual understanding of soil balancing.

Read more about this study at the Soil Balancing Project Site or the Vegetable Production Systems Laboratory. This work is supported by Organic Agriculture Research & Extension funding grant no. 2014-51300-22331/project accession no. 1003905 from the USDA National Institute of Food and Agriculture.


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Spray Drift 102

A few weeks ago, in Drift 101, I suggested that farmers should be cautious before concluding that sick crops are a result of herbicide drift from neighboring fields. Symptoms attributed to drift may be caused by other factors. Nutrient deficiencies may cause chlorosis (yellowing) and necrosis (tissue death), symptoms easily misinterpreted as resulting from herbicide exposure (https://vegnet.osu.edu/sites/vegnet/files/imce/newsletters/VegNet/5-31-16%20VegNet%20Vol%2023%20Issue%207myedits.pdf).

The herbicides 2,4-D and dicamba invariably cause distorted growth of foliage (Figure 1), but so can various environmental pollutants especially when those are concentrated in the greenhouse (Figure 2).

Figure 1: Typical response of tomato foliage to low-dose (simulated drift) of 2,4-D.

Figure 2: Distorted growth of greenhouse tomato thought caused by fumes from incomplete combustion from wood heater. Photo courtesy of M. Badertscher (OSUE Hardin Co).

Likewise flood conditions, during which root systems are completed saturated with water for prolonged periods, are known to induce leaf twisting and formation of adventitious roots (Figure 3), symptoms associated with exposure to 2,4-D (Figures 1 & 4).

Figure 3: Adventitious root formation on stem of tomato following 3 days of flooding conditions (root zone saturation).

Figure 4: Adventitious root formation on stem of tomato following exposure to low-dose 2,4-D.

Even when herbicides are the cause, symptoms can occasionally be misleading and point incorrectly to a nearby field. Consider the case of glyphosate, still the most commonly used herbicide in the Midwest. Glyphosate is quickly absorbed by crop leaves and translocated to growing points. On most crops glyphosate damage becomes obvious 4 or more days after drift because new growth is chlorotic. On tomato, chlorosis most often appears as bands across the base of the leaflets (Figure 5). However in a small number of drift events, chlorosis may not appear at all; instead glyphosate induces leaf and petiole curling and twisting, symptoms reminiscent of 2,4-D or other synthetic-auxin herbicides.

Figure 5: Characteristic basal-chlorosis of new tomato leaflets caused by glyphosate drift.

Figure 6: Occasional ‘auxin-mimic’ symptoms that occur in a small percentage of glyphosate drift events.

Soil residues of environmentally persistent herbicides used in previous growing seasons must also be taken into account. Trace amounts of herbicides in the ALS and AHAS families can cause symptoms similar to those caused by 2,4-D and dicamba. Imazethapyr (Pursuit) is an AHAS herbicide used on soybean that controls weeds at low doses of 3-6 oz/Acre. In our lab, tomato flowering was sensitive to doses of the herbicide equivalent to 1/1000th of the field dose. This finding indicates that soil residues of imazethapyr applied two or more years before planting may still be sufficient to injure field grown tomato, as may drift of the herbicide from a nearby or not-so-nearby application.

Many more similar examples could be provided; hopefully, the take home message is that diagnosing crop injury symptoms is a complicated matter that must take several factors into consideration and can easily lead to mistakes being made. Tread cautiously.

Spray Drift 101 – May 18, 2018

Crop samples submitted to the OARDC Weed Lab (http://owl.osu.edu) with suspected herbicide drift injury symptoms sky-rocketed in 2017 and early indications are for the same trend in 2018.  This is happening mainly because vegetables and fruits are much more sensitive to the 2,4-D and dicamba herbicides grain farmers are now using to kill weeds that are no longer sensitive to glyphosate alone.  There is always some drift; but when crops have an elevated sensitivity to the compounds moving in the environment, everyone sees the effect.

Already in 2018, most grain fields have received burndown sprays containing 2,4-D (and some with dicamba) mixed with glyphosate. Predictably samples from injured fruit and vegetable fields, and orchards are being submitted for diagnosis.  So far, roughly 50% of the samples received are not showing symptoms characteristic of herbicide drift, and that’s significant because our lab rarely receives the samples until they have cleared a pathology screen.  So, while crop injury from drift is up significantly, there are still relatively few instances of damage where drift is clearly the culprit.

A general deterioration in farm community relationships has already occurred in some states, as a result of recent conflicts arising over drift.  Grain, vegetables, fruits, landscapes and natural ecosystems have been damaged sometimes with tragic consequences, following the expanded use of 2,4-D and dicamba. Human nature being as it is, we all tend to look outside to explain our problems before we look inward.  Considering this, it is important to consider other possible causes of crop injury before assuming occurrence of drift from the neighbors’ fields.

  • Keep in mind that the injury symptoms associated with 2,4-D and dicamba are indirect responses of the plant to stress, and may have other causes. For example injury caused by glyphosate, used by most vegetable farmers, can sometimes resemble injury caused by 2,4-D.


  • Volatile by-products of incomplete combustion (think of heating a greenhouse or high tunnel) are also known to cause symptoms on bedding plants not unlike those caused by herbicides.


  • Inadequate decontamination of spray equipment used to apply glyphosate, 2,4-D or dicamba may leave behind trace amounts that are enough to cause injury in a subsequent application to a sensitive crop.


  • Carryover of herbicides used on previous rotational crops, sometimes going back two growing seasons, may cause symptoms on vegetables that can be confused with those resulting from 2,4-D or glyphosate drift.


  • Environmental conditions, in particular flooding during and after crop establishment, may induce symptoms that can be confused with herbicide injury, or they may exacerbate the effects of exposure to trace amounts of residual herbicides in the soil that would normally have no affect.


So, ‘what’s a farmer to do?’, to reduce the likelihood of being hurt by drift from nearby fields?


  • Communicate! Talking with the neighbors is first and foremost. This can be difficult considering consolidation of grain farms and rental of land; but communication is vital in protecting your crops.  Explain that the crops you grow are very sensitive to herbicide drift, and that drift can result in a complete yield loss of a very high value harvest.


  • Communicate! Sign up with the Ohio Sensitive Crop Registry.  Steve Smith, Director of Agriculture with the Red Gold Company, reports that incidents of tomato crop injury from drift have declined dramatically since they required their contract growers to register with the sister-program Field Watch (previously known as Drift Watch).  We know that most, if not all, commercial applicators check the Ohio Sensitive Crop Registry before going out to apply herbicides, so registering is clearly the cheapest (it is free) insurance available.


  • Communicate! Steve also reports that placing “No Drift” signs along field edges helped tremendously.


  • Pay attention to pesticide applications on nearby (and not so nearby) fields. Applications in early morning and evening are especially prone to inversion conditions that keep tiny droplets of spray suspended and prone to drift. It will be prudent to keep a written list of observed applications with date, time and observed conditions (are tree branches in motion, flags straight out, etc?). Photographs taken with a smart phone can be location, date and time stamped.


  • Scout your fields at least every 2-3 days paying special attention to field edges where incoming drift will likely have the greatest effect. Symptoms of glyphosate injury usually take 3 + days to become apparent.  Symptoms of 2,4-D and dicamba can develop in less time, often within 24 hours, when growing conditions are ideal.


  • If you see injury anywhere scout the entire field and the hedgerows. Look for patterns.  Drift usually leaves a path of injured weeds, shrubs and trees (look up) along the way. Trace the path to its apparent origin.


  • Photos are very important but equally so, you need a written description of what each photo is attempting to illustrate and where it was taken. Photograph injured plants and plants that are healthy.  Check each photo to verify that it shows the symptom you are trying to capture.  If it doesn’t, take another in different light, from a different angle, or distance.


  • Create a map of the field or mark an existing map, showing where photos were taken and outlining areas affected and not affected.


  • Communicate! Talk to the neighbor, or the applicator if it was a commercial job. Explain that you have injury, that you have reason to suspect it was drift, and that you are monitoring the situation.  Ask them to identify what pesticides were applied.


  • Maintain normal growing practices. If you seek a settlement you must have yield data, and if yields are lower than expected you do not want your failure to maintain the crop to be the reason.


  • The decision to contact the ODA is personal, although an argument can be made that pesticide applications resulting in drift should always be reported. If you contact ODA there is no cost to you for their services, including their analysis of crop tissue for pesticide residues, but once you have contacted them be certain that they will conduct an investigation.


Thinking about the above points it should be obvious that maintaining high quality and complete field records, year in and year out, is important when drift occurs.  Being able to 1) substantiate your own crop and pest management practices validates that you were not the cause of the problem, and 2) past yield records from the field affected will help support a claim for lost yield.


Remember to communicate with your neighbors each year.  Keep in mind that many grain farmers have no idea how valuable an acre of produce can be.


Finally sign up with the Ohio Specialty Crop Register.  Past experience indicates this may be the single most important step you can take to protect yourself.