Note: This is not an article to frame a debate for or against herbicide use, but rather highlight some commonly occurring issues that often show up around now.

Second Note: There are lots of smart(er than me) folks out there with useful things to say about this topic. This is what I know and have observed.

There are a plethora of weed killing and suppression products that have been developed over the years. Their development has allowed the agriculture industries to expand and deliver good yields of high-quality products at a lower price. Herbicide use has meant that there has been an overall reduction in the amount of mechanical cultivation and tillage in conventional crop production, which has improved soil quality, from a soil structural perspective and perhaps in other ways. Careful and judicious use of herbicides is one of many tools available to address the perennial challenges of managing weeds. I have personally benefited from the reduction in hand labour required to maintain vegetable plots.

With that being said, are herbicides perfect? No. Sometimes they work the way that they are intended, and other times, they do not, or not exactly. Sometimes herbicides end up where they were not intended, through misapplication. Sometimes they cause damage to the plants they are being used to help. Sometimes they affect crops that are planted the following season. Sometimes they find their way into areas where more sensitive plants are growing, with unfortunate results. And that is where our story starts.

Framing the scenario

In my mind, for the entirety of this story, I am picturing my favorite (or most familiar) vegetable plant. The mighty potato (or maybe its relative, the tomato). They are notoriously sensitive to herbicide residues, and feature most often in the herbicide injury scenarios that I encounter.

Waking up to a bad hair day

Herbicide injury can occur at any point in the growing season, from the first point of crop emergence through to harvest. Sometimes injury will only appear the following season, in the offspring of the exposed plants.

Herbicide injury can look like many things. In some cases, it is a marginal tissue burning or localized tissue death. Sometimes it is complete plant death. Sometimes it is serious stunting and weakened growth. Sometimes there is a discolouration or disruption of the plant pigments. The most dramatic cases are where the affected plants look like they are having a very weird, very extreme bad hair day. In some cases, leaves might twist or curl, with leaves and plant parts becoming shorter or longer than normal. They might have a string-like appearance, or they might corkscrew or curl up tight like a fiddlehead (un-emerged fern frond), giving the stalk of leaves/leaflets the appearance of having a bunch of balled up leaves. Sometimes it looks like someone pulled a string on the edges of the leaf, cinching it up.

The severity of the damage can vary hugely, from a few leaves twisting a bit, or one or two plants being mildly affected, to an entire part of a field having serious symptoms. Sometimes symptoms show up only on one side of the plant.

Sometimes damage only shows up after a period of stress, such as a sudden heat wave following cool, wet conditions. Maybe it will show up right off the hop in the early season, or once plants start extending their roots downward and outward. Whenever it shows up in a year, in 2020 there have been an uptick in observed cases right now.

Pinpointing the cause

Determining the culprit herbicide can be extremely challenging, unless you happen to get “lucky” and know just what was applied, where and how. Otherwise, even if a tissue test in a lab was not exorbitantly expensive, as well as almost harder to find than a mountain view in Saskatchewan, the accuracy of such a test can be tough. In “simple terms”, you must accurately determine the probable active ingredient, and then know the derivative compound(s) for that particular product. And you have to hope that the product will still be present at a detectible level in the tissues (or whatever it is that you are testing – e.g. water, soil, etc.) when you sample.

Pinpointing the source

In my experience, when dealing with a possible/probable herbicide injury situation, it becomes an exercise in asking a thousand questions. While knowing the specific herbicide that was applied is important and would be extremely useful, if you happened to obtain that information, it is but one bit of data that needs to be collected. More important is to understand all the different ways that an herbicide might have been introduced to a crop, whether deliberate or indirectly accidental. Knowing the “How much” and the “When” can be just as important.

Here are some of the questions that might be asked to gather information and arrive at a cause (and a possible solution) through a process of elimination.

• What herbicide(s) might have been applied directly to the field in the current year? When was that applied?
• What might have been applied in the previous 3 years?
• What herbicide(s) might have been applied to land adjacent to the field?
• What amendments have been added to the site at any point in the past 2-3 years, including this year? This includes manure, compost (rotted or other), clippings, chips/mulch.
• If amendments were added, how much was applied, where was it applied in the site, what was done to the amendment after application (incorporation, etc.)?
• Ask questions about what might have happened to the amendment, such as “Was it treated?” (deliberately or accidentally), or “Where might an herbicide have been introduced to it?”, and other related questions.
• Was any water applied to the site, and where did it come from? You might need to ask questions about the irrigation source, if there appears to be any sort of potential issues.
• Where are the injury symptoms appearing in the overall site?
• Are you seeing damage on the edges of the planting, in groups, or in random spots?
• What patterns are apparent? Did the symptoms appear in low spots, on the edges, on high spots, etc.?
• Where are the symptoms appearing on the individual plants?
• How have the symptoms progressed? Did it start with one plant, the spread outward from that point? Or was everything affected at the same rate, starting at the same time and place?
• What crops are affected? How severe are the symptoms? At what point did they appear?

These and many more questions might be asked as you gather information. Knowing the answers to these questions might provide the necessary clues to be able to find out what happened to your crop(s) and what you might expect going forward.

What is the solution?

As a caution and a caveat, even if an exact cause is determined, in most cases, there is no easy, magical fix. Mostly, you need to understand how much residue might be present, how long that might be expected to last in the soil in differing situations, and how permanent the damage is in the present. Based on what you know, you can make some educated guesstimates about duration and formulate a response plan. Spoiler alert: Most of the plan will be waiting and watching.

If the damage is minimal, with only minor symptoms observed, it is possible (maybe even likely) that the plants will grow through the injury and survive. Yields will be affected. If things are more dire, with severe symptoms and widespread issues, expect a major yield hit and limited to no recovery.

I should note at this point, that legally, damage in a crop from an herbicide (applied directly or indirectly) is considered a misapplication and an unregistered use, and that crop should not be sold or consumed. I am sure that there are charts somewhere that show what sort of acceptable limit there might be for derivatives, etc. but the short answer to the unasked question (“Can it be eaten?”) is No. That is the legal answer. What a private person decides to do is up to them, but legally, consuming product treated with unregistered product is not allowed.

What about going forward? If you are hooped for the current season, then your thoughts (and those in this situation) inevitably turn to future years. This is where things get complicated. The answer will depend on what the weather is like for the next few years, the amount, and the nature of the active ingredient (e.g. how fast it degrades and breaks down), as well as the crops that are grown on that space.

You can potentially use different plant sensitivities as an indicator of potential severity and duration. For example, potatoes and tomatoes are sensitive, with 4 parts per billion (ppb) causing some injury. In my experience, the next most sensitive crop might be beans, and perhaps some cucurbits. Once everything is showing symptoms, you know that you are dealing with direct application or higher levels.

If the problem showed up due to direct misapplication in the site, then it is likely that the concentrations will be higher, and the problem will last longer, simply because there is more product to be broken down. If introduction occurred indirectly through amendments or something like drift, then the duration of impact should be shorter, theoretically. In dry conditions, microbial activity (microorganisms will be responsible for breaking down the products) is reduced and the residual duration will be extended. If things are moist and warm, the rate of breakdown is more normal and will not be as long.

Other than waiting, there are things that can be done. You can make sure that there is no further introduction to the site. Grow less sensitive crops in the next couple of years. In raised bed situations, you can replace the soil (if that is what is needed). Replanting of perennials might be required. If you are not sure where things are at, a simple bioassay with sensitive plants and a sample of soil can help you to know if you are in the clear.

Herbicide injury is frustrating, but not as uncommon as would be preferred. Take the time to gather the information you need to make an informed and educated decision.

Damage manifesting in potatoes suspected to have been exposed to Odyssey (imazamox / imazethapyr)

Photo by Robert Spencer

Tubers from suspected picloram exposed plants

Photo by Robert Spencer