NOTE: Additional photos of herbicide damage will be added soon!
Column Title: “Northwest Missouri Horticulture”
By: Tim Baker
Release Date: May 17, 2012
Throughout my Extension career, I have seen many instances of herbicide damage on crops and gardens. Sometimes this has been damage from spray drift. This occurs when an herbicide is applied to a nearby field, and air currents carry the chemical in enough concentration to damage a crop or garden. This can occur at the time that the herbicide is being applied, or later through volatilization.
A few times, the grower didn’t get his spray rig cleaned up well enough, and he caused the damage on his own crop. This usually caught the grower by surprise, since he thought he had cleaned his equipment sufficiently.
But over the past several years, I have been seeing increasing damage to crops from herbicide-contaminated soil which was brought into a crop-growing area, or even compost, which contained enough herbicide to cause significant damage to crops.
Before I get to herbicide contamination, I would like to cover the topic of moving soil into a greenhouse, high tunnel, or garden. There are good reasons to do that, and good reasons not to attempt it.
Yes, there may be valid reasons to move soil into a greenhouse or other setting. Sometimes, especially if soil-borne plant diseases have occurred, you may need to replace the soil. This is especially a problem after many years of continuous cropping in a greenhouse or high tunnel.
Sometimes, growers feel that the greenhouse soil needs to be leveled out. That is best done when the greenhouse is established, not later. It’s best in this situation to use the same soil that was originally in the location, or nearby, and not mix soil types.
This brings up the subject of greenhouse or high tunnel locations. Try to select an area that is as level as possible, and does not need major earthmoving to make a good site. No, you don’t want a lot of elevation variability in your greenhouse. This leads to its own set of problems.
If a lot of earthmoving is required, you may run into a situation where there are non-uniform soil types within the greenhouse. On one end, you might find more clay where you may have dug deeply, for example. The other end may have much more topsoil. This can affect plant growth, and make your greenhouse less uniform, leading to fertilizer application and watering problems.
I have even seen growers bring in soils from remote locations, with a very different soil type. This can produce major problems when you want to test your soil, since they vary so greatly. It’s best to have a uniform soil type throughout the entire house.
There are other problems that can arise when moving soil. I once visited a greenhouse with dying strawberries. The grower had moved soil into the greenhouse, forming nice raised beds. Unfortunately, the soil chemistry was very high in salts, which the strawberries could not tolerate. So you really have to be careful when moving soil into a crop-growing situation. You may end up with more trouble than you had expected.
Moving soil into a greenhouse or high tunnel may also bring another unwanted result: herbicide contamination. I have seen several instances of commercial growers who decided that they needed additional soil in their greenhouses or high tunnels. They found what appeared to be good soil, and brought it to their greenhouses. Unfortunately, it had been contaminated with herbicide, and they saw significant damage occurring on their tomatoes.
The first situation that I encountered occurred in 2010, when a grower contacted me about the tomatoes in his greenhouse. It happened that James Quinn, an Extension Horticulture Specialist from central Missouri, and Dr. Jaime Pinero, an Extension IPM Specialist, both happened to be in the area that day, so we stopped by.
The tomatoes had distorted leaves, which looked very much like typical chemical injury. This could have been from a chemical applied to the crop, drift from an application elsewhere, or even wood smoke injury from his wood stove. However, when talking to the grower, we could never determine the most likely cause. None of the pieces of the puzzle seemed to fit together.
My plan in the case of this grower was to keep an eye on the greenhouse, to see if the problem started getting better or worsened. Basically, it got worse. In further conversations with the grower, he said that he had brought in soil from another location to raise the soil level in his greenhouse. After hearing this, I asked him to inquire further with the landowner who owned the pasture. It turned out that the landowner had applied a broadleaf herbicide to the pasture, and that soil brought into the greenhouse contained the herbicide, unknowingly contaminating his tomatoes.
In addition to the distorted leaves, the tomato fruits began to show distorted growth as well, taking on a definite pear shape. This grower had a disastrous year, with very few marketable tomatoes.
A second greenhouse in the area had also brought in soil from this same pasture. The grower was seeing similar damage in his house, although not as great. This was further evidence that this contaminated soil was the source of the tomato injury.
Composts, including those with manures, are a time-honored way of providing fertility to your crops. There’s nothing wrong with using them when handled properly. But modern herbicides have thrown a few wild cards into the use of these products. You need to be aware of these issues, and keep an eye out for them. There are actually two situations that we have observed in the past few years.
The first situation is that of herbicides actually surviving the intestinal tract of an animal, in a high enough concentration to cause crop damage. In this case, a broadleaf herbicide is sprayed on a pasture, creating lush grasses for the animal to feed on. When the manure is collected, the herbicide is still there.
Tom Fowler is my fellow MU Extension Horticulture Specialist based in St. Joseph. In 2011, he received a complaint about tomatoes showing injury in a greenhouse. The grower had used commercial compost, consisting of buffalo manure, its main component. The symptoms appeared to be chemical injury, but as in the case of the greenhouse mentioned earlier, none of the pieces worked in this puzzle as well.
So Tom sent some of the “Buffalo Compost” down to Dr. David Trinklein, MU Greenhouse Specialist. Dr. Trinklein was able to duplicate the damage to tomatoes in a growth chamber. Further inquiry discovered that the herbicide had also been applied to the pasture that the buffalo had been feeding upon. A quick check with a MU weed scientist revealed that yes, herbicides such as this can survive the intestinal tract of an animal, and be in high enough concentrations to cause damage in plants.
Recently, I encountered another kind of herbicide damage with compost. Several growers had obtained manure/compost from a nearby feedlot. Some of them used more of the compost, some less. In the greenhouses with heavier amounts of compost, there was significant damage in the leaves. Growers using less of this product saw less damage. I am still investigating this situation, but it appears that a product containing 2,4-D was sprayed around the feedlot, and perhaps on the manure pile itself, to control weeds. The tomato leaves were certainly showing classic 2,4-D damage symptoms, so this makes sense.
As you can see, it again makes sense to know about the history of any material that you are adding to your soil, whether it is additional soil or compost. If there is any doubt, and you have time, you can plant a test crop in the soil or compost in question to see what happens. Keep in mind, though, that symptoms may not express themselves until the plant is approaching maturity.
Herbicide contamination will eventually go away on its own, as the chemical breaks down over time. But it could be a long time. Many of the herbicides that we have discussed may have several years indicated on their product label before it is safe to plant vegetables. Is there a way to speed up that breakdown process?
One option for the grower is to remove the contaminated soil and replace it with good soil. But that is a lot of work. And none of the growers in the particular situations that I have described opted for this method.
When herbicide contamination occurs in a greenhouse or high tunnel, the situation is even worse. The breakdown of the chemical is often enhanced by rainfall. But you don’t get precipitation inside these structures. So another suggestion was to remove the plastic from the greenhouse, and let the rain and snow come down all winter. The grower who had brought in contaminated soil decided to take this route.
Another suggestion that we made was to use activated charcoal in the soil, mixing it as best as possible. Activated charcoal has been shown in several instances to absorb herbicide. The grower also decided to use this approach.
In all of these suggestions, there were no guarantees. These herbicides can be long-lived, but measures like these could help speed up the breakdown process. Fortunately, they did.
In 2011, I followed up with both greenhouses suffering the herbicide damage on their tomatoes. You had to look hard, but both houses still showed slight evidence of damage on the new crop. However this was so slight, it was hard to find.
The grower with the most tomato damage in his greenhouse had not harvested much at all during 2010 in this greenhouse. After taking the suggested measures, his 2011 growing season was much better, close to a normal year.
I have also taken a look at the greenhouse that showed the worst damage again this year. Believe it or not, I could still find a leaf or two that showed the herbicide damage symptoms. However, this will not be affecting his yields this year.
If you would like to see what some of this damage looks like, take a look at this article on my web site at: http://extension.missouri.edu/nwregion/hort/newspaper.shtml
I’ll close by making a couple of observations. It is amazing to me, that such a small residual of the herbicide in the soil could cause such damage, but it did occur. Even more amazing is that buffalo feeding on a pasture sprayed with this herbicide could pass enough of it thorough their intestinal tract to cause damage in tomatoes.
My final observation is really a warning. Be very cautious. Find a trustworthy source of compost or soil (if you need it) to use on your crops. If in doubt, ask questions. You can’t be too careful.