One of the routine maintenance tasks for athletic field management is the control of weeds. This is not just for aesthetic purposes. Sometimes the weeds can result in reduced lateral shear strength and increased chance for athlete injury. Herbicides, when used according to the label, have been shown to present minimal risk to end users and are typically employed by athletic managers to selectively remove different weeds. However, we are increasingly seeing laws and regulations being passed aimed at reducing exposure to pesticides, including bans of pesticide use on public lands or on school property. In these areas the use of synthetic herbicides is not permitted and alternative management strategies need to be used.
In some locations, laws have been written to exclude the application of any product intended for use as a pesticide. In these areas your options to control weeds are to do nothing, pull by hand, or use some type of thermal product to burn the weed or optimize your cultural practices to favor the growth of the desired turf species.
In most areas the laws that ban synthetic pesticide use allow for the use of certain alternative products. Complicating our attempt to determine which products may or may not be used is that, quite frankly, there is no universal consensus on what can be considered an organic herbicide. Technically, in the minds of an organic chemist, an organic molecule is anything that contains carbon. Thus the herbicide 2,4-D, which is short for 2,4 dichlorophenoxy acetic acid, is technically an organic molecule because there is a 6 carbon ring (a phenyl) with an oxygen attached to it (thus it’s a phenoxy) and there are chlorine atoms attached to the number 2 and number 4 carbon atoms on the ring (that’s why the 2, and 4- are part of the name).
Obviously, the typical home gardener, advocate, activist, government official or regulator does not adhere to the technical definition of what an organic molecule is. In the minds of them and most, organic means something more like “natural.” Ironically, there are plenty of natural compounds that are quite toxic. We used to use some of them as pesticides (like nicotine). Other terms that used to describe non-synthetic (“organic”) pesticides include “non-toxic”, “low-impact”, or “minimum risk”.
The EPA keeps a list of active ingredients that are eligible to be considered minimum risk products, including herbicides. These are exempt from federal registration and thus do not have an EPA registration number. Any active that appears on this list can probably still be used when synthetic herbicide use is not permitted. Commonly seen active ingredients for weed control in turf that appear on this list include corn gluten meal and sodium chloride. The EPA also has a list of approved biopesticides. These are reduced risk products but do not meet the criteria necessary for EPA exemption. Chelated iron is an example of an herbicide in this category.
Unfortunately, there is a lot of confusion regarding some other products that may be being sold or used as low impact or minimum risk herbicides. Examples include not only chelated iron containing herbicides, but also acetic acid, and pelargonic acid. These products may or may not be permitted in areas where synthetic pesticides are banned. The bottom line is that there are too many laws that are too variable to summarize here. So if you are considering the use of organic herbicides because a law was passed in your area that says you have to, you need to check specifically as to what products you are allowed to use. There is, of course, also the chance that you might be considering the use of organic herbicides because either you or the users of the field want you to. Thus the goal of this article is to compare some of the expectations, advantages, and disadvantages of organic herbicides compared to their synthetic counterparts.
Organics Compared to Synthetics for Nonselective Weed Control
Traditionally it is recommended that glyphosate be used for nonselective weed control because it has systemic activity. There are a couple of other synthetic herbicides that are non-selective. However, each of these is a contact herbicide. Included in this list is pelargonic acid, which is sometimes sold by vendors of organic herbicides. Pelargonic acid is very fast, with control sometimes achieved within 30 minutes. However, it is a contact herbicide meaning that repeat applications are required in order to get control of perennial weeds. The majority of products that are from the EPA minimum risk list that are marketed for weed control are non-selective, including cinnamon oil and eugenol. Acetic acid is sometimes used as an herbicide but does not appear on the EPA minimum risk list. Again, it is important for you to determine what is and is not allowed if you are in an area where synthetic pesticide use has been banned.
Organics Compared to Synthetics for Preemergence Weed Control
In a typical field management program, for general preemergence control of grassy and broadleaf weeds, one of several registered herbicides, such as oxadiazon, pendimethalin, prodiamine, and dithiopyr. All of these are older chemistries and are post-patent so there are several different formulated products. Each of these actives will typically give at least 80% control of crabgrass for about 90-150 days, depending on use rate and weather conditions following application. Control exceeding 95% is possible with the right product and if other cultural management practices are optimized. In addition, these active ingredients are labelled for preemergence control of many annual broadleaf weeds.
The option available if using organic products is corn gluten meal. It is a by-product of the wet milling process of corn and is used, among other things, as an ingredient in animal feeds. Technically, it is fit for human consumption. It’s just not very appetizing. Corn gluten meal was discovered by accident in the late 1980’s by Iowa State University Turfgrass Professor Nick Christians. Work conducted in his lab during the 1990’s determined that corn gluten meal contains a large amount of bioactive dipeptides that mimic the action of some commercially available synthetic herbicides. Other work determined that corn gluten meal had preemergence activity against a variety of different weeds in turfgrass, such as crabgrass but also dandelions. Remember that some perennial weeds are relatively short lived and rely on new seedlings in order to perpetuate the population. In these cases a preemergence herbicide can, over time, decrease the population of weeds by decreasing newly germinated seedlings. In field studies it was determined that the best strategy for use of corn gluten meal is to make two applications per year. One is in the spring to prevent crabgrass and other spring germinating weeds. The other applications should be made in the fall to provide at least partial control of germinating broadleaf weeds.
Since the application of phosphorus has become controversial, an additional benefit of corn gluten meal is that it is 10% nitrogen by weight and contains no phosphorus or potassium. The recommended application rate of corn gluten meal is 20 pounds per thousand square feet. This application thus provides two pounds of slowly available nitrogen and applying in both fall and spring would therefore provide four pounds of your annual nitrogen fertility requirements.
Corn gluten meal also has preemergence activity on grasses such as perennial ryegrass that we use as athletic turf. So, if you plan to overseed your field it is best to use corn gluten meal in the spring and then switch to a different fertilizer source and overseed in the fall.
The reported effectiveness of corn gluten meal has been quite variable, with some reporting great results (up to 90% control or better) and others reporting almost no activity against weeds. There is also some evidence that corn gluten meal is more effective in cooler parts of the country and less effective in the transition zone and for southern turf. Results are typically not very good in the first year of use (around 40% control) and then improve significantly in the second and subsequent years, so if trying corn gluten meal you should be prepared to use it for at least two seasons before evaluating its effectiveness on your fields (Figure 1).
If you are considering or are required to switch to an organic field maintenance program then corn gluten meal should probably be the foundation of your fertilizer and herbicide management program. It is more expensive and requires significantly more labor to apply compared to synthetic weed and feed products. However, if used correctly as a part of comprehensive field management program, preemergence control of crabgrass and certain other weeds can approach the levels observed when synthetic herbicides are used. The number of different weed species controlled is similar to that of synthetic herbicides. Duration of control also can equal or exceed that of synthetic herbicides.
Organics Compared to Synthetics for Postemergence Control of Grasses and Sedges
As of this writing, one of the main challenges for organic field managers is selective postemergence control of crabgrass, annual bluegrass, other grassy weeds, and sedges. Simply put, while there may be organic products marketed for this purpose, they are either nonselective or have not been tested extensively by university researchers in order to determine their effectiveness.
Organics Compared to Synthetics for Postemergence Control of Broadleaf Weeds
For broadleaf weed control, including weeds such as dandelion, white clover, prostrate knotweed and ground ivy, the typical strategy is to control these weeds postemergence with an application of a combination herbicide in spring and/or fall. Less expensive options include products that contain mainly phenoxy herbicides, like 2,4-D, MCPP and the related compound dicamba. These products will typically give 90% control of dandelions and white clover for about 60-80 days. Control of ground ivy and knotweed is possible with these products, but can be less consistent. Other options include the pyridinoxy herbicides triclopyr or fluroxypyr. Many combination herbicides on the market combine phenoxy and pyridinoxy herbicides. Newer, more expensive combination herbicides combine two or three phenoxy or pyridinoxy herbicides with a protox inhibitor such as carfentrazone, sulfentrazone, or pyraflufen ethyl. These products offer the most complete and longest duration of control or weeds, including more difficult to control weeds, that is generally available.
Two organic products that are registered for selective control of broadleaf weeds postemergence are sodium chloride (A.D.I.O.S.) and chelated iron (Fiesta™). Fiesta™ can potentially give up to 100% control of dandelion within 24 hours of application (Figure 2). It also works rapidly on weeds such as white clover, the plantains and ground ivy (Figure 3). Control is typically in the 75-90% range. So, for speed of control, the best option is actually an herbicide more often used by organic lawn care managers. On the other hand, chelated iron is a contact herbicide. While this might be effective with one application if targeting young annual broadleaf weeds, for perennial broadleaf weeds control is only of the top growth and the weed typically begins to recover within about 3 weeks. We conducted research at the Ohio Turfgrass Foundation Research and Education Center showing that you can get up to 100 days of control by making three applications of chelated iron 3 weeks apart. However, as with corn gluten meal, chelated iron is more expensive. Three applications to the entire field or multiple fields may or may not be within your budget. You also have the option of spot-spraying. One thing to consider though is that chelated iron will also cause the turfgrass to become a darker green color for a period of time (just like when you apply chelated iron for this purpose). When using chelated iron as a spot spray, you may notice spots of darker turfgrass for a period of time, whereas a blanket application would provide a more uniform darkening of the turf.
A.D.I.O.S. is a mixture of different salts but primarily sodium chloride. It has herbicidal activity but its selectivity is a bit less than that of chelated iron. In our research trials at the OTF Center we found that A.D.I.O.S. could offer some short term control of broadleaf weeds but control is variable. The product is also a bit more injurious to turfgrass. Both of these problems were much less of an issue when we used A.D.I.O.S. in late fall (after mid-November). Thus our recommendation with this product is for broadleaf weed control in late fall. You will also want to test your soil to make sure that use of this product is not going to result in too much sodium, which can damage the desired turfgrass.
In summary, organic weed control in turfgrass has advanced considerably but there are still some management challenges. Speed of control can be just as good or better compared to synthetic herbicides. However, duration of control and completeness of control lags that of synthetic herbicides. Some of these products are very safe to turf, such as corn gluten meal. I was involved in trials where we applied 160 pounds of corn gluten meal per thousand square feet and we saw nothing but nice green grass. On the other hand, there are other organic products that you still have to be careful about non-target injury. Lastly, while prices have come down, organic options tend to be more expensive than their synthetic counterparts.
There is an accompanying Power Point Presentation to this article, which can be accessed here:
Author: Dr. Dave Gardner, Professor of Turfgrass Science.