One of the routine maintenance tasks for athletic field management is the control of weeds — and it isn’t just for aesthetic purposes. Sometimes the weeds can result in reduced lateral shear strength and increased chance for athlete injury. Herbicides, when used per the label, have been shown to present minimal risk to end users and are typically employed by field managers to selectively remove different weeds. But 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 isn’t permitted and alternative management strategies need to be employed.
In some locations, laws have been written to exclude the application of any product intended for use as a pesticide. In these areas, weed control options are to do nothing, pull by hand, 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’s 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). There are also chlorine atoms attached to the number 2 and number 4 carbon atoms on the ring.
Obviously, the typical home gardener, activist, government official or regulator doesn’t adhere to the technical definition of what an organic molecule is. To 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 nonsynthetic (organic) pesticides include nontoxic, low-impact and minimum risk.
The Environmental Protection Agency 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 isn’t 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 don’t meet the criteria necessary for EPA exemption. Chelated iron is an example of an herbicide in this category.
Unfortunately, there’s a lot of confusion regarding some other products that may be 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 if you’re considering the use of organic herbicides because a law was passed in your area that says you must, check specifically as to what products you’re 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 synthetic options.
Nonselective weed control
Traditionally, it’s recommended that glyphosate be used for nonselective weed control because it has systemic activity. There are a couple of other synthetic herbicides that are nonselective, but 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. But it’s a contact herbicide, meaning that repeat applications are required to get control of perennial weeds.
Preemergence weed control
For general preemergence control of grassy and broadleaf weeds, several registered herbicides are used, such as oxadiazon, pendimethalin, prodiamine and dithiopyr. 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 percent control of crabgrass for about 90 to 150 days, depending on use rate and weather conditions following application.
Control exceeding 95 percent is possible with the right product and if other practices are optimized. These active ingredients are also labelled for preemergence control of many annual broadleaf weeds.
If using organic products, one option available is corn gluten meal. It’s a by-product of the wet milling process of corn and is used, among other things, as an ingredient in animal feeds. Technically, it’s fit for human consumption – it’s just not very appetizing.
Corn gluten meal was discovered in the late 1980s by Iowa State University turfgrass professor Nick Christians. Work conducted in his lab determined that corn gluten meal contains many 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, such as crabgrass but also dandelions.
In field studies, it was determined 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’s 10 percent nitrogen by weight and contains zero phosphorus or potassium. The recommended application rate of corn gluten meal is 20 pounds per 1,000 square feet.
This application thus provides 2 pounds of slowly available nitrogen and applying in both fall and spring would provide 4 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. If overseeding your field, it’s 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 variable, with some reporting great results (up to 90 percent control or better) and others reporting almost zero activity against weeds. There’s 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 percent control) and then improve significantly in the second and subsequent years.
If you’re 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’s more expensive and requires significantly more labor to apply compared to synthetic weed and feed products. But 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 like that of synthetic herbicides.
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 percent control of dandelions and white clover for about 60 to 80 days. Control of ground ivy and knotweed is possible with these products, but can be less consistent. Other options include the pyridinoxy herbicides such as triclopyr or fluroxypyr.
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 percent control of dandelion within 24 hours of application. It also works rapidly on weeds such as white clover, the plantains and ground ivy. Control is typically 75 to 90 percent.
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 three 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 three weeks apart. But 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.
Another option is spot-spraying. One thing to consider here is that chelated iron will also cause the turfgrass to become a darker green color for a period (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, 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.
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 even better — compared to synthetic herbicides. But duration of control and completeness of control lags that of synthetic options. Some of these products are very safe to turf, such as corn gluten meal. On the other hand, there are other organic products that you still must be careful about non-target injury. Lastly, while prices have come down, organic options tend to be more expensive than their synthetic counterparts.