Options for weed control in turfgrass have never been better. Several new products are registered for grassy weed control and, more importantly to sports field managers, for control of weeds when attempting to seed or overseed turfgrass. In addition, the number of active ingredients registered for post emergence broadleaf weed control in turfgrass has doubled in the last 15 years.
However, pesticide use in turfgrass continues to be controversial and, in certain parts of the country, applicators are banned from using conventional pesticides on playing fields, lawns and playgrounds. Unfortunately, terms such as “organic” or “alternative” pesticides are open to interpretation, and some of the products mentioned here are considered organic by certain groups, but not to others. Where laws were enacted, the goal was to minimize pesticide exposure to children and restrict the use of pesticides to minimal-risk ingredients.
The products mentioned below can, in most cases, be used on these areas. Having said this, you should be familiar with the laws and regulations in the areas where you make applications.
Natural weed control products that are non-selective have been around for many years, primarily for use in home lawn care. These materials are made of various plant oils or herbicidal soaps. Commercial-grade vinegar (acetic acid) also has been used as a non-selective herbicide.
An obvious disadvantage of these products is that, since they’re non-selective, they also harm the desired turfgrass (Figure 1). In addition, while weed control with these products is very rapid, it also is, in most cases, temporary because these are contact herbicides. Contact herbicides provide no residual control, thus plants with underground structures, such as perennial broadleaf weeds and perennial grasses, as well as some well-established annuals, will grow back after application.
Depending on which source you read, this is classified either as a synthetic or a natural product. However, this product is sold by many vendors who market to organic lawn care providers. The active ingredient is pelargonic acid and related fatty acids. These fatty acids are a very fast-acting, contact, non-selective herbicide. The herbicide disrupts cell membranes and results in cell leakage followed by death of all contacted tissue. Burn down of the contacted plant tissue may be achieved in as little as 20 minutes.
Non-selective herbicides can be used with some success as directed spot sprays on weeds that don’t spread or where weed infestations are not severe. Caution to avoid contact with turfgrass should be exercised, and repeat applications will be required for complete control.
Since these products have no residual, a strategy for use might be to control weeds during the off-season and then overseed to re-establish turf in those areas.
With the exception of corn gluten meal, selective organic herbicides are only a recent introduction to the turf market. Being selective brings the obvious advantage of the ability to make broadcast applications. But because of economics, they often are applied as spot sprays or the application is limited to isolated weedy areas in the turf.
Corn gluten meal
In 1986, Dr. Nick Christians at Iowa State University attempted to use corn meal as a growth medium to inoculate a newly seeded putting green with pythium. No pythium was introduced, but, interestingly, the bentgrass didn’t germinate on the plots that received the fresh corn meal. Over the next 10 years, a series of studies was conducted that determined that corn gluten meal, a by-product of the wet milling process, contained the highest concentration of the substances that prevented the bentgrass germination in his pythium study.
The substances are dipeptides that mimic the action of some of the conventional pre-emergence herbicides on the market. Corn gluten meal is a natural product. It’s an ingredient in many animal feeds and is fit for human consumption (though not very appetizing). Corn gluten meal contains 10 percent nitrogen by weight. Recommended use rate is 20 pounds per 1000 square feet applied twice per year – in early April to control crabgrass and then again in late summer/early fall to provide partial control of germinating broadleaf weeds. These two applications will provide 4 pounds of nitrogen per 1000 square feet annually. The nitrogen is slowly available and, in test applications as high as 100 pounds per 1000 square feet, didn’t cause burn to the turf.
Corn gluten meal is primarily intended to control crabgrass. During the first year of use, reductions in crabgrass germination are around 40 percent. But, during subsequent years of use, crabgrass control with corn gluten meal can be as high as 80 to 90 percent, which is nearly as good as most conventional pre-emergence herbicides (Figure 2).
In some areas of the country crabgrass control is good, and, in other areas, corn gluten meal is less effective. The reason for this has never been determined. Some control of broadleaf weeds from seed, including dandelion and white clover, is also possible.
A website describing the product in more detail as well as a list of suppliers can be found at hort.iastate.edu/research/gluten/.
An issue with the use of corn gluten meal is that it also has preemergence activity on grasses that you may be using to overseed your fields. Because of this, the same precautions that you take when using conventional pre-emergence herbicides on overseeded fields should be exercised when using corn gluten meal. After application, don’t attempt to overseed your fields for about 12 weeks. If possible, use corn gluten meal in the springtime to control annual weeds and then use a different fertilizer source and overseed in the fall.
Whether or not Fiesta should be considered an organic product is debated. But, it is permissible to use it in many areas where conventional herbicide use has been banned, and it may become an important tool for turfgrass managers under these restrictions.
The active ingredient in Fiesta is a proprietary chelated iron that acts as a selective postemergence herbicide against a wide spectrum of broadleaf weeds. Weed control can be very rapid, and, often, nearly 100 percent burn down is achieved within 24 to 48 hours (Figure 3). Though Fiesta has the advantage of being a selective herbicide, it’s important to note, that Fiesta is a contact herbicide. In order to address this issue we conducted research looking at several different programs with different rates and timings of application.
One set of treatments examined whether sequential application in the spring would result in long-term control. But, since many of our broadleaf weeds are perennials and more vulnerable to pesticide applications in the fall, we also tested a program of two fall applications followed by two spring applications. At each of these times we applied 1.25, 2.5 or 5 gallons per 1000 square feet of a 2 percent, 4 percent or 8 percent solution (nine total rate/percentage combinations).
Our results from these studies suggest that three applications every 21 days in the spring provides better long-term control than two applications in the fall followed by two applications in the spring (Figure 4). A side benefit of this is the reduction in cost associated with fewer applications.
Burn down is quite rapid (around one to three days) so the total length of time of control when making three applications may actually be a little longer than with a traditional herbicide that might take up to 28 days to achieve similar control. In addition to the weeds we tested, Fiesta is labeled to control black medic, common chickweed, moss, algae, shepherd’s purse, thistles and veronica.
Something else to consider when using Fiesta is that the main injury symptom is to make affected plants appear black (actually ultra-dark green). The chelated iron is selective of broadleaf plants but will cause some darkening of the turfgrass. If the rate is too high or weather conditions are not favorable at application time (temperatures above 80 degrees) this can appear like a blackening or phytotoxic response to the turfgrass. This can be greatly reduced or avoided if used in cooler temperatures, such as in the 60s or 70s. The blackening will go away after a few mowings, but can be very noticeable until then. Our tests suggest that the severity of this response also varies by turfgrass species. For example, Kentucky bluegrass and perennial ryegrass were quite tolerant of Fiesta even when applied in warmer weather.
ADIOS is manufactured by Herbanatur and is a patented herbicide used in Canada but also gaining popularity in those parts of the United States where application of conventional herbicides are not allowed. This product shoudn’t be confused with Adios® from Arysta, which is a cotton defoliant or ADIOS, which is a canceled formulation of DSMA.
Among the active ingredients listed on the ADIOS label is sodium chloride, which can be effective as a selective herbicide against broadleaf weeds such as dandelion, white clover and ground ivy when applied at high rates. After application, considerable chlorosis and necrosis of weed tissue is usually observed within 24 to 48 hours. In our late-fall trials, ADIOS application resulted in up to 100 percent control of dandelion, white clover or ground ivy with minimal discoloration to the turfgrass. But, when applied in spring, around 60 percent control of dandelion and 80 to 90 percent control of ground ivy for 2 to 3 weeks (Figure 5) is a more typical result. Some injury to the turfgrass is more likely when applied in the springtime, probably due to warmer temperatures. Turfgrass injury symptoms typically last about 7 to 10 days. Of course, this could be minimized by using the product as a spot or directed spray.
Because of the way this product is used a large amount of sodium chloride is applied to the soil. Thus, it’s important that you monitor for any potential long-term deleterious effects on the soil. High concentrations of sodium chloride will cause soil particles to disperse, which will negatively affect the structure of the soil and cause the soil profile to drain more slowly. At too high of a concentration, turfgrass growth may be negatively impacted. Kentucky bluegrass is most susceptible to salt injury, followed by ryegrass and tall fescue.
Future organic herbicides
Several promising herbicides under development have been shelved due to high production costs. But, research in this area is active and many other materials are being developed as organic herbicides that may be released in the next few years.
PHOTOS: CAMERON WHITMAN/ISTOCK AND DAVID GARDNER