Joseph M Hannan
Production Instructor
Center for Arkansas Farms and Food
09.06.2024
Edited by Luke Freeman
Introduction
In my last post, I wrote about organic insecticides, how to use them, and which insects they control. There is much more that needs to be added to that article, but we will save that for a later issue. For now it’s worth moving on to discuss organic herbicides, especially in a season where we have experienced periods of significant rainfall and weed pressure.
Organic herbicides cannot be – nor should be – your sole weed management strategy on an organic farm. Organic herbicides must be used in conjunction with other cultural practices such as ground covers, cultivation, or burning. But in a year like this with multiple periods of significant rainfall, timing cultivation passes can be challenging, making organic herbicides a very useful tool to have in the toolbox. There were multiple weeks this spring and summer at the CAFF Farm where we simply could not get a tractor through our fields, and it appears that many other Arkansas farmers had a similar problem this year.
At first glance, there are several organic herbicides that can be used under fence rows, around buildings, under tree fruit, and in annual production fields. However, they all have the same basic mode of action, even though they have different active ingredients. All organic herbicides are simple burn-down products that are only effective at killing green, vegetative plant material they come in contact with. They do not systemically move throughout the plant to kill the roots like glyphosate (aka RoundUp). Because organic herbicides do not translocate to the root system, they work best by targeting small weeds, generally no larger than 6 inches tall. The smaller the weed, the higher likelihood that the plant will be killed. Organic herbicides have the potential to do a great job of cleaning up weeds, but if the weeds are taller and more mature, the root system will likely survive application and will push up new leaves requiring subsequent sprays.
As mentioned above, organic herbicides work by effectively burning any green vegetation the product touches and should be applied during hot, sunny, calm conditions. The hotter and sunnier, the better! When applied at the correct rate and weather conditions, green vegetation reaches complete burndown within hours to a few days. Under optimum conditions, green vegetation sprayed with an organic herbicide at 2:00 pm on a hot, sunny afternoon will be completely dead and brown by 4:00 pm. Repeat applications every 4 weeks or so, depending on weed pressure, is needed to maintain clean, weed free conditions.
However, there are some MAJOR caveats to using organic herbicides. First, for use in annual crops, organic herbicides can ONLY be used in shielded applications. If any of the herbicide touches the crop leaves, they will be burnt the same as the weeds. This means organic herbicides will only be good for use between the rows or along plastic mulch edges, but will not be suitable for in-row management. Cultivation equipment is still best for in-row weed control in organic production. Second, organic herbicides are prohibitively expensive for use in commodity row crop production. There just isn’t enough value in corn and soybeans to justify the cost of organic herbicides. Third, the labeling and OMRI approval varies from year to year considerably. For instance, Scythe used to be OMRI listed but has not been listed for quite a few years. Another product, Final San-O, which is no longer available on the market, was OMRI listed and labeled for use around buildings and infrastructure but was not labeled for a shielded application in the field. A new version, FinalSan is OMRI listed and labeled for use in the field, however.
Each of these herbicides also have a couple of unique, often overlooked uses. They can be used for sucker control under fruit trees, grapes, and other woody perennial crops. The organic herbicides will kill any green vegetation from the suckers, but will not harm the woody trunk tissue. In addition, these products can safely be used as plant desiccants. For instance, you could use an organic herbicide to burn down potato foliage prior to mechanical harvesting without harming the potato crop.
Homeplate
Homeplate (active ingredient caprylic acid) is manufactured by Certis Biologicals. It has been on the market for 15+ years and is one of the more unique options on the market even though the method of control is the same as other products. It is an amazingly effective product that does what it says it does when used according to the label. However, it is quite expensive at $78 per gallon. Homeplate, like FinalSan, Axxe, Avenger AG, Scythe, and others, is best applied at the upper end of the application rate with lots of water to saturate the leaves nearly to dripping. At the CAFF Farm, in order to kill large, mature weeds, we needed to apply Homeplate at 8 gallons per 1,000 sq ft and 10 oz of product per gallon (80 oz of product per 1,000 sq ft). However, our weeds were quite mature and above the recommended growth stage on the label. With smaller weeds, the rate and water volume likely can be reduced but we have not yet had an opportunity to dial in that rate
FinalSan
FinalSan (active ingredient ammoniated soap of fatty acid) is manufactured by Belchim Crop Production corporation and uses the most common active ingredient found in organic herbicides. Similar products with different names include Axxe and Scythe (not OMRI approved). Application rate, per the label, is similar to Homeplate: 4 to 12 oz per gallon and 1 to 8 gallons per 1,000 sq ft solution for small weed control, with a higher rate for larger weeds. Under optimum application conditions, efficacy is equivalent to Homeplate. However, the cost of FinalSan varies tremendously and ranges from $40 per gallon to $80 per gallon.
Avenger AG
Avenger AG (made from d-limone) has similar characteristics as FinalSan and Homeplate. It carries a similar price as FinalSan, but it can often be found at local farm stores, such as Tractor Supply, making it much easier to source locally.
Vinegar
Vinegar, or acetic acid, is another burndown herbicide option. It works exactly the same as Homeplate and FinalSan. Off-the-shelf kitchen vinegar does not have a high enough concentration to be effective. You must use a high concentration form available in 2.5 gallon containers and 50 gallon drums. There are many acetic acid formulations available, be sure to check the OMRI listing for products and talk to your certifier before applying.
Final Comments
Organic herbicides can have a role in weed management plans on specialty crop farms, especially during wet years or along plastic mulch edges. The specific product of choice from an efficacy standpoint is irrelevant, at least from the author’s perspective. They are all equally good. Adequate coverage with plenty of water and size of weeds controlled are more relevant to success of weeds control. Starting with good clean fields through use of cultivation and weed suppression will greatly improve efficacy. Finally, when using any of these herbicides, be sure to thoroughly clean out your tanks and pumps. Acids, especially vinegar, eat rubber seals and can destroy a pump and fittings if not properly cleaned after use.