Herbicide Modes of Action – Cell Membrane Disruptors and Contact Herbicides

One class of herbicides that is frequently used in vegetable production systems is cell membrane disruptors. Cell membrane disruptors' mode of action is described as a Photosystem-1-electron diversion. Electrons from Photosystem 1 are used to form destructive free radicals, which results in the destruction of the cell membranes. Photosynthesis is required for Photosystem 1 activity, so light is required if the herbicidal activity is to be observed. Paraquat, marketed as Gramoxone, is one of the most frequently used pre-plant burndown herbicides in vegetable production systems. As a restricted-use herbicide, this product requires that a grower must have a private pesticide license and must attend an industry-sanctioned training session before the purchase and use of this herbicide.

Paraquat is applied to the foliage of weeds and is a non-selective herbicide with no soil activity due to its affinity for adsorption onto soil particles. Paraquat or Gramoxone is used primarily as a burndown herbicide, but it also may be used as a harvest aid on labeled crops in some production systems.

Weeds treated with paraquat (Gramoxone) develop water-soaked lesions within hours after application. Wilting and desiccation of the contacted plant tissues will quickly follow. Plant growth initiated after this herbicide application will not be impacted and should develop normally.

Some growers prefer not to use paraquat or Gramoxone on their farms because of its low LD50 (110 to 150 mg/kg in rats) or because it is a restricted-use pesticide that requires a pesticide license. If farmers/growers wish to use a contact or burndown herbicide in their vegetable fields that do not require a pesticide license, they should consider Capric/Caprylic acid-based herbicides like Suppress, Homeplate, or Fireworxx. All three of these products are non-selective, contact, or burndown herbicides, and all have an OMRI label for use in organic production systems.

Capric/Caprylic acid herbicide products disrupt plant cell membranes, remove protective waxes from leaf surfaces, and cause dehydration or desiccation of the plant tissues. Capric/Caprylic acid-based herbicides work rapidly, and weeds treated with these herbicides will typically display injury symptoms in a matter of hours.

I have extensively used Capric/Caprylic acid-based herbicides on my property to control annual weeds in vegetable gardens and landscapes. When applied under hot sunny, dry conditions, the targeted weeds may exhibit bleaching or browning in a matter of hours after application or may die slowly over 7-10 days (T.J. Mason and M.E. Uchanski, 10/20). Perennial weeds will re-grow, and plant foliage that has not been contacted by the herbicide spray may continue to photosynthesize and thrive, so thorough spray coverage is essential if annual weeds are to be controlled effectively. Capric/Caprylic acid-based herbicides are a relatively new tool for organic growers. They are very effective but will cost the grower up to $450 an acre (Fennimore 2019). For additional information on the use of herbicides in vegetable crops, please consult the 2022-2023 Mid-Atlantic Commercial Vegetable Production Recommendations.