In recent years, several organic herbicide products have appeared on the market. These include Weed Pharm (20 percent acetic acid), C‐Cide (5 percent citric acid), GreenMatch (55 percent d‐limonene), Matratec (50 percent clove oil), WeedZap (45 percent clove oil + 45 percent cinnamon oil), and GreenMatch EX (50 percent lemongrass oil), among others.

These products are all contact‐type herbicides and will damage any green vegetation they contact, though they are safe as directed sprays against woody stems and trunks. These herbicides kill weeds that have emerged, but have no residual activity on those emerging subsequently. Additionally, these herbicides can burn back the tops of perennial weeds, but perennial weeds recover quickly.

These products are effective in controlling weeds when the weeds are small and the environmental conditions are optimum. In a recent study, we found that weeds in the cotyledon or first true leaf stage were much easier to control than older weeds (Tables 1 and 2). Broadleaf weeds were also found to be easier to control than grasses, possibly due to the location of the growing point (at or below the soil surface for grasses), or the orientation of the leaves (horizontal for most broadleaf weeds) (Tables 1 and 2).

Organic herbicides only kill contacted tissue; thus, good coverage is essential. In test comparing various spray volumes and product concentrations, we found that high concentrations at low spray volumes (20 percent concentration in 35 gallons per acre) were less effective than lower concentrations at high spray volumes (10 percent concentration in 70 gallons per acre). Applying these materials through a green sprayer (only living plants are treated), can reduce the amount of material and the overall cost (http://www.ntechindustries.com/weedseekerhome.html).

Adding an organically acceptable adjuvanthas resulted in improved control. Among the organic adjuvants tested thus far, Natural wet, Nu Film P, Nu Film 17, and Silwet ECO spreader have performed the best. The Silwet ECO spreader is an organic silicone adjuvant which works very well on most broadleaf weeds, but tends to roll off of grass weeds. The Natural wet, Nu Film 17 and Nu Film P work well for both broadleaf and grass weeds. Although the recommended rates of these adjuvants is 0.25 percent v/v, we have found that increasing the adjuvant concentration up to 1 percent v/v often leads to improved weed control, possibly due to better coverage.

Work continues in this area, as manufacturers continue to develop more organic adjuvants. Because organic herbicides lack residual activity, repeat applications will be needed to control new flushes of weeds.