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Using Adjuvants (additives) to Maximize Herbicide Effectiveness

Herbicides are expensive. Invasive weeds are everywhere, and your available time to go after them is in short supply. In addition, we can’t always choose the ideal time to apply herbicides. Our schedule, prolonged periods of dry weather, hot weather, low humidity, cold weather, windy weather, large plants, impending rain showers, etc. often require us to apply herbicides in less-than-ideal conditions.

The best bet for increasing the effectiveness of your herbicide application over a broader range of environmental conditions is to use adjuvants. Adjuvants generally consist of surfactants, oils, and fertilizers. Adjuvants can improve herbicide performance by influencing a number of factors that are involved in herbicide absorption. Some of these factors include conditioning the water, dissolving the waxy coating on a leaf surface for better penetration, spreading on the leaf surface for better area coverage, sticking to the leaf surface to avoid spray droplet bounce off and minimizing small droplet formation for spray drift reduction.

The world of adjuvants is confusing. There are a wide variety to choose from, and they are not regulated by the EPA. The most effective adjuvant will vary with each herbicide, and the need for an adjuvant will vary with environment, weeds, and herbicide used.

How to know what to use? No worries! Like any good restoration practitioner, you just read the herbicide label! The label will tell you what type or types of adjuvants to use, how much, and how to mix it in the spray solution. Keep in mind that the label may recommend different concentrations of adjuvants with different plant species, as well as different growth stages of the plant. Using adjuvant beyond labeled rates may cause damage to non-target plants.

One of the more common adjuvants recommended by many of the herbicides we use is a non-ionic surfactant (NIS). The main function of an NIS is to increase spray retention, and to a lesser degree, may influence herbicide absorption. Silicone NIS surfactants reduce spray droplet surface tension, which allows the liquid to run into the leaf’s stomata (tiny openings in the epidermis). This “stomatal flooding” enhances herbicide absorption. Methylated vegetable or seed oils (MSO) adjuvants are more aggressive in dissolving wax and leaf cuticle (outer layer of tissue) than NIS, resulting in faster and greater herbicide absorption. MSO adjuvants are usually required for grass-selective herbicides. Some of the most advanced adjuvants combine both a silicone NIS with an MSO for fast and maximum herbicide uptake.

Can I use household soaps and detergents? The short answer is - only if it’s on the herbicide label. The practical answer is that hard water and soap will form scum that plugs equipment. Soft water and soap will form lots of suds. And most household soaps have low concentration levels of surfactants while most agricultural surfactants are in the 80 – 90% concentration range.

In summary, read the herbicide label for the recommended type and concentration of adjuvant for the targeted plant species and growth stage. Buy the adjuvant that meets the label requirements, and follow the label mixing instructions. When choosing among the many adjuvant brands that meet the herbicide label requirements, choose the adjuvant that best matches your application needs, such as drift reduction, rapid uptake, leaf adherence, etc. If there are any questions, consult the herbicide manufacturer.


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