Glyphosate (i.e. RoundUp) has been getting a lot of bad press lately, mainly as it relates to GMOs. This post is not about GMOs, but rather prudent use of herbicides. Herbicides are great tools, but must be used with caution. First of all, the label is the law, so any instruction supplied on the side of the herbicide container is what one must follow for application (an online version of the label IS NOT considered to equivalent to the actual one on the container). Second, make sure the tanks you use are either dedicated for the type of pesticide being applied (one for herbicides, one for insecticides and/or fungicides, etc.). Third, clean out the tank, especially if you are changing products. Residue can lead to unintended consequences. Fourth, understand the mode of action and rotate chemistries to reduce the chance for weed resistance. Fifth, know which weeds you want to control and use the best product for those weeds. Sixth, timing of application is VERY important for control — knowing the weed life cycle and timing the herbicide application with the most vulnerable period will yield the best results. There are some rules to follow when using any herbicide, but since glyphosate is so ubiquitous some closer scrutiny is needed.
Now, on to glyphosate specifically. It is a systemic herbicide, meaning the product is taken up by the plant and translocated within it. Glyphosate inhibits plant enzyme production, thus disruption its ability to synthesize certain amino acids. So, it is very good at killing a broad spectrum of weed species. Unfortunately, if not applied properly, it can be very good at killing fruit and nut plants too. Since RoundUp went off patent, there are many glyphosate products on the market now. Some have very different percent active ingredient. Knowing the percent active ingredient will tell one how much water to mix it with prior to application. Still, sometimes errors are made and a high price is paid.
A recent visit I had to a blueberry field revealed significant damage from glyphosate application. The grower had good intentions and had used glyphosate without problem for years, he had run out of one container and switched to another new one. Several rows had no problems (application with the first container), but the next rows had significant death. Why? The amount of active ingredient was different, but the applicator mixed the same amount for application. The plants may never recover and probably need to be removed. The photo below tells the story. If in any doubt about applying herbicides properly, contact a local county Extension office for help.