Herbicides- What, How, When, and Why?

March 2, 2022 · 7 minute read
Herbicides- What, How, When, and Why?

If you have ever spoken to me in person about the health of your trees and shrubs, it is quite likely that I gave you my soapbox speech regarding the use of herbicides around trees and the danger some products can pose to your woody ornamentals. I personally use herbicides around my home in a judicious manner as I feel that if used properly, their benefits can outweigh their risks, but it becomes hard to know what to use, how much, when, and why. I approach all pesticides from a professional viewpoint as I have been a Registered Commercial Pesticide Applicator in Virginia for nearly fourteen years which allows for a deeper understanding of their usage. Most homeowners do not have this background and rely on product claims, Google searches, and advice from neighbors. While all of this can be better than nothing, it can also spread incorrect information that could lead to long term problems for you and your landscape.

The use of pesticides in general is a very polarizing subject so please understand that this is neither a promotion or denouncement of their use and should be taken only as advice if you choose to use them. It is also important to note that only the general category of herbicides will be discussed as insecticides and fungicides can be extremely selective in their efficacy and can also cause significant harm if used incorrectly. While some of this is also true with herbicides, they will be the focus as the chance for damage to your plants is much greater with misapplication than that of insecticides and fungicides. Finally, brand names will not be mentioned as most herbicides can be purchased under many brand names so chemical names will be the focus to ensure clarity.

First, it is important to know what an herbicide actually is. Herbicide specifically means it is a chemical that kills plants as “herb” is the prefix to denote herbaceous material and “cide” is the prefix that means to kill. This is why we have multiple pesticides (to kill pests) that can range from virucides, to bactericides, up to things like piscicides that are used for killing fish. It is important to remember that whatever pesticide is being used, it is designed to kill something and improper use can harm or even kill unintended targets.  

No matter what herbicide you choose to use, remember to ALWAYS follow the instructions on the label attached to the container. The label with all of its warning and directions are actually legally binding and it is against the law to use a product in a way that is not listed. As an American male, I sometimes fall victim to the mindset that if a little is good, then a lot must be better and that seems to be the natural trend when it comes to most pesticides. If a product says to mix one ounce of chemical with one gallon of water, then it should be done exactly as directed. While it might seem that two ounces would work twice as well, it really won’t and it can actually do harm to off target plants. Even if you don’t cause unintended harm, it is simply wasting money as the products are designed to be effective at the labeled rate and stronger mixing just wastes money. 

Always be sure to follow the instructions on what time of Personal Protective Equipment (PPE) to wear, which is almost always a minimum of closed toe shoes, long pants, long sleeves, chemical resistant gloves, and eye protection. Just because you can purchase these chemicals over the counter at nearly any store, does not mean that they are not dangerous if used incorrectly.  Even though herbicides are designed to kill plants, they can still be dangerous if applied to the skin or accidentally ingested. They are still manufactured chemicals designed to kill so it is important to limit contact wherever possible. By following the label and wearing appropriate PPE, it is possible to occasionally use most herbicides without running the risk of health impacts, but as with everything in life, moderation is key.

Next, after choosing the appropriate chemical, mixing properly, and protecting yourself, you must decide the appropriate time to apply herbicides. Any pesticides that are applied with a sprayer should NEVER be sprayed when it is windy or breezy. This is because sprayers usually apply a fine mist that can easily blow in the wind and kill off-target plants.  There is nothing like applying an herbicide to kill weeds in your sidewalk and finding out a week or two later that all of your annuals are dying because of off-target drift. It is also important to apply when temperatures are cooler and air is moving (but not windy). This helps the products dry on the leaf surface and increase uptake while limiting the chances of off-target damage through volatilization.

Volatilization is the process of a liquid applied chemical becoming a gas that can float up and damage plants in the surrounding area. This commonly occurs on warm humid days when the air is still, which allows the chemical to change phases before the water evaporates and the chemical is taken into the plant. It is quite common in spring to see large deciduous trees that have distorted and damaged leaves on the lower canopy from lawn herbicides that have volatilized and stunted newly emerging leaves. This damage is usually permanently damaging to the leaves being affected but does not pose a long-term concern for the overall health of the plant. In some cases though, heavily damaged plants, especially small trees and shrubs, can be weakened or even killed.  

With proper application and safety precautions taken, it is possible to apply most herbicides without much risk but of course there is always an exception to a rule. That exception is where I find myself time and again on my soapbox denouncing a specific type of herbicide. I personally and professionally would never use or recommend use of any herbicides that are labeled as controlling weeds for an extended period of time.  Most of the major companies have put out products in the past decade that offer multiple months of weed control, up to even one year. These products are usually a mix of multiple chemicals that kill existing vegetation and then sit in the soil, waiting to be taken up by roots of newly growing plants to kill them far after the initial application. Most of these products have glyphosate which will only kill if applied directly to green leaf tissue or paint on cut open wood.  The second chemical is usually imazapyr or imazapic which is soil-active and has a long half life, meaning that it will stick around and keep on damaging long after application.  

Most ‘extended control’ herbicides are labeled for places like patios, driveways, and other hardscapes and that is where they are most useful. It feels good to only have to apply herbicide once a year to your paver patio and know that you won’t have weeds in the cracks, plus you are using fewer chemicals on your property. What most don’t consider though are the roots of nearby trees and shrubs that have grown around and under these areas. These plants can easily uptake the soil-active herbicide from long distances and kill the plants even if they are a dozen yards away.  Sadly, when this damage occurs, there is likely nothing that can be done to fix the damage as the herbicides are now locked inside of the plants and will continue to damage until the product breaks down.  

In one extreme example, a property owner consulted to determine why all of their trees around their property looked sick. When driving onto the property, it was almost immediately evident what had occurred. All of the trees adjacent to the pea gravel driveway and paver driveway apron had either died outright or had extremely small/contorted foliage, a hallmark of herbicide damage. After consulting with the property owner, it was discovered that a readily accessible ‘extended control’ herbicide had been sprayed on the driveway just as the label indicated. While everything was done properly, the trees still died because the trees’ roots had spread under the driveway and absorbed the soil-active portion of the herbicide. At the point of inspection, it was determined that nothing could be done to remediate the damage and that the trees would likely die. If a product can continue killing for a full growing season, then it is obviously a very stable and resilient chemical that is likely to come with significant risks.  

As with everything in life, moderation is key, and that goes for the application of any herbicide. They can be an effective tool to limit weeding and limit horticultural and/or agricultural losses, but if it sounds too good to be true, then it probably is. Herbicides should also be a small part of an IPM program that emphasizes physical control (weeding)  and selection of plants that are strong enough to out compete other unwanted plants. In a busy society, it is far too common to rely solely on chemical solutions to issues be it in your lawn or even your own body health. Judicious and thoughtful applications of herbicides can be very safe and effective but should never be the only means of controlling weeds in a landscape. Be safe and always remember to follow the instructions on the label!