Of all the inventions or discoveries of the 20th century, and there were many, among the most important were synthetic pesticides. This category includes herbicides, insecticides, rodenticides, fungicides and others.
Herbicides are often labeled “weed killers.” Americans apply more herbicides than any other pesticide. It surprised me to discover that we spend more in our efforts to control weeds than we do for problem insects. We even spend more money on weeds than on trying to control plant diseases, which are most often caused by fungi. If you are looking for a product to prevent a plant disease, you probably will purchase a fungicide.
Because they are so widely used, it is easy to take herbicides for granted. Anybody can go into a store, pick up a container, bring it home and spray. When a product says “weed killer” on the label, there is a reasonable assumption that it kills only weeds. That is where people get in trouble. More importantly, here is where plants get in trouble.
Many kinds of herbicides are on the market, and a good number of them claim to kill weeds in turf grass. Not everyone in the Southwest has taken out all their lawns, so a product that gets rid of weeds can save hours of backbreaking labor. Pulling out or digging out dandelions, black medic and sow thistle is hard work and a waste of a perfectly good weekend!
Likewise, how many of us must deal with Bermuda grass? Would a better question be — how many people are not? You can buy herbicides that state right on the label that they will kill Bermuda grass.
The story of herbicides is one where everything is in the details. That is why we say repeatedly: Read the label!
No matter how smart we like to think we are, we can make mistakes, and not reading the label is a big one. Just because something says it will kill weeds does not mean that it will get rid of only weeds. Some of these products are designed to kill certain kinds of plants, such as the pests that appear in turf, or the grasses we really do not want. Some herbicides are formulated to kill nearly all the plant material they touch.
At times, they do not actually kill the plants but only damage them, which may not do anybody any good.
A product that gets rid of sow thistle in turf will harm other plants that have similar physiology, including plants that were carefully and deliberately tended in a landscape. Almost anything that has a daisylike flower will be susceptible, as well as lettuce, endive, chicory — so if you are applying these herbicides anywhere near the vegetable garden, be careful.
Bermuda grass controllers are effective against Bermuda, as well as fountain grass, deer grass, fescue, corn — most grasses. All these are cousins, and what kills or damages one will probably kill or damage others. You want to pay scrupulous attention to where you are spraying or spreading the herbicide.
With these compounds, you need to watch where you are applying. Just as important, here in the great American Southwest, is when we apply them.
Every so often, a person calls or comes into the Cooperative Extension office with a plant that suddenly started to turn brown and wilt — almost overnight. There is no sign of insect, no mildew or other disease, and the plant looked perfectly fine just before. This phenomenon, more widespread than we wish it were, starts in the spring and continues through the summer. It is not voodoo; it’s the magic of chemistry mixed with meteorology.
The label will say not to spray when the wind is blowing more than a certain speed, such as 10 miles an hour, or when temperatures are above a certain level, usually about 90 degrees. Of course, people who do not read the label do not notice that, and they may apply under windy conditions or in the heat of the day.
What happens then is the herbicide does not stay just where it was directed. That is particularly important with anything we apply with a sprayer, whether it is from the end of a hose or a spray bottle. A wind might seem to be drying the product on the leaves, but that is not the only thing happening. The same when it is hot.
The liquid dries, but part of it will often dry into an invisible mist that drifts onto neighboring plants. Unintended damage occurs on nontarget plants. By the time the sudden brown wilt appears, it is too late to do much but hope that the plant has not suffered so much that it cannot recover.
So, take care to direct the spray just where you want it, in the cool of the evening. Guard against applying herbicides to all those plants you spent so much energy growing.
Angela O’Callaghan is the Social Horticulture Specialist for University of Nevada Cooperative Extension. Contact firstname.lastname@example.org or 702-257-5581.