By Charles Kidder
A stroll through the aisles at a big box store will convince you that “better living through chemistry” could well be the mantra of the gardening world. A host of products promise to green up your lawn, produce tomatoes the size of basketballs, kill nasty bugs and rid your garden of weeds forever. Just how effective are these chemicals against unwelcome plants? And even more important, how safe are they?
Many of these products call themselves “weed killers,” even in their trade names. Right off the bat, this is somewhat deceptive. With some exceptions which we will look at later, these concoctions kill vegetation, not just weeds. Spray them on your nice ornamentals, and kiss them goodbye. Technically, this type of herbicide is known as non-selective. RoundUp, Monsanto’s name for the herbicide with the active ingredient glyphosate, may well be the best known of these products.
It would be easy to devote an entire column to RoundUp. It is widely used in agriculture, especially since Monsanto has developed special genetically modified “RoundUp-Ready”crops that can withstand its onslaught. Extensive use has led to the evolution of “super-weeds” that require even more toxic brews. As if that wasn’t bad enough, some studies have linked the use of RoundUp to increased miscarriages among farm women, cancer, etc.
There are selective herbicides, however, although none that truly know weed from wildflower. They work by attacking only one of two major plant groups and leaving the other (relatively) unaffected. The most common products are for use on lawns; they kill primarily broadleaf plants, such as dandelions or clover, but won’t harm turf grass. And some of these products actually do discriminate between weedy grasses and desirable turf.
Either type of product is known as a post-emergent herbicide; that is, they attack the plant after it has germinated from seed. Another broad category of herbicide is the pre-emergents; Preen is a well-known brand. These prevent seeds from germinating, so that they do not become mature plants. Pre-emergents prevent or discourage new weeds, but don’t kill existing ones.
Now that we know some of the basic facts about herbicides, the big question is: should you use them? You might be trusting and feel that “if it wasn’t safe, they couldn’t sell it.” Unfortunately, we occasionally find that not to be true. At the other extreme, you may feel that any use of “chemicals” is an absolute abomination. The simplest answer: use herbicides as little as possible. Or not at all, if your conscience and research so dictate.
If you are going to use them, read the label. Most containers have a label that peels back at one corner to reveal several pages of information that any chemistry major can understand. Pay attention to innocuous-sounding phrases such as, “Do not allow drift onto desirable plants.” Even a light breeze can waft a fine mist onto good plants and cause damage or death.
You might consider “natural” alternatives to the synthetic chemicals. Even though these may contain innocent-sounding ingredients, they are not without risk.
Vinegar is the active ingredient in several such products. (Burnout is one brand name.) Don’t try using this vinegar in your recipes! At 20 percent concentration, it’s four times stronger than what’s in your kitchen and can burn skin and eyes. It reportedly works best on low, slow-growing plants, but robust perennials may require repeat treatment. And supposedly it does not have a long-lasting effect on earthworms and other critters. Does that mean it “only” kills the things it actually touches?
In the pre-emergent category of herbicides, corn gluten is a popular alternative to the Trifluralin in Preen. The effect of corn gluten as an herbicide was discovered by accident at Iowa State University, where it was shown to inhibit the development of a plant’s root system. (Interestingly enough, tests at Oregon State University—not a big corn state—could not duplicate the results they got in Iowa.) Corn gluten is 10 percent nitrogen, and some folks suggest that its effectiveness really stems from its fertilizing properties. On your lawn, it may boost grass growth and overwhelm the weeds. Regardless, excess nitrogen in the environment is not a good thing, so avoid over-use of such products.
In a highly unscientific experiment, I tried a home-brew herbicide on some weeds in my driveway. I mixed some table salt, perhaps a couple of tablespoons, into two cups of water. After pouring it on the weeds, they were decidedly not happy the next day. They appeared dead, although I don’t know if they might have recovered, since I pulled them up a few days later. So, salt water does work. On the other hand, I would not want to dump a lot of it into the environment!
Let me return to a technique I mentioned briefly in the last column, heat. I had speculated on the long-term effectiveness of roasting your weeds with a propane flame-thrower. I happened across one review of such a torch on the Internet. It confirmed my suspicions that fires are possible, and that even weeds flambé may recover in time.
Also in the heat category is using solarization to kill weeds or turf when you want to establish a new garden bed. This consists of cutting down as much existing vegetation as possible, then putting down a layer of clear plastic film and letting everything cook in the summer sun for a few weeks. Plants, seeds, nasty pests, etc. are all killed, at least in theory. The down side: the good critters in the soil are also toast. But you could then reintroduce them with a layer of compost or humus.