Herbicides: What to Know Before You Use Them

It sounds like the setup for a joke, but it isn’t.

More than 2,200 miles apart, two women walk into their local garden-supply stores. They find themselves similarly unsettled to find gallon bottles of “30 percent vinegar” displayed on the shelf among the herbicides.

Acetic acid, the active component in vinegar, can, in fact, help to subdue some weeds. Products from certain manufacturers — much stronger than the typical household vinegar concentration of about 5 percent acetic acid — are labeled for herbicide use.

The punchline, though: The ones these women saw, displayed prominently among the weed killers, weren’t labeled herbicides. Nor did the packaging include any instructions for using them safely and effectively in that capacity.

It was that omission, in particular, that made both women nervous. But only one of them knew the reason behind it.

That woman was Jane Mangold, a weed scientist and professor at Montana State University, in parched Bozeman, Mont. Across the country, in the sodden Hudson Valley of New York, I encountered the same thing, which led to a series of conversations with her.

When I called Dr. Mangold, I wanted to know: Shouldn’t something this dangerous — yes, the word “danger” was in capital letters on the 30 percent acetic acid label, and on the 20 percent solution — come with detailed consumer guidance?

These products are corrosives that can burn skin and permanently damage eyes. Why were the labels so vague? And where do high-concentration vinegars fit into managing a garden?

The first part of the answer was easy, although it seemed to highlight a loophole that could lead to confusion and mishaps. Products like the ones we saw may suggest that they are herbicides — perhaps with a picture of a dandelion on the bottle — but the labels state clearly that they are for cleaning, with no mention of weed killing. That’s because they are not registered with the E.P.A. as pesticides, Dr. Mangold said, so they cannot make herbicidal claims on the label or give instructions about such use.

Do a web search for horticultural vinegars, and the range of concentrations (up to 45 percent) and language on the labels will be positively dizzying. Some are called herbicides; others are not. Some are certified organic; most are not.

To use, or not to use? What emerged from our conversations was not an exhortation on behalf of either position so much as a cautionary tale.

Our strong suggestion is that gardeners who are considering using any pesticide — including herbicides, and even those as benign-sounding as vinegar — first study up, to protect themselves and the environment, and get the best results.

That begins with reading the label, a step too often skipped. But sometimes, as I learned recently, that doesn’t tell you everything, and more homework is required.

Is It a Noxious Weed or Just a Nuisance Weed?

Dr. Mangold’s work focuses primarily on larger landscapes and helping ranchers and public landholders like the U.S. Forest Service devise strategies for tackling invasive species.

“For those dealing with a very abundant invasive plant scattered over a large area, hand-pulling is impractical,” she said. “You have to weigh the risks of using an herbicide versus doing nothing — impacts such as lower biodiversity and other ecosystem effects.”

Herbicides, she said, may be the most effective method in those situations. But some unwanted plants, like an occasional dandelion in a lawn, are just unsightly, and don’t warrant spraying.

We need to differentiate between what Dr. Mangold calls “noxious weeds” and mere nuisance weeds — the ones it would be safest, and most economical, to pull, dig up or hoe. Or, when seeking a fresh slate in a whole garden bed, maybe solarize them under plastic sheeting, letting the sun do the work.

With Noelle Orloff, the weed and invasive-plant identification diagnostician at the university’s Schutter Diagnostic Lab, Dr. Mangold also trains Montanans on proper herbicide use. The two field a lot of backyard-weed questions from homeowners. Their takeaway: Many people use pesticides without much how-to knowledge.

“‘I’m trying to get rid of — fill in the blank,’ they tell us,” Ms. Orloff said. “And then they list all the things they have done so far to the weed, like spraying bleach on it.” (Bleach, should you be wondering, is not a substance that either expert would have recommended, had she been asked.)

Whether dealing with large-scale landowners or backyard gardeners, they try to convey the principles of integrated pest management, or IPM — “using a variety of tools and methods to control an undesirable organism,” Dr. Mangold said. “Which, in our case, means plants that are weeds.”

The IPM decision-making process that precedes any action aims to determine the least toxic solution possible to achieve tolerable levels of pest pressure, whether from weeds, insects or animals.

And if an herbicide proves to be part of a weed solution, “it’s all about maximizing the benefit while minimizing the risk,” Ms. Orloff said. “Because using any herbicide, organic or synthetic, has some level of risk.”

She and Dr. Mangold suggested that anyone tackling weeds get into the IPM mind-set by asking themselves several questions.

Do you know what the undesirable plant is? Without proper identification, it’s impossible to know the plant’s life cycle, including whether it is annual or perennial, which will inform any control strategy.

Do you have a weed that is susceptible to the treatment under consideration, and is it at the right life stage for effective treatment? Horticultural vinegar, for example, is recommended for use on young annuals that have four or fewer true leaves, not on established ones or on perennials that may suffer foliage damage but are likely to resprout from their roots.

With poison ivy or a deep-rooted perennial invasive like field bindweed (Convolvulus arvensis), Ms. Orloff said, “you might have to spray horticultural vinegar every two weeks for five years — not a feasible plan.”

Next: Is the target weed growing in a spot where the treatment you are considering is feasible? A driveway or cracks in the sidewalk might allow for spraying, but weeds within a flower bed or in a lawn can’t be so easily singled out.

Are the environmental conditions right — not too hot (or cold), too windy or when rain is forecast? Some herbicides are recommended for application when plants are growing actively and might not be as effective in times of drought, like the current conditions in much of the West, including Montana.

I’d add another question: Will you be happy looking at the toasty weed remains after treatment? If not, you could find yourself still having to pull them — in which case, the chemical was used in vain.

And don’t forget that an “integrated” mandate must include an aftercare plan.

“Integrating cultural methods is vital when you use herbicides,” Dr. Mangold said. “If you don’t have competitive plants filling in or mulch spread in an area after your weed-control efforts, the weeds will simply come back and colonize the bare ground.”

The Language of Herbicides: Signal Words and More

Not all herbicides work the same way, or on the same types of plants.

Contact or burndown herbicides like horticultural vinegar desiccate just the plant tissue they come into contact with. Systemic herbicides — the one most widely used by gardeners is probably glyphosate — are taken up by the plant and translocated through its parts. Contact herbicides might kill the top of the plant, which works for small annual weeds, but they won’t move into the roots to destroy the whole plant.

Herbicides can be selective (like lawn chemicals that target broad-leaved weeds but spare the grass) or nonselective (like glyphosate and acetic acid).

How long a pesticide is residual in the soil also varies. Characteristics of specific products and their active ingredients are detailed on Oregon State University’s National Pesticide Information Center website, a collaboration with the E.P.A.

Look for critical clues about personal safety, starting with the “signal word” on the front of the label: caution, warning or danger (the most toxic). Signal words, required by the E.P.A. on registered pesticides, describe the product’s acute toxicity, its short-term hazard if absorbed through the skin, ingested, inhaled or contacting the eyes.

Next, read the precautionary label statements, and also what to wear when using the product. To prevent injury with the 20 and 30 percent horticultural vinegars, which have a danger rating, the best practice in personal protective equipment calls for fully covering skin with long sleeves and pants, as well as socks and shoes. Wear goggles or a face shield to protect eyes; a face mask, like an N95; and waterproof gloves. After spraying, rinse the outside of gloves before removing them, then carefully remove other gear and launder it.

This, especially, is the part that makes the Montana weed scientists and me uneasy, imagining gardeners in shorts and T-shirts, with maybe sunglasses on at best, having at the pigweed seedlings between the pavers.

“There are lots of different ways to manage weeds. Whatever you do, do it fully informed,” Dr. Mangold said. “If you don’t know, first ask for help.”

The nationwide cooperative extension system is there to answer questions, she reminds us, if only we ask — rather than go all “ready, shoot, aim” before we do.

Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.

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