By: Jackie Carroll
There are times when the only way to get rid of a stubborn weed is to treat it with an herbicide. Don’t be afraid to use herbicides if you need them, but try other control methods first. Pulling, hoeing, tilling, and digging will often take care of weed problems without the need for chemical sprays. Let’s learn more about using herbicide in gardens.
Herbicides are chemicals that kill plants or prevent them from growing. Their method of killing plants is as varied as the plants they kill. The first step in understanding herbicides is to read the label. Labels tell you how to use herbicides safely and effectively. It is illegal to use herbicides for any purpose or by any method other than as indicated on the label.
Here are some tips to help you use herbicides safely and effectively:
Herbicides can be divided into two main categories: selective and non-selective.
Selective herbicides can be further divided into pre-emergent and post-emergent herbicides.
The type determines when to apply an herbicide. Pre-emergents are usually applied in late winter or early spring, while post-emergents are applied in spring after the weeds begin to grow.
When using herbicide in gardens, take care to protect the plants you don’t want to kill. If you have identified your weed, you may be able to find a selective herbicide that will kill the weed without harming garden plants. Those containing glyphosate are good herbicides for hard to control plants and unidentified weeds because they kill most plants. Protect the other plants in the garden by making a cardboard collar to fit around the weed before applying the herbicide.
Note: Chemical control should only be used as a last resort, as organic approaches are more environmentally friendly.
This article was last updated on
Herbicides ( US: / ˈ ɜːr b ɪ s aɪ d z / , UK: / ˈ h ɜːr -/ ), also commonly known as weedkillers, are substances used to control unwanted plants.  Selective herbicides control specific weed species, while leaving the desired crop relatively unharmed, while non-selective herbicides (sometimes called total weedkillers in commercial products) can be used to clear waste ground, industrial and construction sites, railways and railway embankments as they kill all plant material with which they come into contact. Apart from selective/non-selective, other important distinctions include persistence (also known as residual action: how long the product stays in place and remains active), means of uptake (whether it is absorbed by above-ground foliage only, through the roots, or by other means), and mechanism of action (how it works). Historically, products such as common salt and other metal salts were used as herbicides, however these have gradually fallen out of favor and in some countries a number of these are banned due to their persistence in soil, and toxicity and groundwater contamination concerns. Herbicides have also been used in warfare and conflict.
Modern herbicides are often synthetic mimics of natural plant hormones which interfere with growth of the target plants. The term organic herbicide has come to mean herbicides intended for organic farming. Some plants also produce their own natural herbicides, such as the genus Juglans (walnuts), or the tree of heaven such action of natural herbicides, and other related chemical interactions, is called allelopathy. Due to herbicide resistance – a major concern in agriculture – a number of products combine herbicides with different means of action. Integrated pest management may use herbicides alongside other pest control methods.
In the United States in 2012, about 91% of all herbicide usage, determined by weight applied, was in agriculture.  : 12 In 2012, world pesticide expenditures totaled nearly $24.7 billion herbicides were about 44% of those sales and constituted the biggest portion, followed by insecticides, fungicides, and fumigants.  : 5 Herbicide is also used in forestry,  where certain formulations have been found to suppress hardwood varieties in favor of conifers after clearcutting,  as well as pasture systems, and management of areas set aside as wildlife habitat.
Chemical weed control has been used for a very long time: sea salt, industrial by-products, and oils were first employed. Selective control of broad-leaved weeds in fields of cereal crops was discovered in France in the late 1800s, and this practice soon spread throughout Europe. Sulfates and nitrates of copper and iron were used, and sulfuric acid proved even more effective. Application was by spraying. Soon sodium arsenite became popular both as a spray and as a soil sterilant. On thousands of kilometres of railroad right-of-way, and in sugarcane and rubber plantations in the tropics, the hazardous material was used in tremendous quantities, often resulting in the poisoning of animals and occasionally humans.
Sinox, the first major organic chemical herbicide, was developed in France in 1896. In the late 1940s new herbicides were developed out of the research during World War II, and the era of the “miracle” weed killers began. Within 20 years over 100 new chemicals were synthesized, developed, and put into use. Chemical weed control superseded both plant-disease and insect-pest control in economic impact. In particular, the year 1945 was key to the development of selective chemical weed control. Introduced then were 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), and IPC (isopropyl-N-phenylcarbamate)—the first two selective as foliar sprays against broad-leaved weeds, the third selective against grass species when applied through the soil.
The new herbicides were revolutionary in that their high toxicity allowed for effective weed control at dosage rates as low as one to two kilograms per hectare (one or two pounds per acre). This contrasted with carbon bisulfide, borax, and arsenic trioxide, which were required at rates of up to 2,242 kilograms per hectare (one ton per acre), and with sodium chlorate, required at rates of around 112 kilograms per hectare (100 pounds per acre). However, some of those early herbicides, including 2,4,5-T, were later deemed unsafe for humans and the environment and were discontinued in many countries. Effective herbicides have continued to be developed, and some, such as glyphosate, are widely used around the world.
Since the mid-1980s, certain agricultural plants, known as herbicide-resistant crops (HRCs), have been genetically engineered for resistance to specific chemical herbicides, notably glyphosate. These genetically modified organisms (GMOs) enable effective chemical control of weeds, since only the HRC plants can survive in fields treated with the corresponding herbicide. Such crops have been especially valuable for no-till farming, which helps prevent soil erosion. However, because these crops encourage increased application of chemicals to the soil rather than decreased application, they remain controversial with regard to their environmental impact and general safety. In addition, in order to reduce the risk of selecting for herbicide-resistant weeds, farmers must use multiple diverse weed-management strategies.
Ensure lawn herbicide applications are safe and effective by following the tips below.
A wicking applicator used to apply herbicides directly to weeds.
Photo by Steve Manning, Invasive Plant Control, Bugwood.org
Correctly identify the weeds. Herbicides vary in their control of different weed species. Preemergent herbicides are effective at controlling certain weeds, while regular mowing may efficiently manage other weeds. Choose the most effective control method after correctly identifying the weed.
If this document didn’t answer your questions, please contact HGIC at [email protected] or 1-888-656-9988.
Jackie Jordan, Commercial Horticulture Agent, Fairfield, Kershaw, and Richland County, Clemson Extension, Clemson University
A common misconception about the ideal time for pre-emergent herbicides is that application should coincide with certain events, such as daffodils or forsythias in bloom. In nearly every area of the country, the weeds will have already emerged by that time. To determine the best time of year to apply pre-emergents, note when the weeds begin to sprout this year and count back two or three weeks. That's when you should apply a pre-emergent next year.
If you missed the window of opportunity for applying a pre-emergent herbicide, you can apply a post-emergent product. Post-emergents work by destroying already established weeds. However, take care when applying post-emergent herbicides. Some are selective, meaning they target specific weeds, while others are nonselective, which means they destroy anything and everything green, whether they're weeds, your grass, flowers or shrubs. Therefore read the product label.
Keep in mind that you'll never get rid of all the weeds in your lawn. The wind will blow weed seeds from nearby lawns into your lawn, birds will deposit them and kids running from one lawn to the next will transport weed seeds on their shoes. So do what you feel you must to battle the weeds in your lawn, but do it wisely. Paul recommends using natural pre-emergent herbicides made from corn gluten or natural post-emergents made from vinegar or clove oil. Raise the height of your mower to help prevent weed seeds from germinating in the first place. Fertilize twice a year with a natural product to promote vigorous grass.
In order to select non-contaminated soil amendments, you will need to do some homework. When purchasing soil amendments ask manure and compost producers how they handle herbicide contamination. Ideally, they will reduce use of inputs which may have persistent herbicide residues. Compost and manure producers should also do bioassays on the finished product prior to sale as lab testing can be inconclusive on whether the inputs or a finished product will cause herbicide damage. Another safeguard compost producers may use is to add activated carbon or carbon wood ash to their finished compost which binds herbicides and makes them inactive. Compost producers may specify how to use their products. If they recommend using a product only on lawn or grasses do not use this for susceptible plants as it may contain persistent herbicides.
The best way to determine if the soil amendment you have purchased is contaminated with herbicide residual is to do a bioassay prior to applying to your garden. If you find there is some contamination, return the soil amendment to the place of purchase. You not only should get your money back, but the distributor may not know about the issue. You may save countless other people from having contaminated soil. If the distributor does not respond to your request, contact the Montana Department of Agriculture about the issue.
These instructions are intended to determine whether the test material (compost, soil, manure or other material) causes plant growth symptoms consistent with growth regulator herbicide or auxinic herbicide damage in susceptible plants. These instructions were created by Washington State University Whatcom County Extension.
Figure 3. Results from a bioassay done with known concentrations of aminopyralid six weeks after planting. At high levels there was no germination or plant death. At five parts per billion (ppb) there is leaf curling. The 0 ppb (control) pot shows what a healthy plant should look like under the same growing conditions without herbicide damage.
In a sugarcane field, what type of herbicide is used? nony May 30, 2011
@hamje32 - I have a dog in the backyard and none of the stuff I’ve read on the labels reassures me that it’s okay for pet consumption. I’ll have to take my dog to the vet if he accidentally swallows some of that stuff.
I’ve heard that there are organic weed killers available which will not harm animals and I may look into that. For now, I leave the weeds alone in the back where he plays and just treat the front lawn. hamje32 yesterday
Make it a practice to read the herbicide labels before doing anything. They will tell you what kinds of weeds the herbicides target and whether they will kill only the weeds or the weeds and grass. They also explain how the herbicides work and give you detailed instructions on how to apply them for the best results. NathanG May 28, 2011
@miriam98 - I don’t broadcast spread my herbicide. Instead I use the Roundup herbicide product to kill the weeds in my lawn. This allows me to avoid the kinds of accidents you described.
Roundup has worked well for me, and works better than some of the other stuff I’ve used in the past, like Spectracide and Bayer Advanced Weed killer. Of course if you have a really big lawn, I suppose you would need a broadcast spreader or have to hire a professional lawn service. miriam98 May 27, 2011
You have to be careful when using any herbicide product. I used a fertilizer recently with herbicide in it, and accidentally spilled a bunch of the stuff on my lawn. I now have brown patches in the lawn in the areas where I burned it. I flushed it out with water and dug up some of the soil, and laid down some new soil and compost. I am hoping I won’t have to completely replace it with sod.
I have Bermuda grass so I am expecting that by the end of the summer the neighboring grass will overtake the bare spots. Anyway, that’s a word to the wise. Be sure to set your broadcast spreader to the lowest recommended setting when spreading the herbicide. Once a lawn burns it takes awhile to get it back looking good.
talk about the side effects of herbicides on the environment. anon71829 March 20, 2010
Where can I buy herbicide?
How much will it cost me to enroll for a degree in treating land from herbicides and pests? Please advise. jimdan February 2, 2010
Where can you purchase herbicides? anon20957 November 8, 2008
what substances made up an herbicide? overreactor April 27, 2008
For small areas I prefer to pull weeds by hand rather then use herbicide. Of course that would not work on a large property.