- June 13, 2016
If you are at all like me, some of your garden beds are a mess. In my neck of the woods, the forested northeast, that means a healthy population of garlic mustard, dandelions, common waterhemp, common mullein, and common chickweed–to name a few–are threatening to overrun my beds. An application of a broad-spectrum herbicide to eliminate these pesky weeds is tempting; and yet, I hesitate, not least of all because my young child will soon be playing in my gardens, eating the soil, and the vegetables it produces. What do I need to know to make an informed choice on the best strategy to tackle these weeds?
Gardeners have easy access to many of the same herbicides put to use in commercial agriculture. A subcategory of pesticide, “herbicide” means to kill plants (herb = plant, cide = to kill). Some herbicides are systemic and some are contact herbicides. Contact herbicides kill parts of the plant that come in contact with the chemical. Systemic herbicides are absorbed into the foliage, or the roots, and then move through the plant.
How an herbicide affects a plant is referred to as the mode of action. Some herbicides inhibit a weed’s ability to photosynthesize, like atrazine, which is commonly used to kill weeds in corn and soybean fields. Other modes of action inhibit amino acid development (protein synthesis), inhibit root development, or cause abnormal cell development. In industrial agriculture it is common to use a spectrum of herbicides employing a range of modes of action as a way to minimize weed resistance to herbicide applications.
Some herbicides are selective, such as 2-4, D which is toxic to broad-leafed plants but less toxic to narrow-leafed ones like grasses. Other herbicides are nonselective (broad spectrum), such as glyphosate, the active ingredient in Roundup. Nonselective herbicides are used to kill both broadleaf plants and grasses. Using genetic modification, scientists have bred plants that are not susceptible to the mode of action glyphosate uses to shut-down amino acid production, which eventually leads to the death of the plant. This includes commonly used varieties of corn and soybean in the United States. You can imagine how useful this is to farmers, who can now plant these “Roundup-ready” crops (ie – crop plants that are resistant to glyphosate and can survive being sprayed), and spray their fields with Roundup to kill-off the non-crop plants growing in the field that are competing for water, nutrients, and space.
Herbicides have become an important part of our agricultural system because they increase crop production and minimize tillage.open_in_new One recent study calculated that herbicides increase agricultural yields by about $43 billion in the U.S. and Canada, where roughly half of the corn and soybean crop would be lost without their use.
Applying herbicides also minimizes the amount of topsoil loss.Without herbicides tilling is often used to suppress weed growth. Tilling results in the loss of topsoil because wind carries away the topsoil when heavy machinery turns it over (see the image above of a heavily tilled field). This soil is then lost from the fields, removing valuable nutrients and damaging the soil composition.
The benefits of herbicides are pretty-well established in industrial agriculture, even if they are not without great controversy and debate. Here, however, we want to focus on what you have direct control over: making decisions about how to use chemicals in your own yard, or even influencing how they are used in your community at public parks, offices, and schools.
Herbicides are put to use in non-agricultural contexts on a pretty regular basis for a variety of reasons from invasive species management to cosmetic weed maintenance. What do we need to know in order to make an informed decision about herbicide use in these scenarios?
What is a Weed?
Weeds are contextual. That is, something is a weed not so much because of its inherent characteristics, but because it is someplace we don’t want it to be. Formally, a weed is a wild plant growing where it is not wanted and in competition with cultivated plants. For example dandelions, rich in vitamins and minerals, have multiple uses from the medicinal to the culinary. In a different context a plant like this might be intentionally cultivated; yet, numerous products on the market containing 2,4-Dichlorophenoxyacetic Acid, Dicamba, or Methylchlorophenoxypropionic Acid (MCPP or MCPA) are aimed at removing them from lawns, where they compete with grasses, and are often considered a nuisance by home owners.
By far the most common herbicide is Roundup, whose active ingredient, glyphosate, accounts for approximately 300 million pounds of the 500,000 tons of pesticides annually used in the United States; but, Roundup use isn’t just limited to agriculture. It is also widely used by homeowners.open_in_new
Research continues, not only on the effects of the “active” ingredients, like glyphosate, but also on the seemingly inert ingredients used as part of the Roundup formula, and it is not clear if the benefits outweigh the risks when it comes to cosmetic use in residential and urban settings.
High concentrations of herbicides in freshwater systems have been shown to transform the structure and function of these ecosystems.open_in_new Like their counterparts in terrestrial systems, aquatic plants are also killed when they come into contact with high enough concentrations of herbicides. Urban systems have a high percentage of impervious ground cover, which leads to high rates of runoff, making nearby natural areas particularly vulnerable to storm water management issues and increased concentrations of herbicides, like glyphosate, that end up in the runoff from residential areas.open_in_new
Herbicides may also affect soil health by indirectly interfering with the bacteria, fungi, and invertebrates that are a part of thriving soils.open_in_new Plants are symbiotic partners with these micro/macro organisms that break soil down into compounds such as nitrogen, phosphorus, and potassium that are needed for plant growth and repair. Disrupt the soil life and you potentially weaken the plants growing in the soil.
Herbicides may also affect the health of animals. Dogs are suspected of being more susceptible to bladder cancer if they are exposed to herbicides on lawns.open_in_new Measuring the concentration of commonly-used lawn herbicides in dog urine reveals widespread uptake of these chemicals–even in those whose owners did not use herbicides on their own lawns. These pesticides were detectable up to 48 hours after application to a nearby lawn .
We often hear about the impact of pesticides on bee populations, but research has primarily focused on insecticides, like neonicotinoidsopen_in_new, and not on herbicides. Some investigations reveal mixed results on pollinator populations, where impairment in navigation is documented, while others report no impact.open_in_new Some research also suggests that use of herbicides may impact the availability of host plants for some pollinator species, like milkweed for monarchs. Read our article on milkweeds to learn more about the complex ecology at play in these kinds of plant-insect interactions.
Selective stump painting
When Herbicide Use Might Make Sense
Sometimes just pulling the weeds is not an option when the “weed” is a large, old, well-rooted shrub or small tree. Cutting it down often results in multiple stems vigorously re-growing from the stump. In such cases application of an herbicide to quickly eradicate the invasive woody plant may make sense.
For the best results, with the least amount of herbicide, selective-stump application could be used.
- Cut the trunk or stem of the plant about 2 inches above the ground.
- Immediately apply a thin coat of Roundup (active ingredient, glyphosate) to the outer rings of the stump, just inside the bark. This ring is the cambium layer where active growing takes place and herbicide anywhere else is unnecessary.
- Ideally, do this procedure in the Fall, at the end of the growing season, when risk of contact with other plants is minimal and woody plants are actively transporting to their root system.
The decision to apply herbicides for landscaping should be based on a cost-benefit analysis when working toward your ultimate landscape goals. Using a minimal amount of herbicide specifically timed and placed can remove an invasive species quickly and provide healthier, more productive habitat.
Herbicide application requires education. This goes for chemicals used on farms as well as those applied to private property. How many people fail to read the application directions when they buy a new bottle of herbicide? Most of us are guilty of at least once failing to read directions and jumping into the task at hand. If households across the United States are applying chemicals without following dosage and application directions, how many people and landscapes could be inadvertently and negatively impacted?
Considering the possible negative outcomes of chemical herbicides and the potential for error in application and dosage, other alternatives for controlling unwanted weeds are recommended for smaller property owners.
Alternatives to Commercial Herbicides
- Vinegar Application
- Mechanical control
- Flower/Seed Removal
- Clove oil
- Corn gluten meal
- Plant spacing
Mulching can be a very effective method for eliminating weeds, sometimes just as effective as herbicides. There are various forms of mulch available from wood clippings to grass and straw. The added bonus of using mulch instead of applying chemicals is that you are providing nutrient-rich organic material that will eventually decompose and nourish your soils. The challenges to using mulch are the expense, the laborious application, and the need for annual reapplication.
Vinegar, which is technically acetic acid, can be an effective alternative to other chemical applications. In a backyard experiment we tested vinegar treatments against Roundup treatments and found that vinegar was an effective, quicker, chemical for eliminating weeds on a portion of her sidewalk. The potential problem with vinegar is that it turns the soil acidic. If you are using vinegar, we recommend using it in areas like walkways, driveways or patios, and avoid exposing large areas of soil where the pH might be altered. If you do use it in gardens consider spraying only the leaves to minimize contact with the soil. Most plants require the soil pH to be between 6.5-7 or neutral, soil testing and soil amendments may be required to make sure the soil pH is not decreasing. Vinegar is also a contact pesticide so it will not kill the root system, meaning aggressive reapplication will likely be required.
Mechanical control of weeds, ie., pulling them, is highly effective, especially if combined with mulching and being diligent to remove the entire root system. Different weeds have different root systems and will require different techniques and tools for removal. Some are taproots such as burdock (Arctium) and dandelion (Taraxacum), while others are fibrous. The difficulty in removing the weeds often depends on the root structure and size of the weed. Weeding has the added benefit of being excellent, meditative work, so get pullin’!
Related to mechanical control is flower and seed removal. This strategy can be helpful for nonnative, herbaceous perennials and biennial flowers. When these plants flower, remove the flowers manually and compost. This ensures that new seeds are not produced. This can also be done once plants go to seed (if you do not catch the flower). Remove the seed heads and dispose of them–do not compost in case the seeds survive.
Clove oil’s active ingredient is eugenol. This oil has mixed results as an herbicide. Application of the oil is most effective if the spray coverage is thorough, it is applied when temperatures are 75 degrees or higher due to the viscosity of the oil, and if the weeds are treated when they are small. Like many of the alternatives, clove oil will kill the top foliage, but will not address issues of root vitality.
Corn gluten meal is another product that has mixed reviews. Corn gluten is a bi-product of cornstarch manufacturing and needs to be appropriately applied to be effective. Spreading this material before weeds emerge (pre-emergent herbicide) will suppress the germination and establishment of weeds. This is most effectively done in the early spring before seeds germinate. Corn gluten has large concentrations of nitrogen (see picture 9-0-0), so if applied to mature or already seeded plants, it will have the unwanted effect of feeding them rather than killing them. Corn gluten needs to be used in conjunction with other techniques if used in areas with pre-existing, mature weeds.
Plant spacing is an ideal way to combat weeds. One participant minimized the amount of space weeds could creep in by creating gardens that are so dense weeds are outcompeted! This featured site demonstrates that over time you can have gardens that produce healthy, dense native plantings that establish and thrive so quickly that weeds are unable to grow. This particular gardener removed their turf grass and, over the period of several years, produced thick native plant gardens. When successful, this is the most beneficial weed control for wildlife-habitat gardeners.
Finally, the old adage–accept the things you cannot change and change the things you can–comes into play with weeds. Weeds are a part of gardening. For better or worse, they are plants, just like the beautiful roses and gladiolus we love, the native wildlife habitat we are creating, or the vegetables we are harvesting. They need the same things to thrive, and so, they will thrive right along-side our desired plants. You cannot change the presence of weeds, but you can change how you deal with them, and in doing so, minimize use of chemical herbicides.
Tell us about your herbicide use on your property.
We collect this data at the Site level in the Characteristics. Click on your site outline and then click on the green Info button in the site explorer to the left.
The characteristics will open and you can select whether herbicides are never, rarely, sometimes, or often used.
If your site does not use herbicides, a red circle with a line through it will appear under your site characteristics for synthetic herbicide.