Weed control is critical to keep unwanted pests from competing with grass, flowers, or veggies for sunlight, water, and soil nutrients. While organic gardening and organic lawn care continue to gain popularity, choosing the best natural weed killer can be confusing. To help you make the best choices for your yard and garden, here’s a look at organic herbicides and how they’re used.
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What Are Organic Herbicides?
Herbicides are a class of chemical compounds used to kill or inhibit the growth of unwanted weeds.
Chemicals are classified as inorganic or organic, based on the compounds they contain. Inorganic herbicides are synthetically created in a lab; organic weed killers are made from naturally occurring chemicals. Glyphosate, commonly sold under the name Roundup, is a well-known inorganic herbicide. Horticultural vinegar is a well-known organic herbicide.
The National Pesticide Information Center claims there are hundreds of different herbicides on the market for agricultural, horticultural, landscape, or garden settings. Organic herbicides make up a small percentage of the products available.
How They Work
Simply put, chemical herbicides disrupt biological processes within the cells of targeted plants to kill weeds.
Herbicides are classified into different categories, based on how and when they affect plants when applied. Understanding these categories allows you to choose the best herbicide for your yard and garden.
Contact and Systemic Herbicides
Herbicides come in two major classes:
- Contact herbicides disrupt the biological processes only in the plant tissues they contact. They are fast-acting, but in most cases require multiple applications to kill the entire plant.
- Systemic herbicides act through absorption. The weed’s vascular system carries the poison from the shoots to the root system to kill the entire plant.
Organic herbicides fall into the category of contact herbicides. They are also non-selective, meaning they’ll kill perennial weeds, broadleaf weeds, dandelions — whatever they touch. So careful application is key.
Pre-Emergent and Post-Emergent Herbicides
Organic herbicides can be either pre-emergent or post-emergent.
Pre-emergent products prevent weed seeds from sprouting above the soil surface, stopping the problem before it starts. Their active ingredient works as a mitotic inhibitor, halting cellular division and inhibiting the growth of the root, the shoot, or both.
Post-emergent weed killer targets weeds that have germinated and are actively growing above the soil surface.
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Organic Herbicide Active Ingredients
Just a few organic herbicide ingredients exist. Corn gluten meal, one of the earliest, remains the most popular. Corn gluten meal (CGM) is used as a pre-emergent weed killer, primarily in turf.
Dr. Nick Christians of Iowa State University discovered CGM’s weed-killing properties while researching a pathogen found on golf course turf. Patented as a natural herbicide in the early 1990s, the corn-milling byproduct inhibits root formation for broadleaf weeds, like dandelions, and grassy weeds, like crabgrass. However, it works better in some climates than in others.
Other organic weed killers are post-emergent products. The following contain active ingredients that work by removing the waxy cuticle on plant leaves or damaging cell walls to cause desiccation (loss of moisture) and senescence (loss of the ability to divide and grow).
Active ingredients in organic herbicide products:
- Herbicidal soaps containing long-chain fatty acids
- Vinegar or acetic acid (In high concentrations, it is often called horticultural vinegar. Regular household vinegar is ineffective as a weed killer.)
- Citric acid
- D-limonene, a common terpene found naturally in citrus peels such as orange, lemon, grapefruit, lime, and mandarin
- Essential oils such as clove, summary savory, cinnamon, and red thyme
- Chelated Iron. This consists of the iron product generally used as a soil supplement. However, it is bound to a chelating agent (HEDTA or hydroxyethylenediaminetriacetic acid) that keeps it liquid. It’s effective against broadleaf weeds.
Organic Herbicides List
When compared to conventional herbicides, the market offers relatively few organic weed killers. The list includes some of the commonly used organic herbicides:
- Weed Pharm (20% acetic acid)
- Weed Zap (45% clove oil + 45% cinnamon oil)
- Avenger Weed Killer (70% d-limonene; the first Environmental Protection Agency (EPA)-approved, Organic Materials Review Institute (OMRI)-listed organic herbicide)
- Sunday Weed Warrior (3.68% Ammoniated soap of fatty acids)
- Whitney Farms Lawn Weed Killer (1.5% Iron HEDTA)
Homemade Organic Weed Killer
If you can’t find an organic weed killer, or the budget won’t allow for one, you can make your own. Two popular methods are:
Vinegar: This is the most popular homemade weed killer and is usually combined with soap (but avoid dish soap). However, the cider or white vinegar in your pantry likely isn’t strong enough to do the job. You’ll want a vinegar with an acetic acid concentrate of up to 20%. One gallon of 5% – 20% vinegar and 1 cup of castile soap, and you’re ready to take on the weeds.
- Caution: Horticultural vinegar can cause blindness and burns on your skin, so follow all safety recommendations before using.
Boiling water: If you can boil water, this DIY organic weed killer is within your reach. This method requires a kettle, water, and a stove. It works well on young weeds and is a good choice for patios and driveways. Pour the water directly — and carefully — on the weeds so as not to burn yourself or kill the wrong plants.
Tips for Effective Organic Herbicide Use
Organic herbicides present challenges not found in inorganic products, so keep the following in mind when utilizing them.
- Use in conjunction with hand weeding or other cultural weed eradication methods for best results.
- Organic herbicides typically are contact herbicides, so they must cover the entire plant.
- Perennial weed plants have a greater ability to recover, even if herbicide is applied well.
- The best time to apply is when temperatures are above 75 degrees on a sunny day.
- Reapplications may be necessary for best results.
The sooner, the better once weeds begin to emerge. Organic herbicides work best on younger, smaller weeds. Research by Thomas Lanini of the University of California has proven the best results at cotyledon or first true leaf stage.
Mulch is a good way to prevent weeds, and keeps moisture in the soil. Spread 2-3 inches of mulch in the area you want to keep weed-free. Commercial weed-blocking fabric also works and can be applied under mulch. For a less-expensive landscape fabric alternative, try a few sheets of newspaper or cardboard.
Flame weeding, with a propane torch or other fuel-burning implement, works on young weeds and on solid areas, such as driveways. But don’t set the weeds on fire. Instead, just apply a brief burst of heat that ruptures the weed’s cell walls. Works well on dandelions and other broadleaf weeds.
Use extreme caution with this method: Wear appropriate clothing, be careful when handling the torch, and do not flame weed on windy days, near dry grass, or other flammable materials.
When to Call in the Pros
It’s important to choose the right organic herbicide for your needs and to apply it properly. If you’re not confident about doing this, consider calling a lawn-care professional. A local pro experienced in using organic herbicides can keep the weeds at bay and help you achieve a more sustainable lawn.
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