Natural Weed Killers: A Guide to Organic Herbicides

Weed killer on lawn

Ask anybody who toils in their own yard what their biggest headache is and they’ll tell you, “weed control.” Keeping a lawn or garden free of perennial or annual weeds is tough work and time-consuming. Most people don’t like resorting to chemical use to control weeds but truth be told, herbicides serve a useful purpose.

Allowing weeds to grow rampantly in your yard or garden isn’t a viable solution; weed control is critical to keep unwanted pests from competing with your grass, flowers, or veggies for important resources such as sunlight, water, and nutrients in the soil. This is especially true when implementing organic practices.

Organic gardening and organic lawn care continue to gain popularity amongst homeowners adopting more sustainable lifestyle choices. Organic weed control isn’t an easy fix though. It adds an extra layer of difficulty to the basic yard and garden maintenance when you go looking for organic products. They may not always work as well, since the properties that make them organic may limit their availability and efficiency.

What are organic herbicides?

Herbicides, also known as weed-killers, are a class of chemical compounds used to kill or inhibit the growth of unwanted weeds.

Chemicals are either classified as inorganic or organic, based upon the compounds they contain. Inorganic herbicides are synthetically created in a lab; organic weed killers are made completely from naturally-occurring chemicals.

The National Pesticide Information Center claims there are hundreds of different herbicides on the market available for purchase to use, whether it be in agricultural, horticultural, landscape, or gardening settings. Organic herbicides make up a small percentage of the products available.

How they work

Simply put, chemical herbicides work by disrupting biological processes within the cells of targeted plants, causing plant dieback and hopefully plant death.

Herbicides are classified into different categories, based upon how and when they affect plants when applied. Understanding these groupings allows users to choose the best herbicide for their situation.

Contact Vs. Systemic Herbicides

Herbicides come in two major classes:

  • Contact herbicides disrupt the biological processes only in the plant tissues they come in contact with. They are fast-acting but in most cases may not kill the whole plant, requiring multiple applications to eradicate the whole plant.
  • Systemic herbicides act through absorption. The weed’s vascular system carries the poison from the shoots to the roots to kill the entire plant.

Organic herbicides fall into the category of contact herbicides. They are also non-selective, which means they will kill perennial weeds, broadleaf weeds — whatever they touch. So careful application is key.

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 is used as a pre-emergent weed killer, primarily in turf. Dr. Nick Christians of Iowa State  University discovered its weed-killing properties by accident while researching a pathogen found on golf course turf. He used the corn-milling byproduct as a growing medium and by chance discovered it controls broadleaf and grass weeds by inhibiting root formation. Patented as a natural herbicide in the early 1990s, it remains at the top of the list of most-popular and effective organic herbicides.

Others include 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).

  • Herbicidal soaps containing long-chain fatty acids.
  • Vinegar or acetic acid. (In high concentration. 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.

Commercial Products

When compared to conventional herbicides, the market offers relatively few organic weed killers. The list includes the following commonly used, organic herbicides:

Timing

Pre-emergent products control weeds by preventing weed seeds from germinating, 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 herbicides kill weeds that have germinated and are actively growing above the soil surface.

Weed research says young weeds are easier to kill than established weeds.

Organic Herbicides Work Best on Young Weeds
Broadleaf weed kill rate 12 days, 19 days and 26 days after they emerged
ProductWeed age
12 days19 days26 days
GreenMatch Ex 15%89%11%0%
GreenMatch 15%83%96%17%
Matran 15%88%28%0%
Vinegar (Acetic acid 20%)61%11%17%
WeedZap 10%100%33%38%
Untreated weeds0%0%0%
Source: University of California-Davis Nursery and Floriculture Alliance

Tips for Effective Organic Herbicide Use

Organic herbicides present challenges not found in inorganic products so it’s best to keep the following in mind when utilizing them.

  • Use in conjunction with hand weeding or other cultural weed eradication methods for best results.
  • They work better on younger, smaller weeds. Research by Thomas Lanini has proven the best results at cotyledon or first true leaf stage.
  • Organic herbicides typically are contact herbicides, so they must cover the entire plant.
  • Perennial plants have a greater ability to recover, even if applied well.
  • They work better when temperatures are above 75 degrees Fahrenheit and there is full sunlight.
  • Lack of residual activity means reapplications may be necessary for optimal results.
Amanda Shiffler

Amanda Shiffler

Most comfortable with soil under her fingernails, Amanda has an enthusiasm for gardening, agriculture, and all things plant-related. With a master's degree in agriculture and more than a decade of experience gardening and tending to her lawn, she combines her plant knowledge and knack for writing to share what she knows and loves.