I was browsing the internet the other day and came across a web-site that told me I could kill weeds without using pesticides. Naturally, I was intrigued and decided to read up on DIY chemical weed control. I won't be posting any recipes on this blog (Sorry! There are plenty of web pages out there for you to peruse.), but I will discuss what home-based products are routinely prescribed, as well as their efficacy and safety.

Do they really mean no pesticides? Well, what do we mean by 'pesticide'? A pesticide is generally defined as a substance that is used to control a pest (be it an insect, a microbe, a weed, a rodent, a bird...you get the drift); so, in the purest sense of the word...no. If, however, you take 'pesticide' to mean a synthetic product purposefully manufactured by a large, multinational, agrochemical company...then, yes. 

So, what kinds of herbicidal products are we talking about? A quick Google search will turn up any number of substances, the most common being: vinegar (acetic acid), table salt (sodium chloride), lemon juice (essentially, citric acid), corn gluten mean, baking soda (sodium bicarbonate), Borax (sodium tetraborate), bleach (sodium hypochlorite) and various essential oils alone or in mixtures. Additionally, numerous web sources have also advocated using boiling water, fire, newspapers (or other mulches), and solarization for the management of weedy pests. Note:  'all natural' or 'home made' herbicides are not necessarily certified for organic crop production; see the USDA's web site for a list of allowed and prohibited substances.

Figure 1. Screen capture of a portion of the USDA's list of allowable and prohibited substances for use in organic crop production.

Are these products effective at controlling weeds? That is an interesting question as efficacy can mean different things to different people. One person's idea of successful weed management might be another person's eyesore. That being said, relatively few experiments have actually compared weed control using home made herbicides to conventional, commercial products. One such report found that the use of vinegar to manage weeds in landscape beds reduced future handweeding time relative to the untreated check, although it wasn't as successful as glyphosate (Roundup) or mulch at weed suppression.

People choosing to apply home made herbicides for weed control should remember a few things in order maximize product effectiveness:

• These products are not systemic i.e. they don't translocate in the plant to control roots, rhizomes, or tubers; instead, they are contact materials, meaning they control the leaves, stems, and other tissues that they are sprayed on.
• Good spray coverage is needed to maximize control success.
• Smaller is better! Young, annual weeds are more easily controlled than larger or perennial plants.
• Applications need to be made under favorable weather conditions (low/no wind, no imminent rain) to ensure that the sprayed materials remain on the treated plants (and don't drift onto desirable species) to cause injury. These products are non-selective meaning that they can injure most plants that they come into contact with. That being said, grass weed species may be less readily controlled than broadleaves because the growing point may not be exposed to your herbicide.

Are these products safe? Again, another interesting question. What do we mean by 'safe'? I think it is fair to state that choosing to be free of synthetic agricultural chemicals does not mean that you are free from personal harm. As Dr. Andrew Kniss pointed out in his blog, the LD 50 value for acute dermal (rabbit) exposure to acetic acid (1060 mg/kg) is lower than the LD 50 for glyphosate (>2000 mg/kg) (lower is more toxic).

Unlike commercial herbicide labels, home made herbicide recipes are not legal documents with specific instructions to ensure applicator safety. Vinegar is acidic and can burn both the skin and eyes if splashing occurs. So, too, can bleach, which can also injure the respiratory tract. The physical injuries that could arise from handling boiling water or flames for weed control should be self evident. Home made herbicides can also negatively impact your garden environment. Boron (from Borax) doesn't dissipate readily and may accumulate if used too frequently. Salt, which is highly soluble and mobile in water, can move from treated areas and injure desirable plants.

Anyone choosing to use herbicides should seek the guidance of their local farm advisors, or other trained individuals, to ensure proper application. Always wear personal protective equipment (including gloves, safety glasses, long sleeves, pants, and close toed shoes) to prevent skin or eye contact, never mix or apply products in a poorly ventilated space, keep children and pets away from treated areas, and never, EVER, store any product in an unlabeled bottle.

Figure 2. The herbicides in my kitchen.