Most organic weed killer formulas are fundamentally acids.

Weak solutions of sulfuric acid, nitric acid, pelargonic acid (from grapes, apples and carrots as found in the commercial weed killer WOW! Plus) or acetic acid (as found in lemon juice and vinegar) are all effective at suitable concentration.

Acids work by lowering pH level on target weeds just enough to kill them.

The acid only persists for a few minutes after contact with herbage or soil, quickly oxidizing.

The residues left behind by these natural weed killers are either harmless or useful to plants as fertilizer.

Sulfuric acid, for example, oxidizes to form sulfates of any mineral element it comes in contact with.

If the element is calcium, for example, the reaction results in gypsum, a useful fertilizer and soil conditioner.

Similarly, nitric acid oxidizes to form nitrates, which are also the natural end result (albeit in higher concentration) of the nitrifying bacteria found in soils.

The Weed Option of Last Resort

Use weed killers only for intractable weed problems. Good husbandry of providing ample water and food to your plants, with a little help from hand tools and manual weeding (or let your chickens or ducks do the work for you while fertilizing at the same time) are still the preferred options for most weed problems.

There are also other weed management methods available.


Vinegar Weed Killer

Acetic acid is a great organic homemade weed killer and can be found in vinegar, as well as lemon juice.

While most vinegar has an acid content of around 5%, a more concentrated solution of 10% to 20% will more effectively kill weeds.

At the right strength this organic weed killer will kill the leaves of any plant it comes in contact with, but not the roots.

Because of this it is most effective on young weeds which don’t have enough energy stored in their roots to make a successful comeback. Repeated applications will be needed to permanently disable more established weeds.

Vinegar Weed Killer Recipe

• 120 mls (4 ounces) Lemon juice concentrate

• 1 liter (1 quart) white or cider vinegar

Simply mix the two ingredients together in a spray bottle and you have your organic weed killer formula.

Spot spray it directly on the weeds, being careful not to spray desirable plants. For the most effective result the best time to spray in during the heat of the day.

Bleach Weed Killer

Bleach has a similar mode of action to the acid-based organic weed killer formulas in that it causes an extreme shift in the pH of the target plant. The main difference is that bleach shifts pH to strongly alkaline (the opposite direction to a shift to strongly acid).

Being a compound of the elements sodium, chloride and oxygen, bleach in the environment soon oxidizes to leave a residue of common salt (sodium chloride).

While salt itself inhibits plant growth, if used sparingly, the salt residues left behind by bleach soon dissipate with rain or irrigation. The treated area should be OK to plant after two days.

Only a weak bleach solution (around 4% of active ingredient) should be used to make this organic weed killer formula. Just pour into a spray bottle as is and spot treat the weeds you want to remove.

Take care to avoid contact with skin or eyes. Store safely away from children. If you accidentally spray yourself or desirable plants, immediately flush them with water.