Vinegar
Vinegar is distilled through a fermentation process from grain, apples, or grapes. Common household vinegar has an active ingredient called acetic acid, usually diluted to a 5% concentration. This is sometimes labeled as 5% acidity.

Like most strong acids, Acetic acid is a desiccant. Meaning it removes moisture from things it is exposed to, like when sprayed on plant foliage. The water in the leaves is drawn out, and the top growth of the plant is killed. Whether or not the root is killed depends on the type of plant and its maturity.

The strength of the solution of acetic acid determines how fast and how completely it will kill weeds. Full strength vinegar that is not diluted with water, will be strongest and therefore the most effective. Vinegar with a higher acidity is available, although not commonplace (although pickling vinegar is normally stronger than the normal stuff; around 9% acidity). Caution must be used when creating a homemade weedkiller with stronger vinegar as the higher percentage acetic acid present will mean it is more likely to cause harm to sensitive areas like eyes and skin etc.

Vinegar is not a selective weedkiller. It has the potential to kill any plant foliage and insectlife on the plant. This means that if you spray weeds on a  lawn, the grass will die as readily as the weed. If you spray weeds in your flower bed or vegetable plot, the wanted can die as well as the unwanted. As a home made weed killer, vinegar may have limited application. Application must done with great care and will require that valuable plants be protected.

Some plants may be more resistant to absorbing it. Foliage with a waxy or hairy covering may absorb less of the solution and suffer less damage. Some weeds will appear to die but send up new growth from the root. As vinegar will become pretty much inert as it hits the soil the new growth will also have to be treated. Treatment/application is most effective on hot, dry sunny days.

Salt
Many of the homemade weed killer recipes include salt. Most suggest regular table salt, some say dishwasher salt, others mention rock salt. To be honest salt is salt and whatever kind it will kill plants. If vinegar alone is proving ineffective for some hardier weeds the inclusion of salt could win the battle.

Salt also acts as a desiccant, drawing moisture out of the plant. Most recipes are intended for use as a foliar spray, spot treatment. A few will instruct you to drench the soil around the weeds, expecting to kill the root more successfully.

Salt is problematic. It will get rid of your weeds alongwith everything nearby, it is not selective either. It can hang around in the soil, creating difficulties when you want to grow desirable plants. You may have heard the term “burned a plant with fertilizer”. That is because regular fertilizer is a salt. Apply it too heavily and plants die. Salt can remain in the soil, even affecting roots from distant plants.

Using salt in your homemade weedkiller will certainly add to its potency but defeats the effort of trying to develop healthy soil. So try to avoid using large quantities in soily areas where you wish to grow other things. Instead use it in spot treating weeds that pop up in cracks in your driveway, patio or pavement. Just use it sparingly, as it can leach into the area where good plants live.

Soap
Soap is added to homemade weed killers to help improve the absorption of the spray. Liquid dish detergent (not dishwasher or hand soap) is the most effective at this.

Soap is able to break down the cuticle or waxy surface found on many weeds. This increases the effectiveness of the active ingredient i.e. acetic acid, sale etc by weakening the weeds defenses. The soap will also break the surface tension of water, which inturn helps it to stick to the leaves instead of just running off. This of course allows the killing agent to stay on the weed for a longer period and thus increases its effectiveness.

Soap is often an oil derivative. Oil kills plants. Some soaps may even kill plants themselves if they were applied in a strong enough concentration. A benefit of adding soap to homemade weedkiller is that it is easier to see what areas have been treated. Treated areas will look soapy and shiny.

These results provide good reason to add soap to a home made weed killer. The amount of soap required as an additive need only be small. As a sole ingredient to kill weeds, the concentration would have to be much higher.

Alcohol
There has been some talk of gin and bourbon being an effective weedkiller ingredient. There are quite a few reports supporting this. No doubt the alcohol is acting as a desiccant. Any alcohol works in this manner. Rubbing alcohol will be just as affective if not moreso but I wouldnt have a clue where to buy it!!

Citrus Oils
Citrus oil; orange and lemon oil and similar products are acidic. The acidity would act like the vinegar but there is also the added benefit of the oil which as already discussed is a good weed killer itself. Citrus oils will cost more but may be more effective?!?!

Corn Gluten Meal
This is an all natural weed and feed product that deters weeds spreading by seed. Effective on crabgrass, dandelions, curly dock, knotweed, lambs quarters, pigweed, plantain and other weeds. Can be used on lawns, flowerbeds, bulb beds and vegetable gardens. Make sure to use CGM that is labeled as “pre-emergent herbicide“, stuff from the feedstore doesn’t have enough protein to work. Keep in mind that using CGM affects seed growth for all plants, not just weeds.

The  difficulty with any home made weed killer formula is the quantities or concentrations to use. A lot of it is just guess work and trial and error. Some will be effective on some weeds and not on others.

The results can also be affected by the weather. Usage of the ingredients above during a sunny day will provide better results than a dull or rainy day.