One of the most effective ingredients for killing weeds is vinegar. Plain white or apple cider vinegar will work equally well. Use it straight from the bottle, no need to dilute. Pick a dry day to spray some on a weed and it will immediately start to droop. The acetic acid in vinegar will kill the above-ground part of the weed, but it will not kill the roots. One application is usually enough for young plants, but more established plants will need additional treatments.
Combine salt with vinegar to create a lethal plant killer. Use about a cup of salt to a gallon of vinegar for the best effect. Sometimes, salt can be a little too effective. In large quantities, it will kill everything in sight and make the ground sterile and unusable for years.
A couple of cups of boiling water will cook a weed right where it sits without harming the environment or affecting future growth in the same soil. No need to get fancy, just boil a pot of water on the stove or in the microwave and pour it on the plant. It will kill the roots of nearby plants, though, so be careful when you pour.
Dishwashing soap doesn't kill weeds, but it helps weed killer solution stick to the plants. You only need to add about 3 tbsp per gallon of weed killer. Pick a dishwashing liquid that does not contain bleach to avoid possible chemical reactions.
Corn Gluten Meal
Corn gluten meal, which is not the same as corn meal, is a weed preventive. It prohibits seeds from starting. To prevent weeds from growing, scatter it in an area you plan to plant mature plants in, like a flowerbed. But it also will prevent volunteer flowers, so you'll have to replant when a plant dies if you use this method.
Combining methods will produce the most amazing homemade weed killer of all. Boil water, vinegar and salt, add a dollop of dishwashing soap, and it just doesn't get more lethal.
Method Of Delivery
Large lawn sprayers set on a narrow stream work well. You can also use an empty spray bottle washed out well enough to remove any chemical residue, a watering can or even a gravy boat for small areas. Anything with a narrow spout or a targeted stream of spray will do the trick.
Like many commercial chemical fertilisers, homemade weed killers can't differentiate between weeds and other plants. Use your homemade weed killer very carefully to avoid killing plants you want to keep. Have a container or sprayer of water handy to rinse off plants that accidentally get splashed with weed killer.