Many weed killers labeled as organic contain sodium chloride, which is just plain table salt. What a lot of people don't know is that too much salt is actually poisonous so while it may be organic using too much could possibly do more harm than using a synthetic herbicide.
The other main active ingredient in organic weed control products is concentrated vinegar, usually 10% to 30%. Your standard household vinegar is only 5%; distilled vinegar is generally 10% to 15%. A 30% concentrate can be considered more toxic than a synthetic herbicide.
Using vinegar is usually considered the best organic weed killer but there are a few things you should know before using it. The first thing to know is that vinegar will kill not only your weeds but most of your small plants and grass in the area you use it on, it will lower the pH of the soil making the soil inhospitable to plants for at least a week.
The other thing you need to know is that vinegar can eat concrete, so you don't want to use too much around your sidewalks and driveways.
There are also many other types of organic weed killers, including cedar oil, garlic oil, and many other types of natural plant oils. Mint, cinnamon, and citronella plants are also considered weed killers. It may help to plant of few of the mentioned plants in areas that you want to prevent weeds from overtaking.
These plants have also shown the ability to repel some insects. All of these plants and oils are considered very mild weed killers and may not have an effect on certain weeds that are in your area.