Weeding is probably everyone's least favorite gardening task and any shortcut is always welcome. Herbicides--materials that kill plants--can be helpful assistants in the war on weeds, but many chemicals can be damaging to the environment as well. Organic materials that break down can be both effective and non-toxic in the long term. Be aware, however, that they will not kill every plant they contact.
Vinegar, acetic acid, is an extremely effective liquid weed killer; though ordinary vinegar, a 5% solution of acetic acid, may not be strong enough to kill established weeds. Products with up to 20% acetic acid may be needed. It acts on the leaves, interfering with photosynthesis, so you may need several applications to kill deep rooted weeds.
Boiling water is effective on weeds without deep roots, but if you add salt to the solution it will act more quickly and will desiccate the tap roots of plants such as dandelions.
Choose a time when you can expect two or three days of dry weather to spray your organic herbicide mixture. Rain will wash it off the leaves before it has a chance to work--or, in the case of salt, wash it out of the root zone.
Try to spray weeds before they have a chance to mature and become woody. All herbicides are more effective if applied to fresh, new growth. You may have to spray or apply boiling water several times to remove old ones.
Vinegar will lower the pH of your soil, making it more acidic. This may be beneficial if you want to grow blueberries or potatoes, but detrimental if you want to grow spinach. Do a pH test before and after if you'll be growing plants in that area and use agricultural lime to raise the pH if needed.
Salt will dissolve and leach out of the soil, but makes the area you spray toxic for any plant--ornamentals included--as long as it is present. It's best used on gravel pathways and other plant-free areas. Be sure that there are no tree roots beneath the area also, since these may sustain long-term damage too.