Citrus-based weed control
Several weed control products with concentrates of lemon and orange oils are also available to home gardeners. I have used four or five of these products and found them to be effective and fast-acting. Like the vinegar-based products, they are most effective when used on dry soil on hot days. Some products combine both citrus oils and vinegar concentrates or include other ingredients, like clove oil. These products are all foliar sprays that kill most broad-leaved weeds on contact. Large, established, taprooted weeds like dandelions may need several applications of the citrus oil/vinegar blend to kill them.
Clearly, because of their high concentration and their effects on the soil, these products have significant limitations. Any products that change the soil pH are not safe for use on septic fields, near streams or ponds, on edible crops, or anywhere they may affect the roots of established trees or shrubs (which generally extend past the plant's drip line). Also, vinegar concentrates are nonselective herbicides, meaning they will destroy anything they come in contact with, including wanted plants and soil organisms. So use caution when spraying near garden plants.
Despite these limitations, vinegar products, especially the sprays, are extremely useful for spot-killing difficult weeds, including invasive ones, in a garden bed. They are also terrific for clearing up weeds in paving cracks, such as between bricks or stones and on gravel driveways. They don't leave any residue on concrete or stone but may leave a temporary stain on bricks. In addition, vinegar products are safer for the environment than traditional weed-control chemicals because they affect only the soil they come in contact with and don't leach into the groundwater. Furthermore, their effect is completely reversible at any time by amending the soil.
Even botanically derived herbicides like vinegar- and citrus-based products need to be used with caution, however. Like any herbicide or pesticide, they can be harmful to people, beneficial insects, nontarget plants, and wildlife that may be accidentally exposed to direct spray. Because of the high level of concentration of the vinegar in these products, it is a good idea to wear eye protection and gloves, and to avoid exposing the skin to the spray. Be sure to wash your hands well when you are finished applying one of these sprays.