Lye, also known as sodium hydroxide, has many uses and is more typically found in drain and oven cleaners. It can also kill grass and weeds, though precautions should be taken.
Lye is a strong base. In the oven and drain cleaners in which it is typically found, lye has a pH of 13 or 14, making it extremely alkaline.
When applied directly to a plant or a weed, lye does serious damage to its living tissue. Death is the inevitable result. For those seeking a quick solution, lye can be beneficial.
Avoid pouring lye directly on the soil, as you can unintentionally kill or harm other plants growing in your garden. When poured directly on soil, it raises the pH of that soil, thus depriving plants from getting the nutrients they need to survive. Lye should thus be used carefully and in moderation as a plant and weed killer.