Hand weeding is time consuming and—let's face it—hard work. Chemical sprays and granules work, but may not be safe for humans or animals. Vinegar works too, but is difficult to find in strengths strong enough to quickly kill weeds. But if you've ever noticed that plants in an area where humans or animals urinate seem to wither up and die, you may wonder if urine—plentiful and safe—is a decent weed killer.
Many people, especially pet owners, notice that where dogs urinate, grass often dies. Bushes that dogs favor for urination may also eventually die. This has led to the misconception that dog urine is acidic and the acid in their urine makes a great weed killer. However, dog urine—as well as most animal and human urine—isn't very acidic at all.
Although not acidic, there are other components in urine—especially dog urine—that do kill plants and give it potential as a weed killer. According to Steven Seefeldt of the MadSci network's Agricultural Science department, urine is often high in nitrogen. This is why it sometimes kills plants.
Urine is an ancient fertilizer; in fact, Finnish researchers are reviving the practice of using urine to give necessary nutrients to plants. It is only with repeated application of urine that plants begin consuming so much nitrogen they turn yellow and die.
"A little urea is very good for plants. … As with most chemicals, toxicity is related to the concentration. A little salt on the french fries is very good, [but] a couple of large glasses of very salty water will kill you very quickly," writes Seedfeldt. Urine is only a plant or weed killer if applied in concentrated levels. Since it is difficult to train an animal to urinate on selected weeds, the trick is collecting the urine so a human can apply it as desired. It's easier to collect human urine, but in most cases that urine is not as high in nitrogen (primarily because we eat less meat, say veterinarians Foster and Smith). Therefore, human urine is a less-effective weed killer than dog urine.
The Amherst Small Animal Hospital offers tips for collecting dog urine, but none are especially convenient. For female dogs, they recommend placing an aluminum pie tin under the animal's urine flow. For less mess, hold onto a broom handle tied to the tin with wire. For male dogs, they suggest straightening a wire hanger and tying it around a plastic cup; again, a human must place the cup in the dog's urine flow.