Beneficial weed
A beneficial weed is an invasive plant that has some companion plant effect, is edible, contributes to soil health, adds ornamental value, or is otherwise beneficial. These plants are normally not domesticated. However, some invasive plants, such as dandelions, are commercially cultivated in addition to growing in the wild. Beneficial weeds include many wildflowers, as well as other weeds that are commonly removed or poisoned. Certain weeds that have obnoxious and destructive qualities have been shown to fight illness and are thus used in medicine. For example, Parthenium hysterophorus native to northern Mexico and parts of the US has been an issue for years due to its toxicity and ability to spread rapidly. In the past few decades, though, research has found that P. hysterophorus was "used in traditional medicine to treat inflammation, pain fever and diseases like malaria dysentery." It is also known to create biogas that can be used as a bioremediation agent to break down heavy metals and other pollutants. These are erroneously considered to compete with neighboring plants for food and moisture. However, some "weeds" provide the soil with nutrients, either directly or indirectly. For example, if they are colonized by certain bacteria (most commonly Rhizobium), legumes such as white clover, they add nitrogen to the soil through the process of nitrogen fixation. These bacteria have a symbiotic relationship with the roots of their host, "fixing" atmospheric nitrogen by combining it with oxygen or hydrogen to make the nitrogen available to the plant as NH4 or NO3. Others use deep taproots to bring up nutrients and moisture from beyond the range of normal plants so that the soil improves in quality over generations of that plant's presence. Weeds with strong, widespread roots also introduce organic matter to the earth in the form of those roots, turning hard, dense clay dirt into richer, more fertile soil. Some plants like tomatoes and maize will "piggyback" on nearby weeds, allowing their relatively weak root systems to go deeper.
