Nut grass (Cyperus rotundus), also known as "motha" or "purple nutsedge," has been used in various traditional systems of medicine, including Ayurveda and Traditional Chinese Medicine, primarily for its purported anti-inflammatory, analgesic, and antimicrobial properties. In the context of canker sores (aphthous ulcers), the historical usage is based on the belief that nut grass can reduce inflammation and promote healing of oral lesions. Some traditional remedies suggest using nut grass extracts or powders as a topical application or mouth rinse for mouth sores, citing its cooling and soothing effects on mucosal tissues.

However, the scientific evidence supporting the use of nut grass specifically for canker sores is minimal. While a few laboratory studies have identified antimicrobial and anti-inflammatory compounds within nut grass, there are no robust clinical trials or systematic studies demonstrating its effectiveness in treating canker sores in humans. Most of the available support comes from anecdotal reports and historical texts, rather than empirical research. As such, while nut grass is traditionally used in some cultures for oral health issues, including mouth ulcers, its efficacy for canker sores remains largely unproven by modern scientific standards.

In summary, nut grass's use for canker sores is justified mainly by tradition, with only weak supporting evidence and a lack of high-quality clinical validation.