The use of Polygonum species (such as Polygonum cuspidatum, also known as Japanese knotweed, or Polygonum aviculare, commonly called knotgrass) for influenza is primarily based on traditional medicine, especially within East Asian herbal systems. In Traditional Chinese Medicine (TCM), various Polygonum species have been utilized to "clear heat," "remove toxins," and manage symptoms associated with respiratory infections, including influenza-like illnesses. For example, Polygonum cuspidatum is included in some classical formulas intended for febrile diseases and is believed to possess antiviral and anti-inflammatory properties according to TCM theory.

Scientific research on Polygonum species reveals that certain constituents, such as resveratrol and polydatin (notably from P. cuspidatum), exhibit antiviral activity in vitro against a range of viruses, including influenza viruses. However, robust clinical trials in humans demonstrating efficacy for the prevention or treatment of influenza are lacking. Most of the available evidence is preclinical, involving cell cultures or animal models. Thus, while there is a plausible biochemical basis for potential antiviral effects, the level of scientific validation remains limited.

In summary, the use of Polygonum for influenza is justified mainly by tradition, with some supportive but insufficient scientific exploration. The overall quality and quantity of evidence do not meet modern clinical standards, placing the evidence at a 2 on the scale.