Hair testing is a very poor method for detecting cannabis use at any range but all of the research indicates it is not effective in detecting infrequent use. 1, 2, 3
THC, and the main metabolite THC-COOH, do not incorporate to a great extent into hair. TCH-COOH is not highly bound to melanin. Hence, concentrations in hair are much lower when compared with other drugs of abuse. As an example, cocaine is incorporated into hair at a rate 3600 times that of THC.
While any given case can't be predicted the research is very consistent on this issue. Heavy smokers, daily chronic users are detected at the rate of about 60% and infrequent users are seldom if ever detected. 3
1. Musshoff 2006
2. Galanter & Kleber, 2011
3) Huestis, 2008