Members of the genus are similar to those of the genus Alectoria.:148 A test for the different is that the branches of Usnea are somewhat elastic, but the branches of Alectoeria snap cleanly off.:148
Many species have been described. A three-volume series by Józef Motyka published in 1936 and 1947 distinguished 451 species. Many of these are now regarded as morphological varieties and adaptations to local circumstances. The taxonomic categorization of many members of this genus remains uncertain. The number of recognized species in Finland is decreasing for this reason, from 34 in 1951 to 25 in 1963 and only 12 in 2000. It is now noted as including more than 600 species and being one of the largest genera within the Parmeliaceae. The species Usnea longissima was renamed Dolichousnea longissima in 2004.
Like other lichens, Usnea often grows on sick or dying trees due to the pre-existing loss of canopy leaves, allowing for greater photosynthesis by the lichen's algae; this leads some gardeners to mistakenly blame the lichen for the tree's leaf loss and illness.
Usnea is very sensitive to air pollution, especially sulfur dioxide. Under bad conditions they may grow no larger than a few millimetres, if they survive at all. Where the air is unpolluted, they can grow to 10–20 cm long.
Prescientific medical systems included use of Usnea in the belief it had healing effects for at least 1600 years.
Usnea was one ingredient in a product called Lipokinetix, promoted to induce weight loss via increase in metabolic rate. Lipokinetix has been the topic of an FDA warning in the USA, due to potential hepatotoxicity, although it is unclear yet if any toxicity would be attributable to the Usnea. Lipokinetix also contained PPA, caffeine, yohimbine and diiodothyronine.
There is reason to believe that Usnea, in high concentrations, could possess some toxicity. The National Toxicology Program is currently evaluating the issue.
Usnea species have been used to create orange, yellow, green, blue and purple dyes for textiles.
Usnea barbata has been used in cosmetic production for its antimicrobial and antifungal properties as a preservative and deodorant.
^Batty, Lesley C., & Hallberg, Kevin B., ed. (2010). Ecology of Industrial Pollution. Ecological Reviews. Cambridge University Press. p. 49. ISBN9780521514460. Retrieved 4 December 2012. Usnea spp., at one time widespread and luxuriant, almost entirely disappeared from a major area of England and Wales covering at least 68 000 km² and at least 6 000 km² of lowland Scotland, mainly as a result of the increase in atmospheric pollution.
^"Safety Alerts for Human Medical Products > Lipokinetix". MedWatch: The FDA Safety Information and Adverse Event Reporting Program. U.S. Food and Drug Administration. November 20, 2001. Retrieved 5 December 2012. FDA has received multiple reports of persons who developed liver injury or liver failure while using Lipokinetix. The product contains norephedrine (also known as phenylpropanolamine or PPA), caffeine, yohimbine, diiodothyronine, and sodium usniate.
^Jellin, JM; Gregory P., Batz F., Hitchens, K., et al. (2000). "USNEA". Pharmacist's Letter/Prescriber's Letter Natural Medicines Comprehensive Database (3rd ed.). Stockton, CA: Therapeutic Research Facility. pp. 1048–1049. ISBN0967613647. Adverse reactions are uncommon in appropriate amounts. Poisoning can be possible, although signs of poisoning have not yet been described.Cite uses deprecated parameter |coauthors= (help);|accessdate= requires |url= (help)