Description - from Wikipedia

Cymbopogon nardus, common name citronella grass, is a species of perennial aromatic plant from the family Poaceae, originating in tropical Asia. C. nardus cannot be eaten because of its unpalatable nature and is an invasive species that renders pastureland useless, since cattle will starve even in its abundance.
It is the source of an essential oil known as citronella oil, which is widely used for its natural insect-repelling properties. Essential oils are extracted from the aerial parts and applied topically or slowly burned (e.g. in a "citronella candle") as an insect repellent, particularly to deter haematophagous insects such as mosquitoes.

References
http://www.fao.org/AG/AGP/AGPC/doc/GBASE/DATA/PF000206.HTM Archived 2017-12-03 at the Wayback Machine

External links
Media related to Cymbopogon nardus at Wikimedia Commons