The floristic richness of the reserve is unique with more than 50% of the entire flowering plants of Kerala, that includes 149 species listed under various threat categories. The degree of endemism and species richness is very high.

Orchids found here constitute more than 65% for the entire Kerala. Of the 1966 species of flowering plants reported from Periyar Tiger Reserve, 515 (26%) are Southern Western Ghat endemics.

ANGIOSPERMS

Orchids – 145 Sps. Habenaria rariflora,
a rare species
Grasses – 168 Sps.
Legumes – 155 Sps.
Balsams – 28 Sps.
Palms – 10 Sps.

Out of the estimated 3800 species of flowering plants of Kerala the tiger reserve has 1966 species which is the highest record from a protected area of the state. The Angiosperms include 1441 species of Dicots and 525 species of Monocots. The Dicots are represented by 613 genera spread over 128 families, while the Monocots are distributed under 210 genera belonging to 23 families. In all, the Angiosperms represent 823 genera under 151 families. Poaceae (Gramineae) with 168 species under 76 genera is the largest family of the tiger reserve. Impatiens with 28 species forms the largest genera of the reserve.

Orchids
Orchidaceae is the third largest family among the Angiosperms of the reserve. It is represented by 145 species under 60 genera that include 53 endemics.
Oberonia with 17 species is the largest genera. The presence of 145 species of orchids out of the reported 216 species from Kerala is the highest number from a district in the state.
The reserve is the only known home for the ground orchid, Habenaria periyarensis.
Among the rare and endangered species are, Vanda thwaitesii (a Sri Lankan species) Taeniophyllum scaberulum (a species formerly considered as possibly extinct), Pecteilis gigantea and Bulbophyllum macraei.