[8] Trillium ovatum is a perennial herbaceous plant that spreads by means of underground rhizomes.

Typically the flower opens white and becomes pink with age, but in the Smith River Canyon area of northern California and southern Oregon, the petals become almost barn-red.

oettingeri by the American botanists Philip Alexander Munz and Robert Folger Thorne in 1973.

[12][13] The subspecific name oettingeri honors Frederick W. Oettinger who collected the type specimen in July 1967.

[16][17] In 2024, the taxon was recommended at species rank based on morphological data and phylogenetic analysis.

[21] The epithet stenosepalum means "narrow-sepalled", the primary difference between it and typical Trillium ovatum.

[24] Trillium ovatum f. maculosum Case & R.B.Case refers to a form with mottled leaves that occurs in northern California.

It also occurs in the Rocky Mountains from southeastern British Columbia and the tip of southwestern Alberta, southward through Idaho, eastern Washington and northeastern Oregon, and southeastward through western Montana.

[7] At Lolo Pass, Montana, it grows under spruce and Douglas fir in ravines along mountain streams.

[34] It does not grow in the dry chaparral inland from the redwood and Douglas fir forests of California and Oregon, nor on ridges where the trees are sparse.

[37] More recently, Wayman et al. (2024) concluded that the taxon "occurs in the Rocky Mountains" of Idaho, Montana, Washington, and Colorado, with the caveat that "additional field and lab work are necessary".

[38] Flowers bloom late February in the southern part of its range, and in March or April elsewhere.

Apparently T. ovatum lacks sufficient winter hardiness to flourish east of the continental divide.

[44][45] All known occurrences of variety oettingeri fall within protected areas in northern California but since the effects of wildfire and climate change are unknown, continued monitoring of existing populations is recommended.