Trillium petiolatum (round leaf trillium, Idaho trillium) is a species of Trillium native to the Western United States; particularly Idaho, Oregon, and Washington. It is somewhat unique among Trillium species as it has round petiolate leaves and flowers close to the ground as opposed to most other Trillium species which flower above the leaves. The plant is small, measuring just a few inches tall. It generally flowers from early April to late May and its flower is typically dark maroon to purple in color. It can be found near stream beds, under scrub brush, and near the edges of forests. Like most other trillium varieties in the Pacific Northwest, this variety prefers acidic soil.
- Scapes: 4-17 cm long, terete.
- Leaves: petiolate.
- Blades: 7-14 cm long x 5.5-10.2 cm wide, not mottled.
- Flower: sessile.
- Sepals: 22-47 mm long x 7-10 mm wide.
- Petals: 30-55 mm long x 4-10 mm wide, red, maroon, purple, green or yellow, not spirally twisted.
- Anthers: 16-20 mm long, with latrorse dehiscence.
- Ovary: 4-9 mm long.
- Stigmas: 7-20 mm long. 
- "Trillium petiolatum". Archived from the original on June 25, 2010. Retrieved February 16, 2010.
- http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=2421020 Retrieved February 16, 2010.[dead link]
- Wallace W Hansen. "Native Plants of the Northwest". Archived from the original on December 15, 2008. Retrieved February 16, 2010.