Once the flower is pollinated the petals change colour from white to pink, perhaps as a signal to pollinating insects to search for food elsewhere. Petals often wither early, but the sepals are persistent. When the ripe, green-coloured capsules first open, the seeds are in a glutinous mass. The seeds have an oily appendage, attracting ants which carry them to their nests. In this way ants are an important dispersal agent for the plant.
Western trillium seems to respond to favourable conditions in some years.
Size of the flower and fruit are greater, and more seed is produced, when conditions are just right. In hot weather, the plants may die back and disappear. A dwarf form grows on the west coast of Vancouver Island. Also, a double-flowered variety, with multiple sets of petals, has been recorded. A phenomenon known as 'phyllody', where the petals and sepals revert to leaves, has been known to occur in trilliums.