[3][4] Occasionally individuals have four-fold symmetry, with four bracts (leaves), four sepals, and four petals in the blossom.[7][better source needed].

The tetramerous condition has been described for several species of Trillium including T. chloropetalum, T. erectum, T. grandiflorum, T. maculatum, T. sessile, and T. undulatum.

[12] As it became clearer that the very large version of Liliaceae was polyphyletic, some botanists preferred to place Trillium and related genera into that separate family.

Others defined a larger family, Melanthiaceae, for a similar purpose, but included several other genera not historically recognized as close relatives of Trillium.

This latter approach was followed in 1998 by the Angiosperm Phylogeny Group, which assigned the genus Trillium, along with its close relative Paris, to the family Melanthiaceae.

[13] However, other taxonomists have since preferred to break up the heterogenous Melanthiaceae into several smaller monophyletic families, each with more coherent morphological features, returning Trillium to a resurrected Trilliaceae.

[14] In 1850, German botanist Carl Sigismund Kunth segregated Trillium govanianum Wall.

[20] As of February 2022[update], Plants of the World Online (POWO) accepts 49 species and 5 named hybrids,[2] all of which are listed below.

Phyllantherum has been shown to be a monophyletic group, but its segregation renders the remaining Trillium subgen.

[34][39][40][35][41] This leads to a four-part concept of Trillium that sharply contrasts with the traditional pedicellate vs. sessile dichotomy outlined previously.

[38] Species in this subgenus have pedicellate flowers (on a stalk) with three distinct stigmas (no style) and solid green leaves (not mottled).

[61] Species in this subgenus have pedicellate flowers (except for one variety of T. pusillum) with a definite style and solid green leaves (not mottled).

Just six species are native to western North America: T. albidum, T. angustipetalum, T. chloropetalum, T. kurabayashii, T. ovatum, and T. petiolatum.

In the western United States, species are found from Washington to central California, east to the Rocky Mountains.

In the eastern United States, species range from Maine to northern Florida, west to the Mississippi River valley.

In Asia, the range of Trillium species extends from the Himalayas across China, Korea, Japan, and eastern Russia to the Kuril Islands.

In eastern North America, jack-in-the-pulpit (Arisaema triphyllum) is often mistaken for bare-leaved Trillium.

Both species are about the same height with trifoliate leaves but the former lacks 3-way rotational symmetry and has leaf veins unlike those of Trillium.

For example, the seeds of Trillium camschatcense and T. tschonoskii are collected by ant species Aphaenogaster smythiesi and Myrmica ruginodis.

[81] Sometimes beetles interfere with the dispersal process by eating the elaiosomes, which makes the seeds less attractive to ants.

Yellow jackets are documented seed dispersers for at least three species of Trillium (T. catesbaei, T. cuneatum, T. undulatum).

[83] As of February 2022[update], Plants of the World Online recognizes five named hybrids,[2] four in Asia and one in North America.

[53] The only named hybrid in North America is T. × crockerianum whose type specimen was collected in Del Norte County, California.

[85] Various Trillium species are susceptible to a greening disorder caused by bacterial organisms called phytoplasmas that alter the morphology of infected plants.

In 1971, Hooper, Case, and Meyers used electron microscopy to detect the presence of mycoplasma-like organisms (i.e., phytoplasmas) in T. grandiflorum with virescent petals.

[90] This subgroup of phytoplasmas is associated with various other diseases, including milkweed yellows, Vaccinium witches' broom, and potato purple top.

Laws in some jurisdictions may restrict the commercial exploitation of trilliums and prohibit collection without the landowner's permission.

This is particularly dire in the case of T. govanianum, whose high selling price as a folk medicine has motivated harvesters to destroy swathes of ecologically sensitive Himalayan forests, causing mudslides.

[101] In light of their shared connection to the flower, the Major League Soccer teams in Toronto and Columbus compete with each other for the Trillium Cup.

[103] In the 1990s, the activist Michael Page established the use of the trillium as a symbol of bisexuality,[104] and in 2001, Francisco Javier Lagunes Gaitán and Miguel Angel Corona designed a Mexican variant of the bisexual pride flag, which is emblazoned with an emblem of a trillium.