What is the significance of lilac in "When Lilacs Last in the Dooryard Bloom'd"?

Whitman uses the lilac as a metaphor for both death and renewal.
Since President Lincoln was assassinated in spring, in April, Whitman associates his vision of the flowers with the time when he and the nation mourned Lincoln's death. The whole meaning of the poem is encapsulated in the first six lines, in which the other principal image is presented: the "western star." This, of course, symbolizes Lincoln himself, the Westerner (at that time what we now know as the "Midwest," Lincoln's home, was simply "the West") who saved the nation.
Though Whitman himself was a free thinker, in those lines the metaphor of Lincoln as a Christ figure is developed as well: "Ever-returning spring, trinity sure to me you bring, / Lilac blooming perennial and drooping star in the west, / And thought of him I love." The lilacs, which the poet knows will die but will bloom again and again, are a sign of perpetual renewal, just as the crucifixion and resurrection of Jesus are—which occurred in the same season, spring, as Lincoln's death did.
The lilac is also what Whitman regards as a gift he wishes to present to the dead, not only to Lincoln but to the soldiers who have died in the war. Similarly, the "hermit thrush" with whom Whitman identifies is singing a death song, and a song of life—representing destruction and renewal, just as the lilac does. It is as if Whitman wishes the bird's song to convey his own spirit in order to mingle with the dead: " . . . for the dead I loved so well. . . . Lilac and star and bird twined with the chant of my soul. . . . "
In his other elegy on Lincoln, "O Captain, my Captain!" Whitman gives another presentation of these symbols: "for you the bugle trills," just as the warbling of the thrush does, "for you bouquets and ribbon'd wreaths": the signs of renewal like the lilac. These are more conventional tokens for the dead, just as this poem is more traditional in format, and therefore became more immediately popular, than "When lilacs last in the dooryard bloom'd." Yet the two poems are equally powerful in their portrayal of Whitman's personal grief, as well as that of the nation, which still resounds into our own time.