It's a good thing that Louise Glück entitled this poem "The Wild Iris." Without the title, readers would really be up the creek without a paddle, since the poem itself contains no recognizable references to a flower. But once you connect the title to the lines that follow, you can start unraveling the riddle of the poem.
On a literal level, the title refers to a plant that has tall slender stalks, long pointed leaves, and brightly colored blooms One of the most common types of wild iris is the blue flag. The plant prefers full sun and moist soil.
The wild iris is a perennial plant, which means that it reseeds itself, blooming spring after spring without being replanted. This fact relates to the poem by creating associations of spiritual as well as physical rebirth.
The word "wild" adds another ingredient to the poetic pot. The fact that the wild iris is uncultivated (that means it's not planted or harvested by humans) lends it a certain independent authority as being in Nature with a capital N. Other meanings of the word "wild"—rough, desolate, stormy, violent—convey whiffs of mystery and danger that foreshadow the poem's exploration of suffering and death.