It's a poem about mushrooms, so… it's entitled "Mushrooms." The end.
J/K, Shmoopers. There's a little more to it than that (you knew there would be). Though the speaker spends the whole poem describing mushrooms steadily pushing their way out of the soil, the whole thing is really an extended metaphor for the oppressed. It compares those humble, little fungi to groups, like women, who have been historically suppressed and disempowered. So, the title of the poem basically sets us up for the symbolism to come. Notice that nowhere in the actual poem is the word "mushroom" found. By making this word the title, the speaker has more room to be subtle in the body of the text. All the while we're learning about these poor, quiet, oppressed, but rising and resilient folks in the poem, the title gives us a solid baseline for comparison. Sure, these mushrooms may be downtrodden, but they're set to rise up. Pluck them at your peril.