In a way, the whole poem is an allusion to a tragic real-life event: the death of Eric Garner at the hands of the NYPD. The poem assumes that its readers will know about Garner's story beforehand (or can easily look it up), though this background knowledge isn't strictly necessary. The speaker makes a point of not directly alluding to Garner's death, instead focusing on something more positive about his life—that is, until the last line, which pointedly echoes Garner's last words: "I can't breathe."
This allusion makes the poem's subtle irony explicit: Garner was denied all the things that, in their small and humble way, these plants provide for the world. It portrays the simple act of planting as positive and life-affirming, while implying that the actions of the NYPD, and the officer who killed Garner in particular, were the opposite. The former action improves the world (even at a micro level), while the latter added to the world's tragedy and violence.
The allusion also situates the poem within the wider anti-racism movement. "I can't breathe" has become a rallying cry against police brutality and systemic racism. By echoing it, the poem affirms the commitments of that movement, while showing that Eric Garner was a human being, not just a news story.
Finally, the word "Needful" in the title likely alludes to Robert Hayden's poem "Frederick Douglass" (1966), which calls liberty "needful to man as air" and salutes Douglass's legacy as an abolitionist:
[...] the lives grown out of his life, the lives
fleshing his dream of the beautiful, needful thing.
This subtle callback highlights the way Garner, too, left a legacy that "continue[s] to grow" (line 9). It places his death in the context of a larger struggle for freedom, even suggesting that understanding him is as "needful" (necessary) as freedom itself. (Hayden wrote "Frederick Douglass"—which Gay has called a "masterpiece"—as a politically engaged Black American poet during the Civil Rights era, so the echo here seems fitting on multiple levels.)