In this poem, the title is not separate from the main text: it's the beginning of the poem's one long sentence. The "Small" signals that what follows is—compared with the storm of events surrounding Eric Garner's death—modest and gentle (qualities the poem then attributes to Garner's work as a gardener). "Needful" suggests both that there is a need to remember the human being behind the story and that the actions described in the poem are themselves necessary and important.
The small, needful fact about Garner—which Ross Gay learned from an obituary—is that he:
for some time for the Parks and Rec.
Horticultural Department [...]
That is, he was a horticulturist, or gardening specialist, for the Department of Parks and Recreation (in New York City, where he lived).
This fact is stated plainly, without any poetic devices apart from the enjambment of the first two lines. If not for the line breaks, this could be the kind of spare prose found in obituaries. This understated style contrasts with the heated public language (e.g., in court and on social media) surrounding Garner's death. Its gentleness, in itself, seems intended as a quietly powerful act—like planting flowers in a garden. Amid the debate about his death, the poem seems to want to commemorate something "Small," yet important, about his life.
"Some time" is an important phrase here. It's carefully non-specific, because the poem doesn't presume to know anything definitive about Garner's life. It makes a cautious inference based on one "Small Needful Fact," and resists the temptation to turn Garner into a poetic symbol. This restraint still allows the "Fact" to take on symbolic properties, but in a way that seems considerate and attentive to Garner as a real person. By not knowing—or not saying—the precise details of Garner's work, the speaker subtly reminds the reader that the poem can only speak from a certain distance. Unlike some public commentary, the poem claims no authority over what Garner's life and death represent.