The poem is available at the Poetry Foundation. A Slideshare presentation makes the unsourced claim that the trees mentioned were cut down in the "early 1920s". However, the level of grief expressed within the poem suggests that another event might have influenced the emotions expressed within the poem. I suspect (without evidence) that her sister's predeceasing her by a short period could possibly have influenced these emotions.
Despite this, no source appears to explicitly date the poem, hence the question.