The first line of the poem sets the scene in late August and paints a tempestuous picture, as it describes the "heavy rain and sun" of late summer storms. The rain and sun are needed, however, as the word "given" emphasizes. That is, the blackberries ripen only if there's enough sun and rain, and would presumably fail to ripen without these elements. This emphasis hints at the way that growing up, which will soon be revealed to be a key theme of the poem, requires a certain degree of strife, just as the blackberries need harsh weather to ripen. The first line starts with unstressed-stressed iambic pentameter, which means that there are five poetic feet per line with a da-DUM rhythm:
Late Aug- | ust, giv- | en hea- | vy rain | and sun
This steady meter mirrors the rhythmic turning of the seasons, further highlighting the passage of time in the poem.
The second line builds a sense of suspense with the words "For a full week," as the word "full" implies a sense of anticipation and waiting. The eager blackberry-pickers must wait the full seven days, to the very end of the week, for the berries to ripen. The alliteration ("For a full") and assonance used ("glossy purple clot") create round, robust language that builds up this sense of promise. The speaker uses past tense here ("would ripen"), making it clear that this is a memory being reflected upon from the perspective of an older person.
The first four lines also establish the poem's A/A/B/B rhyme scheme ("sun"/"ripen" and "clot"/"knot"). This is carried throughout the poem, although often in the form of slant rhyme. The poem is full of visual imagery, exemplified in lines three and four as the speaker describes the "glossy purple clot" of a ripe berry and contrasts it to "the others, red, green, hard as a knot." These bright, vivid images bring the stanza to life and emphasize the lively, hopeful atmosphere of the children's berry-picking. A simile is used to describe the texture of the unripened berries, "hard as a knot" (like a knot in a person's back, contrasting with the "just one" ripe berry in line 3. The use of caesura, in the commas around "just one," further sets apart this one ripe berry and heightens the anticipation around eating it, which the speaker will describe in the next line.