How does Seamus Heaney present 'childhood optimism' in the Poem 'Blackberry Picking'?

Seamus Heaney uses the extended metaphor of blackberry picking in order to depict the wilting of childhood optimism. He considers the initial excitement of picking the first blackberries to be joyous and sensual. This is connotated by the use of rhyming couplets in the lines “at first, just one, a glossy purple clot” and “Among others, red, green, hard as a knot.” Rhyming couplets are common in Shakespearian sonnets and are usually positioned at the end of the stanza in order to emphasise the theme of the poem. These lines are present within the first stanza, with the perfect rhyme, repetition of the /l/ sound and the rhetorical device of listing resembling that of a child’s speech. This would lead to the creation of ‘optimism’, and indicate that childhood’s happiness comes from said immovable optimism. The second rhyming couplet at the end of the second stanza states otherwise with “That all the lovely canfuls smelt of rot” and “Each year I hoped they'd keep, knew they would not.” As well as the declarative nature of the lines, the rhyming of negation and a negatively connotated noun in “rot” evidences the influence of pessimism in the wake of adolescence. This also points to the fact the previous optimism held by the poet Heaney was that of his childhood memories, and in adult retrospect the inability to ignore the rotting of the blackberry batch signals his absence of idealism in adulthood.
Heaney’s application of pararhyme creates a sense of childhood optimism being sensitive to time. The off rhyme in the lines “We hoarded the fresh berries in the byre” and “But when the bath was filled, we found a fur” signal the duality of an adult mind and that of a child’s. The noun “berries” is modified with the adjective “fresh” in order to emphasise the raw nature of the experience, with the berries perhaps being a metaphor for a first love. The phonological parallelism, in the repetition of the /b/ sound, is representative of child’s playful thought process feeding the notion of duality. The adult mind focuses on how the berries began to rot, with the fur eluding to the fur like coat berries attain when they begin to sour. The off-rhyme indicates that the process is flawed, but only from the perspective of an adult Heaney. This creates a sense of duality throughout the poem, the initial line signalling the optimism of childhood and the second the retrospective adult consciousness. The second line signals how the optimism of the experience begins to fade as the later part of the picking process becomes more prominent as the child grows older and is subsequently more familiar with it.