Subject matter of a poem, or the theme, can differ based upon an individual reader's interpretation. That said, most critics, teachers, and students tend to agree when a universal theme is identified within a text. The subject matter of Seamus Heaney's poem "Blackberry Picking" is not as simplistic as the title suggests. Instead, multiple themes weave their way throughout the poem. 

Want/Need/Desire

The narrator of the poem is speaking in nostalgic terms regarding his memories picking blackberries as a child. When younger, the children would pick all of the berries, not allowing any of the ripe berries to be left behind. Sadly enough, the berries would not be eaten soon enough, and the berries would rot. This illustrates the children's need, want, and desire to pick all of the berries. Tempted by their sweet flesh, none could be left on the bushes. The greed of the boys insured that the berries went to waste and none were left for later. 

Disappointment

As part of the children's removal of all of the berries on the bushes, the rotting of the uneaten berries brought about feelings of disappointment. Although the narrator wishes that the berries would keep forever off the bushes, he "knew they would not." The narrator felt utter disappointment when the berries rotted, even though he knew they would rot each year.