It's late August which means it's primo blackberry conditions – tons of rain and sun, and the blackberries are slowly starting to ripen. At first, a lot of them are still green and hard, but some are red (getting there) and a few are perfectly ripe and purple.
After the first ripe berry is eaten, it's so sweet that the blackberry-pickers have to have more. They get a bunch of buckets and whatever else will hold the berries, and set off to pick as many as they can.
They hunt everywhere – through hayfields, cornfields, and briars – staining and scratching their hands as they search. They fill the buckets first with the greener, less ripe berries, and then top them off with the ripest ones.
These blackberry-pickers store their stash in a cowshed, yet always, despite their best efforts at preservation, the berries start to rot. The juice starts smelling sour and some of the oldest berries grow furry mold. It's always disappointing for the speaker, who always hopes that all the berries will keep, even though he knows they will rot.