Brassica seed appear at the end of the first growing season, and should be collected before the rains set in or the seed is spilled onto the ground by hot winds.
The seeds rattle in the pod when dry.
The simplest method is to strip them from the plant while it is still in the ground, straight into a bucket by dragging the dry stalks through the hand.
Alternatively, break off the dry stems holding the siliques, and bang them against the side of a deep bucket to shake the seed loose.
Then just throw away the dry seed husks.
Many other seeds can be collected in the same fashion; this bucket full contains parsley seed as well as various brassicas.
Zone 1 - Cool
February , March , September , October , November
Zone 2 - Temperate
March , April , May , August , September , October
Zone 3 - Subtropical
March , April , May , June , July , August , September , October , November