....The wives being often too poor to purchase bundles of rags individually would group together to purchase, clean, sort and sashiko stitch the rags together to make clothing, bedding and household fabrics. It is around this time that white cotton thread appears in sashiko stitching.

How does symbolism in design happen?

...I think its likely that women doing this stitching found it more interesting if they based the designs on things that had meaning for them. This would account for why designs that represent the sea and fishing are found on the coastal fishing peoples clothing (diamond waves for example) and designs having to do with crops (plowed fields, windblown grasses) are found on the clothing of the inland farming peoples.

Wonder about the fabric sashiko was originally stitched on,
and how the designs were transferred to the cloth?