If they're the small viola types then fairly close - 2"/5cms - or a bit more if they're the larger ones. You need the foliage to join up to cover the compost and reduce wind erosion and drying so check the ultimate size and use that as your planting distance.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
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