Iris hollandica

Impervious seed coats. Shake in dry sharp sand, nick with file, rub on a bit of sandpaper (or between two sheets), anything to weaken the seed coat and allow moisture to begin to penetrate.

Warm, cold, cool germinator - 3 steps. Sow 70ºF for 2-4 weeks, move to 40ºF for 4-6 weeks, move to 50-ishºF for germination

While I haven't sown the Dutch, I find most iris to be erratic, meaning not all seedlings will appear at the same time. Or even close, I've had pots of iris seed where germination continued over an entire year after the first seedlings popped up :) Hopefully yours won't require as much patience