Damp vermiculite in closed plastic tub in warm closet, afternoon of 5th Feb.
Day 1, nada. Day 2, nada. Day 3, 2+ cm tap root, too long to slot into rockwool so ended up digging a large hole and clamping small cube straight into larger one, and then straight into DWC basket.
Rockwool had been soaked for weeks in 2ml/litre Ionic Hydro Grow solution.