Those with the tiniest tips of the true leaves showing and roots out of the bottom of the cube are ready to be air pruned. Air pruning details/reasons in the comments

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   Andrew_Higginbottom 3 years ago

   Air pruning is where you raise them up so air flows underneath causing the exposed roots to die back to the edge of the rockwool cube. The plant knowing it can’t grow those roots any further starts pushing out new ones ..until those reach the edges of the cube and tips die back. On and on it does this until the cube is full of roots. Its the plant mapping out what it has to work with.
   Once the cubes are full of roots, I then put them in Kratky style containers, the contained root ball bursts out from all sides of the cube and down into the nutrients. It usually takes about a week for the air pruning to happen. A week lost now but very much a pay back through the plants life.
   Each year I say I will buy a grid for this ..and every year I forget