I put mine under light from the first day that they were in the cubes(no particular reason)...in less than 30 days I had roots and leaves on 42 out of 50 cuttings...and now 14 are transfered into 1gal. pots with no adverse effects...4 or 5 more to be up potted today...I have heard that other members leave the tray in a warm, dark location for a couple of weeks with good success also...I guess I just like walking into my office each day to see them all lit up.