Some strains can start flowering in veg mode, but it depends on the strain. Most don't start fully flowering until the switch to 12/12. The reason is that the dark signals the plant to start growing flowers. So you may see some pre-flower looking stuff, but once you switch to 12/12 you will see flowers. You do have to be patient though, it takes some time after 12/12 to really see any buds forming. Hope this helps!