When to switch light types

How do I know when exactly to switch from florescents to HPS?

use both throughout the whole cycle

I agree you should use both for mixed spectrum. Like the sun. But if your talking about knowing when to go from vegging to flowering that's up to you, when your happy with the plant size you should go to 12/12.

or when it shows preflowers
then you know its sexually mature. Should show on the top nodes between the fan leaves and offshoots

Thats not exactly what i am asking. While they have been babies they have been under florescents. How do I know when its ok to start with the HPS without damaging them... burning them up I guess is what i am thinking.

Monitor your temps. I would have the light 18-24 inches from the canopy of your plants if it's a HPS. Have them fans running.

-Hank.

i have sprouts under 250 watts of MH.

sorry misunderstood
I moved some from flouro to HPS recently while in vegg and the growth stopped for a day or so but they picked right back up after that.