How tall should seedlings be before HPS lighting?

I am gonna start a set of 4 plants

2x feminized White Diesel
2x Auto-AK

I am going for an 18hr light cycle for 2 months to veg out the Diesel and let the Auto do its thing, when its harvested my diesels should be ready for 12/12 and have more room since the AKs are gone.


Anyways, I am gonna be using some tube lighting I bought at a grow shop to grow out my 4 seedlings and I have a 400w HPS bulb and how do I know when they are strong enough to use it over the seedling lamp?

my room is self cooling so no worry on that but how high up should my bulb be from the tips of my plants?


Thanks Ya'll

3 foot over the plants, and i would say 4 inches tall with a set 2 sets of fan leaves (the traditional mj leaf)

As soon as they have sprouted, just keep the light 2-3 ft away and if they seem to be stretching lower the light. All this two node 4 leaf bla bla is BS! Lights are replicating the sunshine, in their natural habitat weed doesnt grow with a little parasol that it gets rid off when its big enough to handle the equatorial sunshine.lol

Yea I don't wanna stretch my seedlings. I think I did that in my last grow (This is only my 2nd one) and I used CFLs for the first month and they stretched alot. Kind of a poor grow.

I got the HPS now and don't wanna burn them

cektor: I put my WW seedlings ona 400 HPS as soon as most seeds sprouted...around 20 inches from the tops. Check out my grow journal, I have pics in there. Enjoy.