How old or tall should seedlings be before safely switching to HID lights?

A customer has a question and I hope we can get some opinions on it, thanks

My seedlings are growing, they are between 1 and 4 inches tall and are about 2 weeks old. I currently have them in a growing terrarium with florescent light. I have purchased an HID (MH / HPS) lighting system.

MH is for veg and HPS is used more for flowering. You can germ seeds under an HID lighting, but you need to keep the light pretty far. I say start it around 36" and lower it if you see any unwanted stretch until it stops.


Since you have a germination station and have the seedlings growing there
You should wait until you need to transplant to larger containers before switching to bigger fixtures
@Aolelon has give good info on what lights to use and when
Posting a picture would help us guide you and joining the forum would be beneficial to your grow :wink:
Happy growing :v:CB

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I appreciate the advice. My first batch of seedling didn’t fare so well… out of 8, 3 remain and are growing ok (future post and pics). Pictured in this post are 6/12 of the new batch, planted in Peat Pellets 5 days ago (all 6 are bangi haze fem and reg… the other strains are still dormant within the Peat Pellets (white widow, hash passion, and fruit). With regard to the 6 seedlings in the picture, my concern is that they shot up extremely quickly and are not very strong. Today I supported them with pipe cleaners. So… my questions please, are: 1. do I concern myself with quick growth? 2. if so, how do I slow it down to strengthen stems? 3. Should I remove them from the terrarium and place them closer to T5 light source? I’ve read conflicting advice on how close seedlings should be from T5 lights. and 4. with regards to the ‘dormant’ seeds, should I keep them warm and in the dark until they sprout, and then introduce them to light? Thank you

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The seedlings can be within a few inches as long as the heat from the light is not too hot. Usually not a problem to have CFLs within 6 inches of the plants. I want my plants to stretch a little under the T5s so I usually keep the plants at least a foot away.