HPS on seedlings... Is it the heat or lumens thats bad?

Or a little of both? Im working with only a 400 but im afraid to put it too close to my seedlings. They seem to be stretching though so not sure what to do. My light is sealed and cooled and placed about 30 inches away right now. My thermometer at the surface reads 75. tia.

that sounds like a good distance. you don't want that hps too close to the sprouts beaming red light at them. heat is not an issue if you can control it. its all in the spectrum that hps throws out. and yes, they stretch like crazy veggin under hps.

liike the above poster stated.. its not really the heat or the lumens you have to look at.. hps bulbs put out light that is more in the red spectrum which is better for flowering than it is for vegging, and mh bulbs put out more of a blue spectrum which is more for vegging than it is for flowering, but people use both hps for vegging and mh for flowering with good results both ways..r
i think that you are getting some stretch as 30 inches sounds a little on the far side to me... as long as you have some good circulation, i would think it is safe to drop that baby about half of that distance... 18 to 24 inches or there about... hold your hand above the plants for a few minutes and if it gets too hot for your hand, than you know it will also be too hot for your plants.. if your able to hold your hand there for an extended period of time, than you know you are in..

mjr99 said:

Or a little of both? Im working with only a 400 but im afraid to put it too close to my seedlings. They seem to be stretching though so not sure what to do. My light is sealed and cooled and placed about 30 inches away right now. My thermometer at the surface reads 75. tia.

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what kind of ballast are you running? if its a digi throw a MH in that mofo. treat those ladies right.

PainWrek said:

what kind of ballast are you running? if its a digi throw a MH in that mofo. treat those ladies right.

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mh bulbs scare the shit out of me after I had a htg bulb catch fire last year. it was partly my fault because i cold started it, but still, im afraid if i get another one and the power flickers or something then i could be in trouble.

yea,my ballast is a htg digital greenhouse. its funny though, when i returned my burnt up bulb and asked for another mh they instead sent me their priciest hps bulb. not sure if it was an accident or if theyve had other mh bulbs do the same thing and they didnt want it to happen to me twice. Im gonna see how this goes and if they stretch too much i may do cfl's for veg next time. thanks for the responses.

how old are they? I had seedlings under a 250 HPS and I was worried about it too, but they are looking great. I have the light about 6-7 inches from them now at 5 weeks..

cowell said:

how old are they? I had seedlings under a 250 HPS and I was worried about it too, but they are looking great. I have the light about 6-7 inches from them now at 5 weeks..

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5 days old.

I think if i moved the light down to 15- 20 inches I could keep the heat in check, but im worried about the rays. The plan at the moment is to move it down 1 inch per day until I get to 12" or the heat gets over 88.

mjr99 said:

5 days old.

I think if i moved the light down to 15- 20 inches I could keep the heat in check, but im worried about the rays. The plan at the moment is to move it down 1 inch per day until I get to 12" or the heat gets over 88.

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I also have a 400 w hps and have my week old plants about 20 inces away and they are doing fine. I think I could even get away with going a bit closer. Btw its not cooled but I have a fan pointed directly at it which def. helps my closet stay cooler. The rays wont effect the plant only the heat will. Even 85 is too hot I think and will probably slow your growth. I think 12 may be a bit to close. My plants were getting lenghty at first but now they seem to be putting their energy into creating more leaves and the leaves are becoming thicker.

For my newly germinated seeds I have put them off to the side of the light so the soil doesnt dry out too quickly.

Just use some cheap CFL's for your seedlings until they get a bit of size, then switch them to a higher power light. Its a good way to keep from frying them (whether the issue be heat or lumens), and it may help fight the stretching a bit (at least until you switch them). CFL's are good at making short internode length...but often fluffy buds, so just use them until you feel you've hit a safe zone for their size, then switch them over.

You'd be good at 15 inches.. I think your plan to move it down gradually is good, I would likely do the same thing.
the optimum range for your 400 is anywhere between 6-21 inches. obviously the closer the light the more intense the light. No closer than 6 though.

If it's too small HPS' will cause too much transpiration and kill the plant (litterally drying it out). It's best to use weaker lighting for seedling till they are about 2-4 weeks old (this obviously depends on the size of the plant).

Ronjohn7779 said:

If it's too small HPS' will cause too much transpiration and kill the plant (litterally drying it out). It's best to use weaker lighting for seedling till they are about 2-4 weeks old (this obviously depends on the size of the plant).

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Although im not the expert in this field at all...im going to have to say i don't think this can be an entirely true statement, perhaps using CFLs could be SAFER option for seedlings as noobs like myself could potential kill a baby, but if done correctly HPS should work fine from seedling to finish, to many other posters in this thread and others who clearly state they have healthy seedlings running under 400wHPS at optimal ranges of 15"+ and lower watts like the 150s under 12" or so and seem to be doing just fine.