MH/HPS - best distance from plant?

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Aug 16, 2005
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Just got a new 400W MH/Agrosun set up. The light is positioned about two feet from the top of my plant. Is that about right? I read somewhere that this was an appropriate and safe distance.

Still, my 400W is relatively cool. And I'm used to getting my plant much closer to flos. Is it possible that at two feet, the light is too far from the plant? In spite of the distance, the MH light definitely seems to be penetrating the plant leaves better than flos did at just 2 inches away.

Max, how old is your plant? I have my light at about 1ft away from my 2week old plants and it doesn't seem to bother them.
Well, it's actually 2.5 - 3 months old, if you can believe it. Only about 8" tall. It's very bushy, but it's a real runt because of the shitty conditions I raised it under. (crappy flos/crappy soil/indirect sun/no ventilation.) I'm changing the conditions for the better, now. I've got the top of the plant 20" away from the MH HID , but think I could maybe move it a little closer. My hand, for example, doesn't feel very hot unless it's within about 6" of the bulb.

Yeah, Bizzy. I saw that pic where you were using books and things to raise your plants. If I recall correctly, Ganja even suggested you move them a little higher. Since I followed GG's advice and got some decent ventilation now, maybe I can go higher too? Thanks again for your help, Bizzy.
Yes, Max if you can't drop the light put something under the pot to get it closer. I would say 20" for that plant is too high. Start with 16" and work your way down to about 10".
THanks, bizzy. I can drop the light, but my hardwood floor is getting pretty hot, so I'd prefer to raise the plant.

I'm at 16" now, will gradually move up 6" over the next couple of weeks. Only going to keep it under my MH for another couple of weeks until I switch to the HPS conversion bulb.
When you start flowering keep the hps at about 8".
Max, where you grow whats the temperature there when the light is on?
bizzy323 said:
When you start flowering keep the hps at about 8".

Oh, okay.

bizzy323 said:
Max, where you grow whats the temperature there when the light is on?

Not too bad, actually. It's stabilized at around 78 degrees F. This is probably due to the fact that I followed y'all's advice about ventilation. I've got an intake, an exhaust, a fan cooling the plant, and a fan cooling the lamp. It's not one of those "can" lamps, though. Just a cheapo 6 inch fan positioned to blow through vents in the hood.

The lamp itself is a 400W MH Hydrofarm. It looks exactly like the one here:


NOTE: That IS NOT my grow!!! :)

Ballast seem pretty big though. The size of a very large car battery.
Overall, the lamp is really nice. The guy sold it to me with 2 bulbs for $100!
It works great so far... what a deal.

Thanks, Bizz.
Is it a hydrofram ballast Max? I think it's the same system i have.
Hey Biz. Yep, a Hydrofarm ballast. 400W MH 120V 4.0A 60Hz. (San Rafael, CA :) )
Yep, its the same one I have, only mine is the switchable one. There nice and don't really get that hot.
Oh... switchable! Very nice. That's the one I wanted, but for 100 bux I won't complain about what I got. (MH only)

Yes, the ballast is warm enough to melt ice, but that's about as threatening as it seems to get. I'm pretty happy with it. In fact, after only two days, my plant seems to be showing results already. Side branches have gained about an inch. Main stem is thickening and is more stable. Leaf blades are getting a bit wider and are a deeper green.

Wow.... THIS is how you're supposed to grow pot! Thanks for the encouragement.

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