light height

What is the correct height for mh & hid?

depends on the wattage, ambient temperature, reflector, ventilation, etc.

generally it wants to be (just) high enough so your plants dont burn.

put ur hand directly under the bulb when its been on a few hours, as a rough guide, if it feels hot on your hand, your hand closer to the light than the plants want to be.

-Yep! -Do yourself a favor and putem on a pully!

Cannabis grows at an optimal rate based upon receiving approximately 25,000 to 50,000 lux. If you really want to know the answer to your question, get a light meter with a silicone photodiode detector, grid your grow, then experiment on a given strain until you find the sweet spot.

If you do, please post your findings so everyone else can take advantage of your pride and joy...

Good call MPD. A light meter is a must. It makes things a lot easier when trying to position everything right. You can get them for like $20 now too. The hand thing is a quick and easy way to tell if the lights too close though and I still use that to this day.

Hmmm, I always found this interesting, I have since said hell with the norm. If I had enclosed hoods, I might change my thinking again, but since my hoods are open. I just keep them now at 2 to 3 feet away. I use to get them around 14 inches from the top of the plants. But I've noticed they taste better after the cure at a farther distance, and my plants grow healthier. I've also have started to start all of my plants under a 400wMH, before I put them under either 2-400wHPS or 2-1000wHPS. I find the quicker I get them under the main lights, the quicker and more yield they will be at the end, instead of putting them under fluorescents for the veg time, then under the HPS's. I find they have light stress more, and this can take a week or so to really correct itself, sometimes you might not even notice it. Even when I gradually move them to higher lights from flo's, some just couldn't hack the light at first, and suffered some. Not to say they didn't grow good, they did, but I just find getting them under the big lights as quick as you can to be my rule of thumb for now on.
Everyone has their opinion on this, and until there are some serious studies, it will continue on. I will say plants love the flo's though, and are lusher, but when the HID's flick on, I find they are slower in the flowering department, and yield is down some.
But the norm is, measuring the heat by your hand, lower gradually, till about 13" from the top of your plants, the more lumens the better, and the farther the light source, the less lumens. But I guess love my different flavors too much. Now if I had enclosed hoods, I would probabaly get them real close.
But I will see what others have to say on this, as it does interest me some.

mpd said:

Cannabis grows at an optimal rate based upon receiving approximately 25,000 to 50,000 lux. If you really want to know the answer to your question, get a light meter with a silicone photodiode detector, grid your grow, then experiment on a given strain until you find the sweet spot.

If you do, please post your findings so everyone else can take advantage of your pride and joy...

Click to expand...

Good info mpd.

I sometimes back my (hooded/AC) lights back to 18-20" if I see some type of stress (yes a FEW times *<80)>....) and let them have a cloudy day or few depending.

If all is well I do like you explained. I wish I could give accurate light intensities, but alas I don't determine it that way because I am more gut inspired.

(BTW-the rolleyes thing is to demonstrate what people do when reading my drivel/I don't take myself that seriously, don't expect others to either)

I am working on an emerging hypothesis at the moment on the whole light thing.

See if this makes any sense:

1. It is impossible to replicate grow conditions unless you are growing in an atmospheric chamber. Humidity, temperature, soil moisture, pH, and nutrient levels all conspire to make one grow different from the next. Period the end. Argue about it all you like, the facts remain the same.

2. The only thing that is constant is the lighting times - 24/0 or 12/12 in most cases.

3. Any grow report that includes a declaration of a grow disaster event (water stressed, nutrient stressed, blocked uptake, pH stress, etc.) serve only to disqualify a given grow report as being a reliable indicator of a strain's performance so each one of these reports must be disqualified from consideration within any database of grow reports.

Therefore, the only reliable grow reports are those that provide us with definitive light intensity settings for a given strain because the intensity of the light at the plant canopy is what will govern the plant's ability to carry on photosynthesis without any other events happening in the course of a given grow that would tend to discredit the report in its entirety.

Accordingly, strain performance must be determined based upon finding the sweet spot for the strain in terms of the lux or fc being rained down on the plant thruout its lifespan without any other event occurring.

What say you?

mpd,
As far as I can tell (stoned) that all sounds fine to me.
I guess I am a little bit less 'intense' (a few foot candles shy of a nova), and just care more about good organic pot. I don't weigh & don't generally show too much.

I think it is wonderful you like to do real scientific measurement, and it helps many (including me) when we are growing new strains.
Keep on with your theory, it is good work (by their FRUITS shall ye know them!).

Think I'll sit this one out.....(Too Rolling Stoned).

so...

how far do you guys keep uncooled 400hps lights away from the tops?

8bit