Lighting Questions

The topic is HID lighting.

Hello everyone,


I've been trying to do some research on lighting but I'm finding it difficult to locate useful information. The problem I run into is that the recommended lighting does not really match up with the botanical data I've found. This has raised a lot of questions for me, and I'm hoping I've come to the right place for answers.

The most common use of HID lighting (from what I've seen) would be MH for vegetative growth, and HPS for flowering.

Why is it that HPS is better for flower?

The only answer I've been able to find to this question is that HPS better simulates the fall sun which tends to be more red in color. While I understand the basic logic of this, trying to replicate nature, I don't see the actual benefits. If the stimulus to initiate (and maintain) flower is a minimum of 12 hours of darkness per 24 hr cycle, why deprive the plant of the higher quality light for the sake of mimicking a duller sun?

Peak absorption for chlorophyll a is 430nm in the blue range and 662nm in the red. Chlorophyll b has peak absorption at 453nm and 642nm. For both a & b the absorption range is much higher, and broader, in the blue range than in the red. Are there any benefits specific to the red and blue spectrum other than absorption?

From what I have gathered, it is better to use a very blue MH, not just for vegetative cycles, but for flower as well. Unfortunately, this contradicts what I have read on hydroponic forums and products sold from vendors. Most MH lights available from hydroponic suppliers fall into the 5000k to 5500k range, but it seems the best distribution comes from a 6,500k MH bulb (which I can't even find in a wattage >400). If switching bulbs between veg and flower is beneficial, wouldn't it be better to go from 6500k MH to 5000k MH?

Sorry my first post is a novel of interrogation, I'd just like to better understand the light needs of our plants.

You're absolutely right that any light on 12/12 will trigger flowering, but you were also right about the light spectrum. Back in the day they pretty much just used MH lights because they are widely available as commercial lighting.

HPS work better for flowering because they give more of the red and blue colors flowering plants want.

You're right about the blue, but you still need red.

Everyone is always welcome to ignore the experience of others and run some side by side experiments of their own... but all the people on RIU probably aren't wrong.

I'm not saying using red is wrong, I'm just trying to find the benefit of the red spectrum. Also, check out these two bulbs:

6500K MH:




2100K HPS:




Seems like the MH has better energy output in both red and blue range.

Your going to get many different opinions. Most like to bloom under HPS, some under LED, few under MH. The difference is yield mainly from what ive seen, HPS being the yield king. Quality of final product with MH should be better then HPS though but ive never tried MH in bloom. LEDs have the best spectrum of light and most HPS growers know this but based on yield there not switching over to LEDs.

Meh spectrum is overrated, the reason why hps kick arse is due to hps bulbs almost putting out double the lumens to the equivalent mh bulb.

Intensity accept no substitutes.

auswolf said:

Meh spectrum is overrated, the reason why hps kick arse is due to hps bulbs almost putting out double the lumens to the equivalent mh bulb.

Intensity accept no substitutes.

Click to expand...

I sorta agree...Most discredit lumens but i find the more lumes the more par getting to the plant, they work together. HPS isnt double MH lumes there about 20% more lumes per watt. Spectrum is important though, more then i thought. In the past ive started plants under 250hps with no issues. Well recently had one of my 600 mh's go out and had to stick a 600 HPS in its place in one of my Veg rooms. Well 5 days later when i finally got a new MH bulb in the damage from the different light spectrum was terrible. The plants all looked like shit, droopy, discolored, weak from switching to the HPS. Took almost 2 weeks for the plants to fully recover from that "mistake". Never again will i ever stick a HPS bulb in a Veg room!