Switching to HID
 

Hello all,

I am a long time stalker on the boards and fairly new to orchid growing. I currently use a T5 set up with a mix of blue and red bulbs. I want to switch to using HID bulbs in order to grow my collection and also add some more bright light loving species, Cattleya, Vanda etc. My question is do I still need to provide blue and red light with the HID's (MH and HPS) or would the MH be enough to get everything to bloom. I am considering a dual bulb HID (Growzilla) but the cost of that light is a little more than I'd like to bear at the moment. Any advice is appreciated....Thank you..

Actually, you are doing your plants a disservice by mixing red and blue bulbs now. If you switch them over to bulbs having a color temperature near 6500°K, you will get a lot more lumens per watt in the complete chlorophyll and carotenoid absorption regions, and they will do much better.

Such lights are fine for higher light plants, you just need to provide more wattage and/or move them closer.

HID lighting is fine, but they don't have a particularly good spectrum match, which is why you have to go with 400 or 1000 watts output.

I noticed a huge increase in the growth and blooming of my plants since the switch to HID Metal halide. Started growing with T12's, then T8's and then T5. Still use the T5's in an isolation area for new arrivals.

I use a dual setup of MH and T5. The T5's flank the MH lamp on each side. It's obviously a bit higher than the T5's from the plants, but this allows a middle space for higher light loving plants, and the sides for less loving plants. You also get the bonus of heat in the winter, but the MH needs a fan in the summer.
Ray is dead on about the red/blue lamps. 6500K or higher works just fine for Catts/Oncs/Phals, and provides a full/near sunlight spectrum for orchids.

Well, I don't know exactly what spectral power distribution is meant by "red" and blue" but still I see no reason for your assertion that using these red and blue fluorescent lamps does any disservice to the plants. LED fixtures for plant growth do just that - use a combination of red and blue LEDs.

Hi David.

Maybe "disservice" was too strong and narrow of a word to use, but here's my logic:

• Plants have evolved to absorb and use light coming from the sky.
• The absorption by chlorophylls and the various carotenoids that feed energy to the chlorophylls matches quite well direct sunlight plus the "blue backscatter" of a clear sky.
• That combination is closely represented as being somewhere in the color temperature range of 6400°-6500°K
• Bulbs of that color temperature match the sky spectrum better than does a combination of other colors.

Concerning the LEDs, the use of "red and blue" might be sufficient for pot growers, and certainly is practicable for orchids too, if you don't mind the horrendous appearance, but there are actually studies going on right now that are using identified physiological responses by plants to specific "trigger" wavelengths, so use- or avoid them using LEDs, to try to optimize the growth of various plants.

"Red and blue" might be sufficient, but it's not optimal.

---------- Post added at 09:49 AM ---------- Previous post was at 09:29 AM ----------

Sure - it's all light-flux related - lumens in the right wavelengths reaching the plant.

A metal halide bulb puts out about 125 lumens per watt, but in a spectrum that is far higher in the red end of the spectrum than the blue.

I'll also bet that you "upped" the applied wattage considerably, too, which you have to do for the blue end to be sufficient for plant growth.

My contention is that if you applied an equal number of lumens from 6500°K fluorescents and from MH bulbs, the plants would do better under the fluorescents.

T5's are usually good for 90 lumens per watt, so if you're using a 1000W MH bulb, in order to match the lumens with 54W, 4-foot T5's, you'd need 125/90 x 1000/54 = 26 T5 bulbs.

I'll bet you didn't have that many...

Yes, _possibly_ not "optimal" but no basis for you to have said:
Lumens is a meaningless metric for stating efficacy of photosynthesis. Lumens is a measure of luminous efficacy - the ability of the human eye to perceive light. For photsynthetic efficacy you need to know the number of photons; if you instead try to use lumens as your metric you will be in error by as much as a factor of 30 - it takes 30 times as many photons at 670 nm to equal 1 lumen as it does 555 nm photons.
Luminous Efficacy

Also, I don't know why you are saying that metal halide light is "far higher in the red end of the spectrum than the blue". Are you using your blackbody calculator?
Light Sources & Color Temperature
That is highly innacurate for anything, like HPS, fluorescent or MH that is not a blackbody radiator. Compare this actual spectrum of a MH light
File:Metal Halide Lamp Spectrum.jpg - Wikipedia, the free encyclopedia
against your value for MH that assumes it is a 4000K black body radiator.
As you will see when you look at the actual spectrum of a MH lamp, it has quite a lot of blue in it:
File:Metal Halide Lamp Spectrum.jpg - Wikipedia, the free encyclopedia
Indoor marijuana growers use MH when they want a light with a lot of blue and switch to HPS when they want a light with more red.

Again, lumens is a useless term for measuring photosynthetic efficacy. And color temperature is also not very useful for evaluating the photosynthetic efficacy of non blackbody radiators such as metal halide, fluorescent and high pressure sodium. Of the light sources that we use, only incandescent and the sun are blackbody radiators.

Black body or otherwise, and no matter what the Wikipedia graph shows (actually confirmed by it), the MH bulbs do not match the absorption spectrum of chlorophylls and carotenoids in plants as well as do 6500°K sources of light.

Saying "6500K" is a meaningless term all in its self. Do you mean 6500K blackbody or 6500K Corellated Color Temperature?

Hi dave.

I understand that fluorescent color temps are corrected to perception by the eye to the equivalent black body radiator, but my experience and that shared by many is that the 6500° bulbs give better overall growth that the equivalent wattage in redder and bluer bulbs.