Can I use light from fish tank to grow orchids ?

[newbie0600]

I'm a 100% total newbie at this, so bear with me. I have a saltwater reef fishtank that I have been cultivating coral in for a year now. I really enjoy it. The tank has a very high power light fixture on it. I reasoned that I may be able to grow unusual plants (I live in northern nevada, so almost anything that blooms is "unusual" for this area) due to the light given off by the tank.

Of course, direct light from the tank is incredibly intense. But the light that is reflected out of the side of the tank may be the right amount. That's what I want to find out. I bought my first orchid 2 days ago, ( I only remember the first name, Dendrobium, I have to look at the tag again to see the last part), I got a book, and all I know so far is they need humidity, air movement, and a temperature drop each night. I have a lot to learn.

My question is this: does anyone know how the orchid may respond to *very* blue light ? I am running an actinic (blue-er than blue) light for 10 hrs/day, and I'm running a 20K (also VERY blue) high power halide light for 5 hours/day (concurrently while the other light is on, so only 10 hrs/day of light). I stress that the light is only intense on the water below, the plant is only exposed to what comes out the sides, which seems similar to what it would get in a window with indirect sunlight.

What are the first signs to look for that the plant is getting too much or not enough light ? Because if I know what to look for, I'll cancel my experiment and just give it regular indirect window light. I suppose that the plant's leaves are green because it wants the green spectrum of light, but I'm very curious to see what happens. Any guidance or suggestions would be greatly appreciated. Thanks!

[Ray]

That's a pretty complex situation to evaluate.

I have no facts to back this up, but I doubt the refracted light is of nearly enough intensity to do much good, and who's to say what wavelength shifting took place?

[newbie0600]

Thanks for the reply. The reflected light is less intense, but the original light source uses 750W of power. Would a lux meter do me any good ? Or do those expect a certain color of light ? Maybe I could borrow one and post the results of the direct/vs indirect light to be more exact ?

[Ross]

Let me step in with a welcome first. Then, yes an accurate lux meter will do the trick. It won't measure color temperature (although there are some exotic meters that do) but will give you a clue as to how much light. For most Dendrobiums you will need to be able to provide at least 12 hours of 20,000 lux at the leaf surface. I wouldn't be too worried about the color temperature so much as light levels. I really am doubtful, but would be curious what you measure. Keep us informed.

[newbie0600]

Thanks. This ought to be interesting. I did some internet research, and came across this article:

LED Grow Lights: 08/20/06


which says


"Most of the light capable of inducing the photosynthesis reaction is either red or blue. In other words, plant leaves mostly reflect green light, while they absorb red and blue with higher efficiency."

and

"The curve that results from plotting photosynthesis efficiency as a function of wavelength is named "Photosynthesis Action spectrum". It is the equivalent of the photopic curve for photosynthesis. The curve is typically double-peaked, with peaks around 420 nm (blue) and 670 nm (red) and a "valley" around 550 nm. The curve drops sharply below 400 nm and above 700 nm."

I've definitely got plenty of blue, although I still need to confirm this with the lux meter, but if what that article says is correct, it's going to really like the blue light. I don't know how it will react to the absence of red, however.

In the mean time, I've got to work on increasing the humidity in here. I'm lucky if I can get it past 50% But the plant is looking good. I did notice one pinhead size blemish on the flower, but it doesn't look like it has grown any. I'm misting the leaves with a spray bottle and watering it with RO/DI filtered water.

I'm a little confused on watering - I have a collection platter below the pot to make a puddle that doesn't touch the plant just to keep its humidity up, but I read somewhere that you should only actually water the plant when its roots are dry, and somewhere else I read that you just need to water it once a day. If it's roots are well ventilated, it probably won't get over watered right ?

Another question - How do I know when I should repot it ? I read somewhere that the roots need to be a little bit cramped in its pot anyway. So how do I know when it's time to give it a new pot ? Its still in the small square one that comes from the plant store.

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[Ray]

1) Light

2) Do not wet the leaves in an effort to raise humidity. Evaporation

Letting plants dry out between waterings is a (false) way we have been taught to treat them. In fact, many orchids stay constantly wet in nature. The problem is that orchids need to "breath" through their root systems (more than most other kinds of plants) ad soaking a too-compact potting medium can block the air pathways, suffocating the roots.

Repot when the plant outgrows (or grows out of) its container, or in the case of organic media ingredients, when they just show signs that it is beginning to decompose.

[goodgollymissmolly]

Newbie, hey you're ahead of the game here because you already know about light intensity and spectral output. You are correct that plants use red and blue at the wavelengths you specified (and Ray's page shows) and plants are green because they reflect green. I suspect that you also know that the Kelvin temperature of the incident light is meaningless to the plant (though it has a lot to do with what you see when you view the plant). There are an infinite number of wavelength combinations that will appear as the same Kelvin temperature but lack significant portions of the spectrum. That is known as metamerism.(The characteristic of the eye that views various spectral combinations as identical).

The one place that I think you might be going wrong is that I think you are suggesting that plants can utilize red or blue rather than red and blue. The two types of chlorophyll use the two wavelengths independently. The plant needs both. I have no idea what you will find out regarding how much light intensity you have external to the tank, but obviously it's going to be blue. If you have enough intensity, you might be able to get the red by hanging an incandescent light nearby. Just an idea.