Light Values

[Bruce]

I thought there was a discussion here somewhere, but I can't find it... Does anyone have a table of vague-ish descriptions of light requirements (eg "bright shade") vs actual foot-candle measurements? I've re-wired my light cart to T8s (with full-spectrum tubes) and have a lot more light than I did. I'm wondering what new sorts of plants I can kil...ermm grow.


Bruce

[iann]

Bright shade = 1,000 foot-candles
Direct sun = 10,000 foot-candles
Give or take 50%.

[Bruce]

Thanks, Ian. So how many foot-candles in "bright light"? Or, to put it the other way around, what sort of name can I put to the 3500 foot-candles my new setup is producing?

[iann]

Call it anything you like. 3,500 foot-candles = 1 Brucie?

Always remember that natural light is not constant. Heavy overcast may be 100 foot-candles. By dusk, even on a clear day, the light level will be down below 100 foot-candles. I've seen "dusk" quoted at 10 foot-candles, which is about the level of normal domestic lighting. 3,500 foot-candles continuously all day is pretty intense, even though it won't produce the same effects as shorter periods of direct midday sun would.

[Bruce]

Yeah, I knew as I was typing it I was asking for trouble... So, what do you think Ian - is one "Brucie" for 10 or 12 hours enough to take another try at lithops?

[Saxicola]

You should consider getting a light meter. They aren't terribly expensive and you would know with certainty the conditions you have. The best Ian or anyone can give you is an estimate of the light levels at your location with your set up.

[Bruce]

@Saxicola - Thanks. I've got a light meter. The problem is I get hard values like "3500 foot-candles" and the guides for various plants say things like "needs bright light". I've been try to bridge the difference before I buy anything completely unsuited for my (newly-rewired) conditions. If I can at least get the lighting right, I stand a chance of growing the plant. If the light is wrong, then I'm just wasting my time and adding another plant to the compost pile. In the world of orchids (where I come from), they associate foot-candle ranges with "phalaenopsis light" or "cattleya light" or "vanda light". It's certainly not exact, but it gives you a starting point. (My 3500 foot-candles puts me on the high side of "cattleya light".) You might see something like this general description (from staugorchidsociety.org) :

Oncidiums – Dancing Ladies
„Light – Somewhat less than Cattleya Light, medium
bright, 2000 – 3000 ft candles, southern exposure,
dappled light, protect from midday direct sun

I suppose I can simply use the orchid light ranges and apply them to the descriptions of cacti (bright light = cattleya light = 2500 to 3500 foot-candles) but I have no idea whether different groups of hobbyists actually mean the same thing in their un-scientific descriptions.

[iann]

I don't like Lithops under lights. They like wide swings in temperature as well as periods of very intense light, and ideally high UV to develop colour and toughen them up. Lithops grown under artificial lights tend to be soft and prone to problems.

Cactus suffer some of the same issues but less so. I think you should be aiming for cooler nights and then seeing what you can do to provide at least short periods of very intense lighting. Although I imagine it gets very warm under that amount of light, it is possible that the plants still aren't warm enough during the day. While the air temperature outside may be 90F or 100F, plants in the sun will easily and regularly reach 120F and more in the summer. I've seen 150F without the plant being killed. Lithops are especially adapted to heat up in the sun, but this won't happen with typical lights.

Just some thoughts. I haven't put a lot of effort into year round growing under lights, just seed raising and overwintering.

[DaveW]

Some random links off the Web:-

http://www.cssainc.org/index.php?Itemid ... &task=view" onclick="window.open(this.href);return false;

http://www.cactusinfo.net/artificial_light.htm" onclick="window.open(this.href);return false;

http://www.ehow.com/about_6134470_plant ... actus.html" onclick="window.open(this.href);return false;

http://www.gardenguides.com/90265-plant ... actus.html" onclick="window.open(this.href);return false;

Somebody on one site claimed:-

"Artificial light can not replace normal sunlight.
Cacti need some 20.000 lux for growing well, you can't get that from TL's or bulbs."

He was not growing San Pedro's for their beauty though, so was mainly interested in maximising their alkaloids!

Me I don't like paying electric bills so they have to wait until the sun comes out!

[iann]

Bruce already has more than 20,000 lux, but that is far short of English direct sunlight, let alone tropical sun at high elevation in clear dry air. A great deal of what you find on the web about artificial lighting for plants is bogus, a mixture of total lack of understanding and a considerable loss of brain cells due to the reason they want to grow plants under artificial lights

[daiv]

He was not growing San Pedro's for their beauty though, so was mainly interested in maximising their alkaloids!

As I understood it, the strong sun is required even more so for producing alkaloids. I could be mistaken on that, however, as I haven't ever looked into it.

[Sharpy]

I am going to be ordering my lights in the next week or 2 and am curious if 2 of these (per box) will be enough?

http://www.hydrofarm.com/product.php?itemid=7661#tabs-2

My boxes (2) will be 6'x24" deep x 20" tall. Completely enclosed with as little light loss as possible. All white formica interior. I know Ian has mentioned many times that the effects are different when you acheave a high level of light containment (concerning the distance from the fixture). Each light will be on its own switch so I can have full control over how many are on. But going by the "watt per sq/ft", 2 of these will only get me 9W/sq ft. The distance from light to plant will be ~15". Not trying to highjack your thread, but this seemed like a good place to put this and get a final answer before I spend the cash.

Thanks

[iann]

I am going to be ordering my lights in the next week or 2 and am curious if 2 of these (per box) will be enough?

http://www.hydrofarm.com/product.php?itemid=7661#tabs-2

My boxes (2) will be 6'x24" deep x 20" tall. Completely enclosed with as little light loss as possible. All white formica interior. I know Ian has mentioned many times that the effects are different when you acheave a high level of light containment (concerning the distance from the fixture). Each light will be on its own switch so I can have full control over how many are on. But going by the "watt per sq/ft", 2 of these will only get me 9W/sq ft. The distance from light to plant will be ~15". Not trying to highjack your thread, but this seemed like a good place to put this and get a final answer before I spend the cash.

Thanks

Not much for twelve square feet. Enough to start seeds, but that's all. Even seedlings will probably start to etiolate after a couple of months.

[Sharpy]

Thanks Ian. I will go to PMs if I have any more questions after this. Don't want to de-rail the thread.

To make it real simple, are these lights a good choice and if so, then how many would you put in there?

[iann]

Thanks Ian. I will go to PMs if I have any more questions after this. Don't want to de-rail the thread.

To make it real simple, are these lights a good choice and if so, then how many would you put in there?

These are good intense light sources, although a little expensive. I would double them at least for your size of box. An alternative would be five foot T8 (58W in Europe, 55W in the US?) tubes, with similar output to the T5s, but should be easier to find at your local box store and much cheaper. You might even be able to fit in a six-footer? Common size for commercial lighting. Watch out though, conventional fluorescent tubes in the US are frequently ridiculously low-powered and useless as plant lights, so make sure you're getting 5,000 lumens plus from each tube.