For flowering what? Almost all plants must reach a particular age and size before flowering. I'll take that as a given.

Most plants are not picky about spectrum. Don't try to extrapolate results for a specific crop to every plant. If you have a specific crop in mind then ask about it, maybe someone here will have the requirements.

Most plants are picky about intensity and won't flower well or at all below a specific level. Rules for intensity haven't changed. HID lamps can provide any intensity you require very easily, up to way beyond levels that will kill a plant. Fluorescents will provide intensities suitable for most plants fairly easily, very high light requirements need careful choice of equipment.

Many plants have specific requirements for photoperiod. Again, we would need to know the plant involved.

Some plants may be induced to flower by a combination of factors or by any one of several different factors. It isn't difficult to flower most plants under any kind of artificial light simply by letting them mature and providing sufficient intensity.

To call one specific lighting technology inferior is unwise. Out of the major plant lighting groups, metal halide, HPS, and fluorescent, (and possibly LEDs now) each have strengths and weaknesses. More precisely, each has niches that they are most suited to, none is best at everything, none if worst, none is best for flowering every plant, none is worst.

Well if ur flowering bushes are big, then HPS is more capable of the high footcandle lux lumens per square foot, and with a nice reflector or reflectorized lamp (Gavita), the intensity does not drop off very much either (known loosely as "penetration"). Depends on what flower ur growing. My rose cuttings and bergamot are doing fine under fluoro, but they're kinda of short plants. For big bushes, like 2-3-4 feet, then
generally HID is more satisfactory.

Still the message isn't getting through. There is no difference on penetration (the relevant measure of suitability for tall plants) between HID or fluorescent lighting, provided that both have suitable reflector systems and both are of the same power.

So HID is not more satisfactory, it is more convenient. Because a fluorescent setup to replace a single 400W HID lamp would contain a minimum of 8 48" tubes (T5 HO 54W) and possibly as many as 14 48" tubes (32W T8 on a 0.88 ballast), all with individual reflectors or within a largely enclosed reflective grow room. HID systems are far more likely to come with a suitable highly collimating reflector system for the small single bulb. HID systems are also far more likely to have light to waste by placing the lamp too far from the plants or by using an inferior reflector. None of these things prevent you getting the same performance from a fluorescent system, they simply make it less convenient.

Shrubs n bulbs, you say, (I haven't yet figured out how to quote previous postings):

"So HID is not more satisfactory, it is more convenient. Because a fluorescent setup to replace a single 400W HID lamp would contain a minimum of 8 48" tubes (T5 HO 54W) and possibly as many as 14 48" tubes (32W T8 on a 0.88 ballast), "

I'm guessing that you based your equivalence of eight fluorescents with a single 400 watt HID light on the power consumed by both, (~400 watts), but aren't HID lights much better converters of electricity to light than fluorescents?

TIA
Q

aren't HID lights much better converters of electricity to light than fluorescents

In a word, no :)

HPS is most efficient in terms of converting electricity to photons by quite a margin in the 400W-1000W+ range. Light output decreases significantly with age but still remains higher than other sources.

Metal halides are most efficient at around 400W, but have good efficiency from 250W-1000W. Light output in traditional lamps decreases badly with age, to far below the best fluorescent tubes. Modern pulse start bulbs and electronic ballasts maintain brightness much better withon only 15% or so loss over the normal life of the bulb.

Modern fluorescents on an electronic ballast match the efficiency of the best metal halides. Fluorescents are most efficient with long tubes but are not practical beyond 8'. In practice few people use them for plants at longer than 4'. Most of the fluorescents used for plants are T8, T12, or high output T5s, all about 10% less efficient than a new 400W metal halide bulb but they maintain their brightness over their whole life. and will be more efficient than the metal halide after 5,000 hours - 10,000 hours. There is so little to choose between the two technologies in terms of efficiency that you should base your decisions on other factors, such as initial cost, requirements for total power, convenience of operation, ease of sourcing bulbs, etc.

Compact fluorescents are always less efficient than a straight tube of the same power and quality. Bulb life and lumen maintenance are also worse. The more bends in a tube, the worse its performance becomes. Simple double-tube compact fluorescents are still less efficient than a single tube of the same power but generally more efficient than separate tubes of the same length. Household spirals and multi tube lamps like the envirolite are about 40% less efficient than the best straight tubes, although only about 10% less efficient than small straight tubes, for example a 24" 20W T12.

High output and overdriven fluorescents are less efficient than normal output tubes. For example, the 46" high output T5 is about 10% less efficient than the normal output 46" T5. Beyond the optimal power level, which is a little under 1W per inch of tube length with current designs, efficiency drops as power increases. Efficiency also decreases at lower power levels but nobody would really want to go that way.

Possibly the most important thing to remember is that older technology of any of these lights is hugely less efficient than the best which is available today. Fluorescent and metal halide light output have nearly doubled in the last 20-30 years, HPS has improved but not by so much. Bulb life and lumen maintenance have improved dramatically and a much wider variety of spectrums is available.

I think what I was getting at is if ur growing a plant that likes much more than 3000 foot candles intensity, then fluoros won't do it. Fluoros however are extremely efficient because
the plant canopy can actually be touching the lamps, the light spread is very uniform over the entire garden, and you can't easily burn ur plants the way you can with HID. However, for growing a 3 or 4 foot cannabis plant for instance, I can't see doing that without resorting to HID. However, the sea of green (SOG) method for cultivating shorter cannabis plants is how the grower utilizes fluorescents for this crop, having them flower when they're shorter plants, and training them like SCROG method for example, and this is a very efficient way to get a good and high yield of bud. Please note I don't grow cannabis myself anymore and don't recommend breaking the law. Too much hassle when you get caught by LEO. Plus they confiscate your equipment and vandalize your place while they're at it. Paul Mozarowski.

T8s are about 3,000fc at the tube. T5HO is a little over 5,000fc at the tube. Beyond that you would have to overdrive which I generally don't recommend. I've never calculated or measured a value right at an HID bulb, so high its scary! In practice that 3,000fc you mention is a practical upper limit. Luckily I never need more than that :)

(Shrubs) "There is no difference on penetration (the relevant measure of suitability for tall plants) between HID or fluorescent lighting, provided that both have suitable reflector systems and both are of the same power."

(Lermer) With t-8 fluorescent tubes, I don't think you're going to get more than 3000 foot candles. With the t-5 high output tubes, you get more intensity and can grow fine up to about 1'. However, 4300 foot candles is usually preferable, and in cases 5500 is optimum. To get these higher light intensities indoors, I think you need to go to HID lighting. The 400w Ceramic Metal Halide has far more intensity and can do fine up to about 2.5'. A point source tends to be more intense, and distribution can be improved by multiple points of light and/or moving the lights. Multiple points of light would increase the number of angles, and overlap the light footprints (much as banks of fluorescent lights work better than a single fluorescent tube).

(Shrubs) "Metal halides are most efficient at around 400W, but have good efficiency from 250W-1000W."

(Lermer) That depends on how you measure efficiency, and the type of reflector. Clearly, 1000w Metal Halide has more lumens per watt than 400w, with a similar spectrum. However it's better to use more points of light because that results in more even distribution of light and fewer shadows on leaf surfaces. Given limitations on total power consumption, and better bulb selection at 400w, I recommend (in most cases) multiple 400w lights. Generally the best 400w light is the Ceramic Metal Halide, not the standard quartz metal halide. CMH has been thoroughly field-tested, and for indoor use is about twice as efficient as standard MH and 50% more efficient than HPS.

(Lermer) If you use LARGE numbers of flourescent tubes, the resultant array gain and overlapping of light patterns means you can grow plants more than 1' tall. However very few growers use a bank of 20 or so tubes. The weight alone is a negative factor, and so many fixtures would block natural sunlight. Still you get only about 3000fc max with t-8s (not counting the edges) and this is less than optimum. It's better to go with a higher intensity light and move the light.

Some college grower on the other thread was asking about whether he should apply foil DIRECTLY to the flourescent tube to get better reflective properties. I actually tried draping a 48"x10" sheet of mylar over the two adv830 T8's in my overdriven shoplight fixture. The lumens, compared to stock (in overdriven configuration)
Stock: 3000 -4000 foot candle, Hydrofarm digital meter. With Mylar drape: 4000 - 5000 foot candle. Holy Sh*t!
I think you could grow some serious crop with this modification - just a simple sheet of mylar draped over the
two T8's boosts the effective light by 20%, with an extremely even pattern. The stock configuration measures like overdriven, and overdrive measures like an HID!!
Paul Mozarowski.

(Lermer) P.M. didn't specify at what distance from the tubes he got the 4000-5000fc reading, or if he was measuring near the center or at the edges. A 20% gain from his homemade mylar reflector is not that impressive. Almost half the light goes upward, so with a reflector you should be able to almost double the intensity. Mylar is only about 88% reflective. It is made of plastic coated with aluminum, and could disintegrate if exposed to excessive heat. You're better off with the German-made aluminum sheet metal, the best is 98% reflective (far better than anything made in the US or the UK). In fact it's possible to buy the fluorescent fixtures with reflective aluminum. I don't recommend putting anything in between the bulbs, that would lessen the array gain (technically called "constructive interference").

Allow me to talk about point vs. line sources of light. HID lights are more like points than lines, and a 4' fluorescent tube is more like an infinite line than a point. Actually, all light comes from point sources. A line is really an infinite series of points. Let us compare a 40 watt 4' fluorescent tube to a series of 4 MH lights of 10 watts each (1' apart). The two would have similar light patterns. The pattern of the 4 MH lights would be a little bumpier or less uniform than the fluorescent tube. Each 10 watt MH light would be a point, but the points combine because their patterns overlap (if all were within 1' of each other). Now increase the wattage of the MH lights to 40 watts each; obviously then the vertical penetration would be greater. Then take away 3 of the 4 lights; the single 40 watt light would have greater vertical penetration than the 40 watt fluorescent tube; the tube would have greater horizontal coverage because of overlapping light footprints (from the infinite number of points in the line). The initial intensity of the 40w MH would be greater because it is more concentrated.

So, fluorescent tubes sacrifice vertical penetration for greater horizontal coverage. However, if you put a 400w CMH on a light mover, you still have the greater vertical penetration, but now you also have the horizontal coverage.

The 8 fluorescent tubes might have more array gain than a single 400w CMH, but you could put four of the 400w CMH on a spinner to get comparable array gain, and with coverage of about a 8.5' circle. About 54 sq ft, and you can turn the circle into a square by placing taller plants in the corners.

Imagine a 400w CMH, stationary, vs. 8 t-4 high output fluorescent tubes.

400 watt CMH
3'x4'=15 sq ft area of coverage (minimum 1000 fc)
with 6' track, area covered 3'x10'=30 sq ft
total lumens = 34,800 or 87 lumens per watt
85 or 95 CRI (depending on brand)
suggested retail price = $265 or $450 (depending on type and brand).

432 watt t-5, 8 tubes 4' each
2'x4'= 8 sq ft area of coverage
not practical to track
total lumens = 40,000 or 92.5 lumens per watt
85 CRI
suggested retail price = $450

Because of the greater intensity of the 400w CMH, vertical penetration is greater. Mainly because of tracking, the 400w CMH also covers at least 3x the horizontal area of coverage.

(Lermer) Pardon my typo, 3'x4' area of coverage equals 12 sq ft. I was thinking of another reflector type, which covers a 3'x5' area.

Also, if you get the Life Light e-ballast, you can also run PSMH instead of CMH. The LL PSMH comes in four different spectrums, 3K, 4K, 6.5K, or 10K. I recommmend the 3K for most of the bloom period (the extra red results in more bloom hormones), with 10K the last week for increased potency.

4 x 6' area, 24square feet, 20 overdriven T8 lamps, 1000 watts, 100,000 lumens, 100lumens per watt. CRI = 85.
Cost retail, on sale 500$ 20$ per square foot.
4000 lumens per square foot, corresponds to measured 4000
foot candles. Lamps last 20,000 hours plus.
"Penetration" = excellent. No spinners required.
Observed actual plant growth (roses, bergamot and peppermint) = excellent. PM.

(Lermer) Thanks for the data, PM. However I would still like to know where you measured, at the center or at the edge; and what the fc reading would be at, say, 4'.

(PM) "'Penetration' = excellent."

(Lermer) You don't say how tall your plants are now. Also, how do you explain the apparent contradiction with your statement (in the LED vs HID vs FLUORO thread):

"I've grown ... under HID, exactly 2 x 400 w per 3'x5' growing closet, Gavita HPS. The plants grow 3-4' tall, they're bushy as hell, very heavy with bud, and when you admire your crop, it's just like you're out in a wide open field. You just cannot accomplish this with fluorescent."

(Lermer) At about 4', I get 760fc with a 400w CMH. You are using 1000w of fluorescents because you are overdriving the t-8s. Do you modify the ballasts to "overdrive" the bulbs? How do you do that? Doesn't overdriving reduce bulb life? How can you calculate lumens, when the lumens rating is for the standard (not overdriven) ballast?

If you're willing to burn 1000 watts, probably you're better off with a 1000 watt super HPS (like Hortilux). With a unique 98% reflective hood, I get a reading of 1000 foot candles at 4' from the broadside of the bulb. So,

3.5'x7.5'=over 26 sq ft
Cost retail, regular price $410.
1000 watts, 145,000 lumens, 145 lumens per watt
CRI=22
5576 lumens per sq ft
Lamps last about 24,000 hours, I recommend replacing every 6 months of use (depending on the value of the crop).
Penetration with HPS is better than with full spectrum, but I think you're better off with plants 2'-2.5' and using two 400w full spectrum CMH.

(PM) "No spinners required"

(Lermer) You wouldn't need to track or spin the 1000w HPS, either. But if you tracked it 6', you would cover about a 9.5'x7.5' area of 71 sq ft. The increase in total yield with tracking/spinning is tremendous.

as well as peppermint and bergamot. The bergamot has nice "colas" on it. I have cut back the light to "regular drive", 0.58 amps per 2 48" lamps. This should be delivering the full 32 watts per tube. When overdriven to 54 watts, some of my roses were getting "bleached out" and yellow leaves. With the regular drive on for 3 days now, the yellow leaves are developing a nice green zone around the leaves, and the new growth is nice and green. The plants are currently about 10 inches tall. Paul Mozarowski.

a unique 98% reflective hood
LOL. So unique it actually doesn't exist. Unless its made of pure silver and polished every day before you turn the lights on ;)

Penetration with HPS is better than with full spectrum
Now you're talking rubbish again. Penetration has nothing to do with the type of bulb nor its power and everything to do with how it is collimated and how far away it is. Until you realise that you will continue to give misleading advice.

I'm using 650 watts at the moment for my 6x4' fluoro garden. It has beautiful and easily controlled illumination. There is no way I could get these results with a 650 watt or 600 watt HID. A 600W HPS will put out 90,000 lumens, with 20 x (32watts) x 3100 this gives 62,000 lumens, but the foot candle intensity in my growing zone is still 2000 - 3000 footcandles. My plants are currently 9 - 10" tall, and they're doing great.
When I turn on overdrive, I use my digital footcandles measurement to confirm that each lamp is putting out 5000 lumens, this corresponds to what zinc mentioned (or it could be someone else) in his overdriving T8 thread. It really does work. Sure the bulb burns out sooner, but it still puts out the same TOTAL NUMBER OF PHOTONS BEFORE DEATH, and it gives me the flexibility to keep using the same setup as the plant gets up to 2 feet in height, or more, which is only about as tall as I want roses to gro indoors.
The lighting intensity is measure ALL OVER THE GARDEN, it is nice and even, just about to the edges, where is starts to drop a bit, due to spilling off the edge, but still at 1500
footcandles. When I had it on overdrive, some of the foliage was getting bleached and yellow due to the intensity, which verifies the light meter reading. Fluoros can be just as intense as HID, with the exact same "penetration", as long as you have a fairly intelligent monster setup. I use one ballast per tube, the ballast is designed for two tubes, and I have a toggle switch on the end that lets the second output through to overdrive it.
The pictures are on the flickr site. Paul Mozarowski.

Here is a link that might be useful: My flickr garden, I actually did it, and it measures like I say it does.