How much light is enough?

[BarryRice]

Hey guys

From my experience with other light-loving plants like carnivorous plants, I've learned that 6 fluorescent bulbs about 30 cm (12") from the plants are fine, and that a mix of cool white and warm white bulbs is entirely adequate.

Is this acceptable for cactus seedlings?

[1bigfruit]

I would use bright but diffuse light for seedlings. In the wild they generally germinate and hide in the shade of bigger plants. Pretty much like wild plants do outside here.

[iann]

Cactus seedlings need less light than almost any other plant you'll grow from seed. Certainly less than you'd expect from their sun-tolerant parents. I use 40W of compact fluorescent bulbs in my 2 square foot propagator for starting seed. I can increase this to 50W or even 60W when they're a few months old, and I have a brighter growbox for yearlings and the minority of cacti that like really bright light.

Most of them need heat too, at least during the day, but watch out for mountain species that need cooler nights or they won't germinate.

[HP22B]

Is there an efficient way to grow adults indoors without severe etiolating? I have no idea how much wattage they would need to stay healthy. I'd think upwards of 500 W/sq ft using a nice metal halide lamp.

[iann]

Is there an efficient way to grow adults indoors without severe etiolating? I have no idea how much wattage they would need to stay healthy. I'd think upwards of 500 W/sq ft using a nice metal halide lamp.

Probably don't need that much, but certainly they need a lot of light.

[CactusFanDan]

Is there an efficient way to grow adults indoors without severe etiolating? I have no idea how much wattage they would need to stay healthy. I'd think upwards of 500 W/sq ft using a nice metal halide lamp.

Have a look for some of Christer Johansson's posts. He grows adult plants under light and manages to get them looking nice and compact.

[One Windowsill]

400W of metal halide per square metre would be more like it. Though a mixture of reddish and bluish is best for most plants. It is difficult to say how much advantage you get with high output fluorescents with multiple sources rather than a single source high wattage lamp. Theoretically you get ⅔ of the light of high pressure lamps from the equivalent amount of HO fluorescents but multiple source illumination and a different spectrum might make up for some of that.

You will need good extraction fan ventilation and a high chamber with a large bulb as well.

[HP22B]

I currently have two small (~6 inch tall, 2 inch wide) Echinopsis peruvianus going in a grow box I made that's about 1.5 sq. ft. and is getting 250 W/sq. ft. of CFL lighting right now and the new growth is visibly skinnier than when I received them so this clearly isn't enough light. I understand metal halides have better penetration but since there isn't really vegetation blocking light right now, I don't think this is an issue. What they're currently getting is way more than 400W/sq meter as you suggest but it's still not nearly enough apparently.

[One Windowsill]

What brand and colour of lamp is that? To get 250W in a ft² you must be using the cramped spiral sort? One 250W lamp? Again you have just one point source as with a metal halide. I much prefer straight tubes.

Is the light hitting the top of the plant or coming from the sides? A columnar cactus has a little footprint, most of the photosynthesis will be going on down the sides of the plant.

You have perfect reflection on all sides and above?

[HP22B]

They're the ecosmart coiled CFLs, 4100K. Two of the 150 W bulbs spread out over an area of around 1.5 (not exactly) sq ft. (hence ~250W/sq ft.) and are not directly above the cacti I mentioned but are located about 3 inches from tops of them so that mostw of the light is hitting the plant at an angle. The box is lined with white paper on all sides and white on top as well.

The setup seems to be doing fine for my seedlings, I just noticed that when I get fast-growing matured plants in there, the etiolation becomes more obvious. I've been meaning to look into some of the straight tube lamps but the only thing is that they're (obviously) missing the 'compact' part and I'd need more of them than what most fixtures can accommodate to supply the same amount of light I'm currently giving.

[One Windowsill]

I am not familiar with 4100K, most of the plant lights available are 2700K and 6500K, so very red and very blue. These are pretty good at providing the wavelengths that chlorophyll uses most efficiently at 450 and 650 nm (off the top of my head, they are around there). Can you find the spectrum output for the lamp? I am guessing that it is much brighter for human vision than for plant growth.