cactus pictures Growing Cactus with Artificial Light

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Growing Cactus with Artificial Light


Lighting plants is really very simple, despite the acres of drivel written about it on the internet. Work out how much light you need for area amount of plants you have, find a lamp that efficiently provides that amount of light, and find a way to shine it all on the plants.

Yet growing cacti under artificial lighting is not easy and may not be very rewarding. OK, maybe you can say it is easy, but you will need to spend several hundred pounds on a kit and then expect enquiries from the police when the electricity company tells them how much juice you started to use.

Most cacti need very intense light, approaching the intensity of full sunlight. Ideally many of them would prefer even more intense light than direct sunlight in Britain. This light intensity can only be achieved with HID lighting, expect to need 100W of lighting per square foot of plants. In practice, you can get reasonable results with a half or a quarter of that, because the light can be maintained at that intensity all day and every day. Make sure you don't block more natural light than you are adding!

Direct sunlight during the middle of the day in summer is 5-10 times more intense than you will achieve with most plant lights. Even summer shade is as bright as most artificial light systems. So use natural light if you've got it. The main problem is that the back side of the plants in a window never gets enough light and they can lean or stretch.

Other than cost, the heat put out by the required level of lighting for cacti can be a huge problem and you will need to run a fairly robust system of fans both to stop the plants overheating and to stop the rest of your house overheating.

You might be better looking for types of cactus or other succulents that will grow well in your western window or outside without much sun. The "jungle" cacti, epiphytic cacti generally, and a number of Carribean or South American species manage on lower light levels. Small globulars, Gymnocalyciums, Fraileas, some Mammillarias, in fact many of the things you might find in bubble packs at B&Q, are tolerant of less than ideal conditions. Or maybe you can come to like Haworthias, Gasterias, or other succulents that don't need blazing sun all day.

Light Requirements

You can have too much artificial light, but you've got to go some to do it!

The amount of light needed for growing a desert cactus full time is upwards of 2,000 lumens per square foot. As a rough rule of thumb, assume 20W per square foot as a starting point, 30W is better. Small tubes such as the two foot 20W ones and compact fluorescents, are less efficient, you would need a minimum of 30W per square foot but I don't recommend them. For comparison, direct noon sun is 10,000 lumens per square foot or more, although it is lower in early morning and late afternoon and lower when it is cloudy. You can compensate somewhat by running the lights at full power for 14 or 16 hours a day, but the plants will not develop the same spination, will not grow as compact, and may be difficult to flower, but would certainly be happy for a time such as over winter. For full time growing of full sun cacti, you could use up to 100W per square foot.

For very small areas, the spiral compact fluorescents can be used, although you'll need about 50% extra power because they are not very efficient. For medium to large areas, straight fluorescent tubes are good, but don't use very short ones unless you have absolutely no choice. You probably don't need metal halides for cactus seedlings unless you have a whole room full, but they're good for large areas of adult cacti. I wouldn't recommend HPS, again unless you have a whole room full, and also because cacti will etiolate under them unless you supplement the light. Don't use incandescent or halogen lights, too much heat, not enough light. LEDs are now an option but difficult for a beginner to know what is needed and whether it is sufficient.

The heat thing is a bit of a red herring, of course a 400W lamp gives out more heat than a 150W lamp. How about a 15W lamp, even less heat, but you can work out for yourself why you wouldn't use it. Pick the lamp that gives you enough light, anything bigger is just wasting money (and that's before you have to turn the A/C up!), and then work out whether you need fans to cool it.

These figures assume that you get the majority of the light onto the plants with a good reflector and side baffles, or with a light used inside an enclosed white growing area. Most standard reflectors would waste probably half the light.

You may be able to use lower light levels if you have some natural light, if you are growing jungle cacti, to raise seedlings, or just for overwintering. I use nearly 2,000 lumens per square foot from 5x36W T8 fluorescents for overwintering some winter growing succulents, 1-2 year seedlings, and anything that likes to be really warm. I run these lights for 10-12 hours a day because some of the plants are sensitive to day length and only flower in winter or after winter. I use 600-1,200 lumens per square foot from two CFLs for starting cactus from seed.

Light Intensity

Intensity is simply the amount of light falling on a particular area. You have to really work hard to get the same intensity as direct sunlight using artificial lights. Even if you can manage it, the light is usually lower in UV. But you can provide the full intensity of light continuously which you can't get from the sun. Still hard to provide enough light for an adult cactus.

Penetration is a much abused term, but let's just stick to plant lights! Very few people understand what penetration means, they have just been programmed to think that powerful lights somehow magically have good penetration and reach the bottom of the plant. Penetration is actually a function of the collimation of light, which we have already talked about. Sources of light which are spreading out quickly have poor penetration, that is the light intensity at the bottom of the plant is much lower than at the top. This is why placing a plant close to a poorly designed light is not a good substitute for stopping the light spreading out. Powerful light sources are generally placed far away from the plants so that there is little difference in light intensity between the top and bottom of the plant. Hence the "myth" that powerful light sources have better penetration than weak ones. The sun is the best example, a very powerful light source placed so far away that there is essentially no difference between the light intensity at the bottom of a plant and the top. A powerful light source placed close to the plants has very poor penetration, the bottom leaves will die from too little light and the top ones will be scorched by too much light.

Diffusion of Light

Fluorescent lights, and all other lights for that matter, don't lose efficiency with distance, the light simply spreads out. Prevent the light from spreading out and you can have the plants as far away as you like. Of course, no system for containing or focussing the light will be 100% effective so mounting the lights at one end of your basement and the plants round the corner at the other end will likely give poor results! Remember, shop light "reflectors" are designed to spread the light over the whole room, not to put it all on a seed or plant tray, hence they are useless for what we need. Similarly, get rid of any diffuser on your lamp.

After all, why would you take a light fitting designed to spread light evenly over a 400 sq ft room and use it on two sq ft of plants? Use a light system designed to focus light on a small area and all those preconceptions about inverse square laws and light losing efficiency go out of the window. Lasers don't lose intensity with distance, searchlights don't lose intensity with distance, why should you settle for a plant light that loses intensity with distance? Or one where three quarters of the light misses the plants altogether?

The simplistic solution of placing fluorescent tubes almost in contact with plants is just about the worst possible approach. You will still end up with half or more of the light not hitting the plants, only useful if you need a very brightly lit basement for 16 hours a day. Also, the difference in light intensity between a quarter inch from the tube and say two inches at the bottom of the plant will be huge, resulting in poor growth. The whole arrangement becomes completely unusable once the plants are more than a couple of inches tall.

There is nothing magic about this. Plant lights produce a certain amount of light, and so long as that light doesn't escape or get absorbed then it must reach the plants no matter how far away they are. In practice, each bounce off a surface loses about 20% of the light so you don't want to go too many feet, but there is certainly no need to press everything up against the tubes. Whatever you end up with, it is important to get all the light onto the plants instead of just turning your basement into an artificial beach. Use a good reflector, the ones that come with fluorescent lights are designed to spread the light out over a whole room, not to concentrate it on a couple of square feet. Surround the lights and plants with as many white surfaces as possible: white shelf above and below; white wall behind it; even hang white card or mylar at the front of the shelf for most of the time.

Ultimately, nobody can tell you how close to put the lights, that entirely depends on the design of your reflector. When the plants start to burn, raise the lights. Try to have the lights at least as far away from the plants as the height of the plants. With an open shelf, try to arrange the plants so that they are all the same distance from the lights. Or surround at least three sides with white sheeting and then it won't matter.

Distance from the lights is irrelevant. What matters is how much of the light makes it to the plants rather than going out the window lighting up the rest of your basement. Putting the lights close to the plants is just the simplest (and worst!) way of getting a lot of the light onto a small area of plants. Very high intensity lights have to be placed away from the plants to prevent overheating and to allow the light to spread out to a sensible intensity.

light enclosure An "enclosure" is a good solution for getting nearly all of the lights onto the plants without loss. If you are using an enclosure, just paint it white instead of going through the pain of using mylar. The latex paint they use to make your roof reflect sunlight is probably the best, but ordinary interior flat emulsion paint is nearly as good. The problem with enclosures for growing adult cacti is that moisture gets trapped and the humidity rises. Heat buildup will be a problem too if you are using enough lights for an adult cactus, so design an extractor fan into your enclosure. Enclosed lightboxes work well for seedlings, they like constant moderate light, high humidity, and steady warmth.

Light Duration

Cacti don't respond strongly to photoperiod, but many of them do set their flowering by a cool dry winter. Put them on a timer to be on during normal daylight hours, because many succulents need to know when its winter. Mesembs are very seasonal and this is triggered by the length of darkness at night.

For seedlings, I start out at 20 hours of light and bring it down slowly to 16 hours after germination. Tall'n'skinny seedlings are caused by too little light intensity and too warm temperatures. Night temperatures are most important, get them as low as possible (without testing their hardiness) once the seedlings have germinated.

Increasing the number of daylight hours of a light that isn't bright enough can make etiolation worse. Cacti grow faster with more hours of light and more hours of warmth but they will only stay compact if you increase the light intensity or decrease the temperatures. For best growth, keep seedlings warm by day (75F-85F for most) and cool by night (55F-65F for most) and give them light just sufficient to keep them compact but not to make them red. If you give them too much light they will turn colours and not grow as fast, even die if you really go mad. You can boost small seedlings along nicely with constant warmth and long hours of moderate light but eventually you need more adult conditions or you might as well have bought the fat plants from Home Depot. You can use lights for fewer hours if you use a higher intensity, but most seedling cacti will stop growing if the light gets too intense.

The Light Spectrum

As for picking the "right" wavelength of light to grow plants "best", that's a bit like trying to pick the right vitamin for people to live off. You can't do it, they're all important for different things. Specific wavelengths, or more importantly specific pigments activated by light at particular wavelengths, have been shown to control the orientation of plant's leaves and flowers relative to the sun, the orientation of chloroplasts within leaves, the quantity of chlorophyll generated within leaves, the size of the leaves, the opening and closing of stomata, the opening and closing of flowers, and the induction of flowering and fruiting. Mostly these wavelengths are towards the blue and green end of the spectrum, at the red end it is mostly just chlorophyll making sugar.

Plant light sellers make a big deal out of the chlorophyll absorption spectrum and how green light is useless. There are many pigments in plants, not just chlorophyll, and plants can make use of light of any colour. Deep red light is the most efficient in terms of simple photosynthesis, but not to the huge extent usually claimed, and plants grown only in red light get leggy and overly lush. The first step is to add a little blue light, then the plants do better. Some people would claim that plants cannot be healthy without light that closely mimics full spectrum sunlight, others that monochromatic red light blows everything else out of the water. I don't see huge differences between them and going to extremes in either direction has big drawbacks.

UV "stresses" plants and not much else. There is certainly no biological need for it and plants can grow without it. In the case of cacti, stressing the plant can be desirable, it produces heavy and dense spination, skin colouration, and "stunts" plants so that they are flatter. Shorter light wavelengths in general produce plants with shorter internodes, smaller leaves, and more branching. Its not quite that simple though, because the primary controller of internode length is the relative levels of red and far red (and near infra-red) light. This is one of the reasons that incandescent lights and HPS lights can produce severe etiolation. This particular combination of light wavelengths is generally ignored in the whole blue/red debate, partly because it is difficult to see the relevant wavelengths and partly because it is even more difficult to control them.

Anecdotally, some cacti will not flower without high levels of UV. The high altitude Andean dwarf Opuntioids are one group, but potentially other mountain species like Pediocactus won't do well without UV. This is particularly relevant for plants which spend pretty much their entire lives behind a piece of glass or plastic which blocks virtually all the UV. It is obviously difficult to separate the effects of UV from full sun at high altitude from just having extremely high light intensity. As you'll see from reading the lizard pages, its hard to get anything like outdoor UV levels from a light. I don't worry too much since my plants get sun in the summer.

Types of Lighting

Light, as much as you can get. Fluorescents for a small area, or metal halides if you want to light a whole room.


No good, too hot, not enough light. There are some metabolic effects where short wavelength infra-red light increases the rate of photosynthesis and some growers have attempted to cheat the laws of physics by adding incandescent light to make their other lights work better. Unfortunately, in almost all cases the incandescent lights are so inefficient that better results are obtained simply by using more or better regular plant lights.

-Fluorescent Tubes

flourescent tubes Good for low to moderate lighting of small to medium sized growing areas, and can be stretched to higher intensities and larger areas.

Four 36W or two 58inch tubes will give you a good even spread over about 10 square feet (in a standard commercial fitting, although you could make it smaller if you build your own) which is good for starting seed or overwintering. I'd suggest the light intensity isn't high enough for successful year round growing of high-light plants. You could get by, but I don't think you'd ever be really happy with the results. Consider high output T5 fluorescents if you want to grow year round, they'll give you nearly twice the light in the same space.

I'm not keen on the little two foot fluorescents. They cost virtually as much as the four foot ones, sometimes more, but they are less efficient and hence less capable of producing sufficiently intense light. Fluorescent lighting can only just about produce the light intensities you need for adult cacti so you really need to be using the best available. Therefore I much prefer to arrange my plants in a shape that can be lit by four foot tubes.

-Compact Fluorescents (CFL)

Compact fluorescents are just a wound up fluorescent tube. Quite a lot of light from such a small area but not usually very powerful so only good for a small number of plants.

A compact fluorescent is always less efficient (hence less bright) than an equivalent straight tube of the same power. In the case of the "Envirolite" type CFL the difference is dramatic, they are comparable in efficiency and life to the small domestic energy saving CFLs. Good straight tubes are about 50% more efficient.

Although a compact fluorescent is less efficient than a straight tube of the same power, it is sometimes more efficient than a straight tube of the same length because short fluorescent tubes are inefficient. A compact fluorescent is a long tube that just loses out a bit because it is bent. Compact fluorescents are also thinner tubes, usually T4 or even thinner, and these generally perform better than fatter tubes. Compact fluorescents also lose output very quickly, much worse than the best straight tubes, worse even than the best modern metal halides and HPS.

The 55W power compact (basically a four-foot tube bent in half) makes a good comparison with the four-foot 54W T5HO tube. The power compact is half the length of the straight tube, gives 25% less light, and lasts half as long. Two straight two-foot T5HO tubes perform almost identically to the 55W power compact, 15% lower power, 10% less light, but a dramatically longer life, although they would take up a wider space. A 55W multi-bend compact fluorescent is less than a foot long, produces barely half the light of the straight tube, lasts half as long to failure, but loses so much output that you should discard it (as a plant light) after 1,000-2,000 hours. Note that standard output (not HO, not overdriven) T5 tubes are even more efficient but the low light intensity compared to T5HO tubes means they aren't usually used for plant lights.

The Envirolite type CFL has just awful performance. The only reason to use them is for a quick and dirty solution for a few hundred hours where low startup cost outweighs the poor performance. Compact fluorescents are useful for very small spaces, but when you want 125W and certainly 250W of light there is always a better solution.

-HID (High-Intensity Discharge) Lighting

HID lamps are just not worth messing with at low powers, inefficient, poor life, hot, and generally not worth the trouble. Normally I would suggest you only use HID lights from about 250W upwards, ideally 400W, for their high efficiency and ability to provide high light intensity over a good sized area. Smaller lights produce less light, lose output fast, have shorter lives, and don't cover a large enough area. They could be used for high intensity lighting on a small number of plants, for example 150W would be good up to perhaps five square feet. That said, I would normally recommend something like a 4 x 54W T5 fluorescent system for someone looking at 150W HID bulbs.

-Metal Halide Lighting

Metal halides have long been the standard choice for growing high light plants, but that choice should no longer be automatic. High performance fluorescents such as T5HO and the newest VHO tubes offer competitive performance and some advantages. Few people will offer an unbiased comparison because they are trying to sell one product or the other. Metal halides offer a small extremely intense light source with high efficiency, but lumen maintenance, lifetime, and cost cannot compete with the best fluorescents. A good 125W pulse start metal halide will offer similar efficiencies to high intensity fluorescent lighting in a much smaller package, but will lose 30% of output in the first 5,000 hours, so there would have to be a pressing need for small size or extreme light intensity to make this worthwhile. Mostly you should stick to the efficient and widely available 400W metal halides which are more convenient than fluorescents for lighting large areas and for providing high light intensities.

Watch out for those metal halide bulbs, they will be putting out a lot less light after you've used them for six months. Historically, plant growers have replaced metal halide bulbs every 6-12 months, although they will run for several years. For a variety of reasons, plant light sellers are often still wedded to an obsolete technology known as probe-start. Modern halide lighting is pulse start, they last longer, they don't lose light as they age, they start quicker, and they are less likely to explode, but they need a different ballast. Pulse start ballasts and bulbs are generally more expensive, but plant lighting is often sold at a huge markup anyway and you should be able to find a pulse start ballast and bulb for less than $170. You'll mostly find them sold as business and warehouse lighting. Another worthwhile efficiency improvement is to get an electronic ballast (often called a digital ballast by plant light sellers although strictly that isn't quite the same thing), they will save about 10% on your electricity bill.

Another very confusing issued much abused by those trying to sell you something is heat. You will get the same amount of heat from 400W of fluorescent tubes as you do from a 400W metal halide. You might get it in different places, but don't be fooled by the fact that one is a tiny raging hot bulb and the other is many metres of moderately warm glass. Still, metal halides inside a home have several disadvantages including fire hazard and fading nearby fabrics. And of course if you don't need 400W of light then there's no point wasting all that electricity and getting way more heat than necessary. Metal halide lamps radiate more heat as infra-red (and UV also) than fluorescents which can be good on an adult cactus but may be too harsh for seedlings.

-HPS (High Pressure Sodium) Lighting

HPS lamps alone would not be a good choice for growing most succulents. They'd tend to etiolate and not grow dense spines or develop good colour. Use metal halides or a mix unless you are just supplementing natural light.

HPS is waaaayyyy better in efficiency than flourescents or metal halide in terms of lumen output but it is in a restricted spectrum that isn't always suitable on its own, certainly not for most cacti although I'd bet that Pereskiopsis would just love it. Metal halide retailers like to compare their products in PAR watts where they can exceed HPS lamps, although this is slightly misleading. When properly measured as a photon flux the HPS are still better.

HPS technology has not advanced as much as metal halide in recent years and the gap is narrower than it once was. Digital ballasts are available, but the basic lamp technology has not really moved on. HPS bulbs have always offered better lifetimes and better lumen maintenance than metal halide but they haven't improved as much so the difference is now much less. HPS lamps don't quite reach peak performance until the 600W-1000W range although 400W bulbs are pretty good. There are an increasing number of crossover HOD lights offering spectrums that are a combination of HPS and MH, or with swappable HPS and MH bulbs in the same fitting, but you always pay a price for the flexibility.

-LED (Light Emitting Diode) Lighting

Many many experiments have been documented using LED grow lights. Performance in terms of plant mass and size is comparable to other light sources at the same light intensity, although it can be very difficult to determine the true intensity of a non-white light source. The typical "red plus a little blue" LED light usually produces different growth patterns, primarily less compact and less branched plants. Note that this is in stark contrast to the claims made by LED plant light sellers who would have you believe that three LEDs running off an AA battery are equivalent to a 1000W metal halide lamp.

I look at LEDs every year or two, but so far I haven't tried them on plants. Until recently they simply haven't been powerful enough or efficient enough at producing light (compared to the best conventional solutions) to make them more than a toy. Now they are available at performance levels which are potentially useful but initial costs are still high, extremely high for commercial LED plant lights. I should probably try a small DIY LED setup but its not going to happen this winter. By next year of course they'll be another 50% better!

The specific wavelengths of LEDs are often given as an huge advantage and used to justify how a massively under-powered light can perform like its bigger brothers. This is 90% marketing hype; LEDs happen to provide light at just a single wavelength so make it into a selling point. In practice, fluorescent tubes have been available for decades providing virtually the same spectrum. The earliest were called Gro-Lux and they were not massively better than simple white light sources, and with significant drawbacks in terms of efficiency and operating life. Today you can still find similar lights being sold, google for "PURple". Gro-Lux fluorescents are still available but they now have a wider spread of light, still mainly blue and red but with other wavelengths also. Experiments have shown variable results from just using monochromatic blue and red light vs white light, sometimes a bit better, sometimes a bit worse, but don't expect the spectrum to change the laws of physics for you. The monochromatic light output, and particularly in far red and blue wavelengths, are very difficult to compare with white light sources. They will look relatively dim because our eyes don't see those wavelengths, and for the same reason the output in lumens is not directly comparable (although it can be converted if you know the wavelength), but few manufacturers will quote accurate photon outputs for their lights. They often won't give you the total light output in any units. Buyer beware!

LEDs produce such strongly directional light, without any need for additional reflectors, that penetration is actually quite good. You would have to check your particular LEDs since some produce a much wider spread of light than others. Many LEDs produce almost their their light within a 10 degree cone, much better than you will achieve with even a very good conventional reflector. An LED with a 45 degree light cone would not be so good and you would have to put it quite close or use additional reflectors. The large arrays that must be used with LEDs naturally produce quite good directional light with good penetration. Even if the LEDs at the edge of the array are sending a little light away from the plants, the vast majority in the middle of the arrays are hitting plants even with their off-centre light. This feature is often used to provide misleading comparisons, where a very powerful conventional light produces similar growth to a small LED light simply because 90% of the conventional light never hits the plants.

There are some other interesting points about LEDs. The long lifetime is often quoted as an advantage. Typically a life of 50,000 - 100,000 hours will be quoted although some of the newer high power LEDs have shorter lives. What isn't highlighted is that the light output of an LED decreases more or less steadily from day one until it is too dim to see. You need to look carefully and see what the actual lumen depreciation is because it may become unusable as a plant light after 10,000 - 20,000 hours. Of course that is still several years and by then LEDs will cost less than a dollar and be brighter than the sun. The best fluorescent tubes lose 5% of output over a 20,000-30,000 hour lifetime, modern HID lamps 10%-20%. LEDs don't really outperform in this respect.

High power LED arrays must be very carefully designed. Cool operation is often given as an advantage, but LEDs actually produce a similar quantity of heat to other plant lights. The difference is where that heat goes. In a metal halide lamp a very high proportion is radiated away, which can be bad for plants that get too close but at least stops the bulb melting! LEDs radiate away relatively little heat, they just dump it into the semiconductor and will rapidly deteriorate without careful heatsink design. Its hard to tell by looking, but cheap LED lights will likely fail long before that 50,000 hours.

I haven't yet seen LEDs for sale which have the same lumen (or photon) output per watt as modern metal halides or fluorescents. Maybe they exist or maybe not, the only ones I've seen are in laboratories or for special order at a special price. Again, the people selling the LEDs like to claim they are more efficient than other light sources, but they also like to use what is essentially obsolete technology for their comparisons. They also like to run "tests" with poor quality conventional lighting where the majority of the light never even lands on the plants. Under those conditions, LEDs blow away conventional lighting at the same power levels, but they don't when you compare best practice setups for both types of light.

The top end LEDs available in the UK far exceed incandescents (although they still can't match the total output from a small area like a normal light bulb), are slightly better than compact fluorescents, are approaching mainstream triphosphor T8s on electronic ballasts, are still some way short of the best T5 and T8 tubes and similar up-to-date (pulse start, ceramic, digital ballast, etc.) metal halide lighting, and way short of HPS lights (which have their own drawbacks). I see reliable data from white light LEDs at around 70 lumens/W, with monochromatic LEDs a little better although you can't measure them in lumens/W. I expect around 90-100 lumens/W from my plant lights and I expect LEDs will reach that level very soon (written in 2009).

-Other Specialist Lights

I try to avoid the specialist fluorescents like the aquarium lights. They have "interesting" spectra but they can be incredibly inefficient and need replacing every few months. Aquarium people have very stringent requirements for colour, UV levels, a difficult job lighting things through some depth of water, and very restricted in getting it all under a small hood. They have really taken to the new technology like overdriven T5 fluorescents and the power compacts which pack a lot of light into a small area. If I grew my cacti all year under lights I'd probably try adding more UV and I'd certainly want to have more light intensity, but for plant . My current lights have what I would call moderate light levels of around 2,000 foot-candles, but the plants seem to overwinter well and some of them even flower.

An increasingly common type of plant light is the so-called purple fluorescent. These appear to be modern triphosphor tubes, usually T5, with some or all of the green phosphor missing. Performance is good, with lumen maintenance and lifetimes comparable to other quality fluorescent lights. They could even perform better than a standard white source due to their spectrum, but it is worth remembering that the red light produced by the red triphosphor is nowhere near the absorption peak of chlorophyll so most of the advertising claims are clearly bogus.

Special Consideration for Seedlings

Cactus seedlings are fairly easy to start under artificial lights. Surprisingly, most of them can't take full sun when they are small so you either have to shade them or grow them under lights. They also, most of them anyway, like high humidity and regular water for the first few months.

I use two spiral compact fluorescents for starting seed, 20W each for germination, up to 30W each for growing on. That propagator is 2.5 square feet. I run the lights for 16 hours a day on a timer.

You can start cactus seeds under just about any fluorescent light. Get whatever is convenient for you and easy to find. Ywo 36W T5s would be enough light for an awful lot of seeds, up to 10 sq ft, although perhaps half that for growing on after the first month or two.

You can also sow in natural light if you have a good bright area not in direct sun. However, you might still want lights for the winter because seeds sown in the fall will still be very small in their first winter.

The standard warm/soft white or cool white fluorescents which you will find in any box store are fine for starting out seeds, but if you are growing on the seedlings for many months like from fall until next spring, you might want to look for 5000K or 6500K tubes which will give you healthier seedlings. You can do it either way but I think you'll find seedlings grown under say a 3500K light for nearly a year will get a bit bloated or tall. Don't worry about the advice your grandfather (or cactus club president!) gives you to mix warm white and cool white fluorescent tubes. This had a useful effect with older designs, but modern triphosphors it is pointless. Pick the shade of white you want and use it.

The "baggie method" is well suited to use in a propagator with artificial light. Pots sealed in plastic can get too hot in the sun so it can be tricky in natural light.

How much light? For cactus seedlings, 15W per square foot would be about right, that's over 100W/sq m. For tiny seedlings just germinated and not with proper spines, half this amount may be enough. For properly growing adult cacti and a few types of seedling, twice as much or more is needed. These figures apply only to the common types of plant lighting, fluorescent tubes and metal halide, and correspond to a little over 1,000 foot-candles or 10,000 lux. Other forms of lighting may produce different levels of light per watt, or even have a spectrum that cannot properly be represented in lumens. For properly growing adult cacti and a few types of seedling, twice as much or more is a minimum and more is better.

Lastly, and most people fall down here, get all of the light onto your plants. If you can see the bulb/tube then the light is shining in your eyes and not on the plants. Use a reflector which almost completely encloses the bulb and focuses the light down just on the plants. Better yet, enclose the whole thing, plants and lights, inside a white or properly reflective enclosure. Cactus seedlings don't mind being enclosed, but monitor the temperature in case it is too enclosed and gets too hot. I use a small extractor fan, actually a PC case fan, but relatively small openings in the top of the enclosure would also be enough for hot air to rise out. Also, after the first few months, try to let the night temperature drop by 20F or more. Constant warm nights and only moderate light levels will produce rapid growth but the seedlings will etiolate.

You could keep almost any cactus seedlings in a somewhat enclosed lightbox for a year or more if you really wanted. Generally they grow faster without the extremes and stresses of natural conditions, but eventually you will probably want to expose them to sun, high and low temperatures, and some water stress so that they don't grow into great green beachballs with no spines For some species such as Strombocactus, a good year or two under lights with high humidity is almost the only way to keep them alive. Others, for example many Opuntias and Echinocereus, are so vigorous that you would generally just stick them in the sun and treat them like an adult after a few months.

Cost Considerations

Last thing to remember, most of the cost of running plant lights is the electricity. Buying a cheap and inefficient system will cost you extra in the long run. Nothing is simple, huh?

Author: Ian Nartowitz*
This article was compiled from several of Ian's posts with his permission.
Index of Articles
Introductory and Naming
     Wherefore Art Thou Cactaceae?
     How to Write a Scientific Name Correctly
     Cacti & Succulent Identification
     Cereus Peruvianus -The Least and Best Known Cactus
     More About Cereus Peruvianus

Cultivation and How-To
     The Sun Burned Cacti
     Grafting on Pereskiopsis
     Making Your Own Cactus Soil
     Growing NON-Hardy Cacti in Cold Climates
     Tools of the Trade
     Growing Cactus with Artificial Light
     Making a Hypertufa Planter
     Cactus Flowers - Fake or Real?
     How-To-NOT Build a Cactus Terrarium
     Raising Cactus From Seed
     Growing Cacti in Terracotta

     Is Cultivation Conservation?
     Interview: Cactus Conservation in Paraguay with Alex Arzberger
     The Fate of the Minnesota Ball Cactus

     A Cactus Odyssey in Arizona
     Mangrove Cactus
     Have a Cup of Cactus
     Opuntia as an Invasive Species in Australia
     The Creating of "Springtime Succulence"
     Making Botanical Illustrations
     Adapt or Perish
     Chasing the Wild Epis
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