A 150W halide will put out as much heat as 150W of fluorescents. The heat will be concentrated in a small bulb which is therefore hotter to touch, but the total heat output is the same.

Small metal halide lamps are not as efficient as larger ones, so I would suggest sticking to efficient fluorescents if this is all the light you need. You will get a more even distribution of light. I don't know about fluorescent brands and availability in Oz, but look for 40W T8 tubes that put out more than 3000 lumens. They might actually be a 32W or 34W tube but they sell them as nominal 40W. Or look for a T5 lamp. Either way, aim for 90 lumens/W or more to get the most light for your dollar.

The only reason to use the metal halide would be to light just one or two pots very intensely since 150W of fluorescents will cover perhaps four feet by one foot while you can get all the light from the metal halide on may one square foot (sorry for old measurements but metres are just too big for what I'm talking about). If you want to light a couple of square metres or more than you should consider a 400W metal halide as the most efficienct method.

If the plants are only a few inches tall then just put the tubes very close to the leaves. If you have tall plants then make sure you have a decent reflector with the tubes so that the light is focused downwards and reaches the bottom leaves properly. You will need good ventilation, probably a fan, especially if you have the tubes very close. Too much heat will be a problem if the air is not moving, all depends on which herbs you grow of course.

shrubs-n-bulbs just gave you a VERY good reply. Pretty much what I would have said. I grow cacti and succulents under a 400w metal halide, and to control heat I just run a fan aimed at the lamp, that way it provides air movement, as well as cools the lamp. For herbs, I'd say fluorescent, as you don't need TOO much intensity. A common theory is that fluorescents are more effecient then metal halide. It all depends how you look at it. For the amount of light, metal halide blows fluorescent out of the water in efficiency. It takes over (40) 4 foot fluorescent lamps to equal (1) 400w metal halide when it comes to the light output. Like mention above, if you go the route of fluorescent, position your plants no more then 6 inches away from the lamps, and make sure they are 100% under the lamps. If they are not directly under, they won't get the needed light, as fluorescents lose intensity VERY quickly the farther you go from the lamp. basically, if you only have enough plants to cover an area about 3 feet by 6", a 2 lamp fluorescent light would work. The reason I say 3' for a 4' lamp is that the light is far more intense toward the center of the lamps, and not at the ends. If you more then that in plants, you may need more lamps in your setup, or you go to metal halide. 150w is really a waste. If you need halides, 400w would be minimum I'd say. 400w can effectively cover an area 4 foot by 4 foot, when positioned 2 to 3 feet above the plants. As for loss of spectrum, all lights lose spectrum. Metal halide lasts a lot longer then fluorescent, but also costs more for the lamps. Fluorescents, if run at 16 hours a day, lose quite a bit of intensity after 6 to 8 months, where halides drop to 85% at around 12 months or so. Also, if you go fluorescent, know that cool white provides primarily blue, which promotes stockey leaf and stem growth, and warm white provides red, which promotes flowering and fruiting. Metal halide is about all blue spectrum. High pressure sodium is Metal halide's red spectrum cousin.

Note: I am an electrician, and know how to build these light set-ups. So PLEASE, if in doubt, DON'T mess with electricity. Know what you're doing to avoid a house fire.

joezkool,
Thanks for that.I have been a bit confused as to which I will use as I have been wondering about heat and light and which makes a plant grow.
So blue encourages growth not flowering....this is what I want.

Esp was the point about making sure that plants are in the centre of the fluro set-up...if I decide to go fluro.

About the 150watt halide if i only had 2 x 1 foot area which is what my plant space is,what would you say to that?150 still to small?

Also i have a contact that had halides for sale but have been in use for 5 years...need new globes?From what I have read they need changing.

maybe be better to go to fluro for such a small area?

Thanks again

Kevin,

For a very small area like 2'x1', even 150W is really too much. A quick rule of thumb is that 20W per square foot is enough for most growing needs. Often you can use less than that, maybe a bit more for real high light situations. So for that small area, what you really need is a compact 40W light source and a reflector to make sure that you are only lighting that area and not the whole room. Metal halides are too powerful and fluorescents are too long, although you could use some 24" fluorescents if you don't mind the cost and low efficiency. I would recommend a compact fluorescent bulb and a reflector, you can get 42W bulbs quite easily, or maybe a bit more if you want high light. I'm not a herb grower, can't tell you exactly how much you need.

On spectrum, the bulk of the light for any type of growing should be red, in the 600nm-700nm range. These photons take the least watts to produce and they are used efficiently by the plant. Blue photons can also be used but it takes more energy to produce each one. In practive you need about 10% of the photons to be blue because they cause the plant cells and leaves to orient properly towards the light and power some internal processes. This is equivalent to about 25% of the energy going into blue light. The physiological balances between plant elongation, branching, and flowering are controlled far more by the relative amounts of red and far red light than by the proportion of blue light.

So it simply isn't true to say that blue light causes growth and red light causes flowering, this is a vast approximation that kind of defines the effects of HPS and metal halide lights. By an odd coincidence (or simply because somebody found out it works), the old cool white and warm white fluorescent combo puts about 75% of the energy into red light and 25% into blue light (ignoring the green bits!) and so do the pinky-purple grow light fluorescents in one tube. Metal halides produce more blue and HPS produces more red. HPS simply produces more photons per watt than other light sources and usually wins out, but the colour distribution can cause plants to elongate rather than grow compact and strong. The light distribution may also encourage plants to flower rather than continuing to grow, but if this can be avoided they will grow just as efficiently as metal halides. Commercial growers of most crops use HPS with blue supplemental lighting (or some natural lighting which includes blue photons) rather than taking the efficiency hit of going to metal halides. Take the hint from these guys because they have to provide quality, but it is their own money at stake.

Metal halide bulbs (and HPS and, to a lesser extent, fluorescents) decrease in brightness over their lifetime. The decrease is not readily apparent to the naked eye unless you compare a new and old bulb side by side but it might be as much as 40% less photons at the rated lifetime. In addition, metal halide lamps have a habit of failing catastrophically if you run them right to failure. There are protective envelopes to minimise damage but failed bulbs can emit dangerous UV and be a fire hazard. Newer bulbs are less likely to explode, but metal halides are typically swapped out on a timetable rather than when they fail. Growers often swap them out at 6,000-8,000 hours to maintain maximum light intensity and efficiency. This might seem excessive and costly, but remember than the bulk of the cost of running any light source is the electricity. It is worth considering that even though you might get a freebie lamp from your friend, it will actually cost you more after a few months if it is more powerful than you need or if it is simply an older less efficient bulb. 150W metal halides, for example, are less efficient than the best fluorescents even with a brand new electronic ballast which your fried's lamp probably doesn't have. Metal halide efficiency peaks in the 400W-1,000W range, with 400W increasingly preferred because they are available with electronic ballasts which improve efficiency by 5%-10%.

So, get yourself a 42W compact fluorescent, or a couple of 25W or 30W ones if you feel you need that much light, and rig up reflectors or white baffles to make sure that most of the light ends up on your small growing area. You can grow under the standard domestic 3000K/2700K warm white colour, but if you have trouble with stretching or bolting then look for a 4100K or 6500K bulb, either as one of the two bulbs or as the only bulb.

The main problem is, plants aren't meant to grow under lights. Will we ever have a 100% efficient way to do it? probably not, but we can do our best. Using red spectrum light will indeed cause tall, lanky plants, and using blue will do the opposite. A combo of both is ideal, and I've read that if you have some natural light, that metal halide is all you need, as you get your red light from the natural sun you get. I don't know facts on this, because I don't have any natural light for my plants in the basement. I use metal halide, and in fall will add 150w of HPS to mimic fall sunlight which is higher in red. As you can see, everyone has ideas and opinions on lighting. As an electrician, I know for a fact that HID lighting is FAR more effecient when high levels of light are needed. Fluorescent wins when the opposite is needed, like in houses or office buldings. But now go to warehouses or street lights. You need more light, lots more. You COULD put tons of fluorescents up and get enough light, but you'd be paying out the wazoo for all those lights, and all the power they'd consume. HID is the way to go when high light levels are needed, such as plant growing. Seedlings need less obviously, so I am talking mature plants. My 400w metal halide draws around 2 amps of power, which equates to 240w at 120v. Works for me!

Something else to consider is that many plants benefit from a period of winter dormancy, with cooler temperatures and shorter days. The best thing for your plants might be to leave them outdoors for the winter and wait for them to bound back in spring, or to build a simple hoop house or cold frame if they require winter protection in colder climates.