Indoors, 2000 lumens per sq. ft. is about as low as you want to go indoors. If you get under this mark, plant growth will certainly not go as fast as possible, and internode/stem length will increase. Also, light distance to hemp plants will be much more critical. Daily adjustments to the lamps will be necessary, meaning you get no vacations.
2500 lumens psf should be a good target, and 3000 is optimal if your going to inject or enrich CO2 levels (more on that later).
High Intensity Discharge lamps are the best solution for most indoor growers. HID lamps come in 3 basic flavors: High Pressure Sodium (HPS), Metal Halide (MH) and Mercury Vapor. Metal Halide is an improved spectrum, higher intensity Mercury Vapor design. HPS is a yellowish sort of light, maybe a bit pink or orange. Same as some street lamps.
HPS lamps can be used to grow a crop from start to finish. Tests show that the HPS crop will mature 1 week later than a similar crop under MH, but it will be a bigger yield, so it’s better to wait the extra week.
The easiest HID to buy, and least expensive initially are the flourescent and mercury vapor lamps. MV will put out about 8000 lumens per 175 watts, and 150 watts of HPS puts out about 15k lumens, so HPS is almost twice as efficient. But the color spectrum from MV lamp output is not as good. HPS is high in reds, which works well for flowering, while the Metal Halide is rich in blues, needed for the best vegetative growth. Unfortunately, MV lamps provide the worst spectrum for plant growth, but are very inexpensive to purchase.They are not recommended, unless you find them free, and even then, the electricity/efficiency issues outweigh the initial costs saved.
400 watt HPS will output around 45k lumens. For every 500 watts of continuous use, you use about $20 a month in electricity, so it is evident that a lamp taking half the power to output the same lumens (or twice the lumens at the same power level) will pay for itself in a year or so, and from then on, continuous savings will be reaped. This is a simple initial cost vs. operating costs calculation, and does not take into account the faster growth and increased yield the HPS lamp will give you, due to more light being available. If this is factored into the calculation the HPS lamp will pay for itself with the first crop, when compared to MV or fluorescent lamps, since it is easily twice as efficient and grows flowers faster and bigger.
Lamp Type Watts Lumens per bulb Total efficiency
Fluorescent Bulb 40 3000 400 watts = 30k lumens
Mercury Vapor 175 8000 400 watts = 20k lumens
Metal Halide 400 36000 400 watts = 36k lumens
High P. Sodium 400 45000 400 watts = 45k lumens
Notice the Mercury Vapor lamps are less efficient than the fluorescent (FL), and can not be positioned as close to the hemp plants, so the hemp plants will not be able to use as much of the MV light. The light distribution is not as good either. MV lamps simply are not suitable for indoor gardening. Use flourecent, MH, or HPS lamps only. Halogen arc lamps generate too much heat and not very much light for the wattage they use, and are also not recommened, even though the light spectrum is suitable for decent growth.
There is a new type of HPS lamp called Son Agro, and it is available in a 250, 1000, and 400 watt range. The 400 is actually 430 watts; they have added 30 watts of blue to this bulb. It is a very bright lamp (53k lumens) and is made for greenhouse use. These bulbs can be purchased to replace normal HPS bulbs, so they are an option if you already own a HPS lamp. The beauty of this bulb is that you do not give up most of the advantages of MH lamps, such as minimal internode spacing and early maturation, like most HPS users do, and you have all advantages of a HPS lamp. One bulb does it all.
Internodal length of hemp plants grown with the Son Agro are the shortest ever seen with any type of lamp. Plants grown under this lamp are incredibly bushy, compact and grow very fast. Son Agro bulbs however, do not last as long as normal HPS bulbs. There is something like a 25% difference in bulb life.
Metal Halide (MH) is another option, and is available in both a 36k and 40k lumen bulbs for the 400 watt size. The Super Bulb (40k) is about $10-15 more, and provides an extra 4000 lumens. I think the Super Bulb may last longer; if so, that makes it the way to go. Halide light is more blue and better than straight HPS for vegetative growth, but is much less efficient than HPS. It is possible to purchase conversion bulbs for a MH lamp that convert it to HPS, but the cost of the conversion bulb is more expensive than the color corrected Son Agro bulb, so I would recommend just buying the Son Agro HPS. Even though it costs more initially, you get more for your energy dollar later, and it’s much easier to hang than 10 fluorescent tubes.
If you have a MH 36k lumen lamp burning at 400 watts and a 53k lumen HPS burning at 430 watts, which is better efficiency wise? Which will provide a better yield? Obviously, the Son Agro HPS, but of course, the initial cost is higher. Actually, the ballast will add about 10% to these wattage numbers.
The Son Agro bulb will prove much better than the MH for any purpose. The MH bulb does not last as long, but is cheaper. Compare $36 for a 400 watt MH bulb vs. $40 for the HPS bulb. Add $15 for the Son Agro HPS. The HPS bulb life is twice as long. 10k hours vs. 21k hours. The Son Agro is 16k hours or so. Still, longer bulb life and more light add up to more for your energy dollar long term.
Horizontal mounting of any HID is a good idea, as this will boost by 30% the amount of light that actually reaches the hemp plants. Most HID’s sold for indoor garden use these days are of this horizontal mounting arrangement.
HPS is much less expensive to operate than any other type of lamp, but comes in the 70 watt size at the home improvement stores. This size is not very efficient, but blows away FL in efficiency, so they might be an alternative to FL for very small operations, like 9 sq. feet or less. Over 9 sqr. feet, you need more light than one of these lamps can provide, but you could use two of them.
70 watt HPS lamps cost about $40 each, complete.
Two lamps would be 140 watts putting out about 12k lumens, so it’s better than FL, but a 150 watt HPS puts out about 18k lumens, the bulb life is longer, bulbs are cheaper and the lamp more efficient to operate. The biggest problem is that the mid size lamps like the 150 and 250 watt HPS are almost as expensive to buy as the larger 400’s. For this reason, if you have room for the larger lamp, buy the 400. If your going pro, a 1080 watt model is available too, but you might find there is better light distribution from two 400’s rather than one large lamp. Of course, the two smaller lamps are more expensive to purchase than one large lamp, so most people choose the larger lamp for bigger operations.
Heat buildup in the room is a factor with HID lamps, and just how much light the hemp plants can use is determined by temperature, CO2 levels, nutrient availability, PH, and other factors. Too big of a lamp for a space will make constant venting necessary, and then there is no way to enrich CO2, since it’s getting blown out of the room right away.
Bulb Costs: the bulb cost on the 70 watt HPS is $24, the 150 is only $30, and the 400 is only $40. So you will spend more to replace two 70 watt bulbs than you will to replace one 400 watt HPS. (Go figure.) Add that up with the lower resale value on the 70’s (practically nothing) and the fact that they are being modified and are not suited to this application, and it becomes evident that $189 for a 250 HPS lamp, or $219 for a 400, might just be worth the price. Keep in mind that for $30 more, you can have the larger lamp (400watt) and it puts out 20k lumens more light than the smaller lamp. Not a bad deal!
Here is the breakdown on prices (from memory):
Type Complete Cost Bulb Cost Bulb Life Lumens
HPS 400 $219 $40 18k hours 50k
MH 400 $175 $37 10k hours 36k
Son Agro400 $235 $55 15k hours 53k
Super MH400 $190 $45 ?? 40k
MH 250 $149 $32 ?? 21k
HPS 250 $165 $36 ?? 27k
HPS agro250 $180 $53 ?? 30k
MH 150 $139 $25 ?? 14k
HPS 175 $150 $30 ?? 17k
If your looking for these types of lamps, look in the Yellow Pages under gardening, nursuries, and lighting for indoor gardening stores in your area