A note on lumens & watts...
There are many opinions on the question of “how much light is needed for growing marijuana indoors”.The info below, on lumens or watts per sq/ft, is only a general guideline to how much light you really need. The type of lighting used (High Pressure Sodium/Metal Halide, Compact Fluorescents, T-5, L.E.D.) as well as the stage of plant growth – seedlings: less light, vegetation: more light, flowering: most light. General rule of thumb is – Throw as much light on them as you can, with respect to heat, safety and affordability.
Any thoughts or questions? Drop us a comment below! Thank you, endive – Your humble narrator
In a nutshell…
Lumens per square foot:
Minimum amount of lighting needed is around 2000 lumens per square foot.
(Two – 23 watt CFL’s, per square foot*)
Mid range is around 5000 lumens per square foot.
(Three – 23 watt CFL’s, per square foot*)
Optimal is 7000-7500, or higher, lumens per square foot.
(Five – 23 watt CFL’s, per square foot*)
*I’m only using CFLs as a reference here, with the assumption they’re kept at a distance of 3 – 5 inches from the plant. It is important to understand that the “penetration” (the farthest measure that quality light reaches down into the plant canopy) will at best be only a few inches deep.
Watts per square foot (wattage per sq ft WILL vary by light source):
Minimum wattage per sq ft.: 30w
Mid Range wattage per sq ft.: 50w
Optimal Range wattage per sq ft.: 50-80w
Remember, the output of usable light (by wattage) will vary based on light source!!!
Technical (but not entirely useless) Definitions
- Lumens - one lumen equals the amount of light emitted by one candle that falls on one square foot of surface one foot away.
- Watts – A measure of the amount of electricity flowing through a wire. Watt hours measure the amount of watts used in one hour. A kilowatt/hour (KWH) is 1000 watt/hours.
- Light Spectrum – In simplest terms – Blue (425nm – 460nm, think a 6500k CFL or MH) & Red (630 – 660nm 2700k CFL or HPS). Blue wavelengths are associated with vegetative growth, and reds with rooting & blooming.
What to do with it all
To determine how many lumens per square foot you have:
- Find out the square footage of your space.
- Width x Depth = Square feet. Divide the lumens available by your square footage.
- This will give you lumens per square foot.
Example: Say your space is 3 feet deep by 4 feet wide, 12 square feet. The total lumens available from your light(s) is 45000 lumens. 45000/12 = 3750 lumens per square foot.
To determine how much light you need in watts?
The general rule of thumb for providing light for an area is a minimum of 30 watts per square foot but 50+ watts per square foot is optimal. You can determine the proper lighting for your area by using this formula:
50 watts (or the total watts you are using) x (your square footage).
Example: You have an area of 10 sq. ft. – 50w x 10 s.f. = 500watts/sq.ft minimum or 50 watts x 10 s.f. = 500 watts/sq. ft. (optimal).
Also, remember that fluorescent’s are weaker and emit less light than an HID. This means you will need 5 times the amount of wattage to equal the output of an HID. So, 30 watts of HID would equal 150 watts of fluorescent’s. This is why it is advised to provide a minimum of 30 watts per square foot for HID lights and aminimum of 150 watts per square foot for fluorescent’s.
In more detail: How much light do I need?
Technology has advanced so much in the last 15 years that we are constantly refining the process and updating what we know works best for growing. Current theory holds that the: » Minimum amount of lighting needed to sustain growth is around 2000 lumens per square foot. » Mid range is around 5000 lumens per square foot. » Optimal is 7000, or higher, lumens per square foot. What if you want to determine how much light you need in watts? The general rule of thumb for providing light for an area is a minimum of 30 watts per square foot. 50 watts per square foot is optimal. You can determine the proper lighting for your area by using this formula: 30 watts (or 50) x ?(your) square feet. Example: You have an area of 10 sq. ft. – 30w x 10 s.f. = 300watts/sq.ft minimum or 50 watts x 10 s.f. = 500 watts/sq. ft. (optimal). Also, remember that fluorescent’s are weaker and emit less light than an HID. This means you will need 5 times the amount of wattage to equal the output of an HID. So, 30 watts of HID would equal 150 watts of fluorescent’s. This is why it is advised to provide a minimum of 30 watts per square foot for HID lights and a minimum of 150 watts per square foot for fluorescent’s.
This is all important because the light intensity will directly affect the quality and yield of your crop. If you have less than optimal lighting your yield and potency will be reduced and buds will not develop as dense. This point can not be stressed enough. You must have the right amount of light for your space to grow high quality bud. The question is often asked, “can I have too MUCH light?“. Theoretically, yes you can, but in practicality you probably won’t. According to the law of diminishing returns, you could theoretically reach a point when your plants just couldn’t absorb any more light but it would be impractical and inefficient to do so. Heat from the lights would become a problem long before you ever reached that point. So use as many lights as you want, just control the heat. Experimentation is the only sure method to determine the best solution for each plant. If plants are not receiving enough light, they begin to grow tall and spindly as if stretching for the light and foliage becomes pale green. Or, if they need to be moved closer to the light, or given a longer light exposure period. Too much light may lead to bleaching of leaves and flowers, browning and shriveling. Leaves would become overly compact and curl under at the edges.
PHOTOPERIOD (how long do I keep the lights on for?)
Your plants should be started and taken through vegetative growth with a 20/4 or 18/6 light regimen (on/off). I prefer an 18/6 to running lights 24 hours solid. The reason for an 18/6 regimen is to give the plants a short dark period to rest, grow roots and will help to reduce your electric bill a bit. Most marijuana strains thrive in vegetation, with at least 16 hours of light a day (Some tropical Sativa strains may require more) . Adjustments should be made according to your individual plant requirements. For flowering – 12/12 is the norm. Again, adjustments may need to be made. 12 hours light – 12 hours of darkness, is the preferred light timing for the flowering or budding process.
Words of Caution
Don’t burn your plants by getting them to close to the light(s), or by not having enough circulating air.
- Fluorescents do not put out much heat and can be as close as one or two inches, but plants will grow into the light quickly, sometimes 2-3 inches a day, check them frequently for proper spacing and airflow.
- HID lights get much, much hotter and will need to be farther away. A good test is to put your wrist, palm-up between the light and the plant. Within a minute, if your wrist gets too hot for comfort, the light is too close.
- LED lighting is only warm to the touch, but its soft intensity can be misleading. Keeping an LED grow light too close to plants can result in bleaching, but is easily fixed by keeping a 8″-24″ (depending on lamp wattage) distance between the tops of the plants and the LEDs.
There are some common lights that may induce a seed to come up, but are worthless for growing purposes. These lights include; Any incandescent (regular) light bulb, halogen lights, black lights and heat lamps. Don’t waste your time trying to grow with these lights, you will only be disappointed.
L.E.D. grow lights
Growing indoors and employing L.E.D grow light technology is one of the newer methods employed in growing your plants effectively indoors. This form of light is completely different than the standard (HPS) high-pressure sodium and (MH) Metal Halide variety of grow lamps in many respects.
Plants convert light energy into plant energy via photosynthesis. There are two primary compounds that achieve photosynthesis: Chlorophyll A, and Chlorophyll B. These compounds absorb blue and red light, while nearly all other spectra are reflected away from the plant and into the nether. There is a term called the “absorption peak” which is the point at which Chlorophyll converts light energy into plant energy the most efficiently. These absorption rises can be measured in units called nanometers (nm).
L.E.D’s (LED grow lights) are light sources that emit narrow wavelengths of light, and can be tailored to nearly any nm that you as a grower desire. By using L.E.D’s at the same nm as each of the absorption points for Chlorophyll, high-end LED grow lights (like the Pro-Grow series we use) plants can convert light energy into plant energy much more efficiently than HIDs, a Pro-Grow 260 LED lamp is easily comparable to a 400 watt HPS, though is consumes less than half the power (180w ±).
High Intensity Discharge lights (HID), like High Pressure Sodium and Metal Halide, emit a very wide spectrum’s of light (much of which is unused by plants in the form of heat), quality LED grow lights, primarily emit the spectra plants require, which results in much higher efficiency in terms of growth per wattage used. By using the ideal ratio of proper spectra L.E.D’s, 95% – 98% of the light generated by the units is usable by the plant for photosynthesis. The bottom line is that your plants will utilize less overall electricity and produce large yields similar to that of HID lamps.