Marijuana Grow Lights
LED grow lights – More detail
HID grow lights – More detail
High pressure sodium (HPS) grow lights – More detail
Metal halide (MH) grow lights – More detail
Fluorescent (CFL) grow lights – More detail
Grow light cycle timing – More detail
Marijuana grow lighting basics
We will take a look at the specific light required by marijuana for growing purposes. Light is on a spectrum like a rainbow, from gamma rays right through the visible spectrum (think rainbows) to radio waves. The visible light spectrum that you and I can see with our eyes is very small actually and pretty much the same spectrum that marijuana loves to grow in.
Let’s start with a bit of light measurement; nanometres (nm) are a measurement of one specific wavelength of light (one specific color, one billionth of a meter on the scale). While Kelvin (°K) which is discussed in more detail later on, rates the ‘temperature’ of a light and offers an indication to the peak nanometers (nm) within the spectrum that each light offers. A spectrophotometer has a more sophisticated system of separating the light into the individual wavelengths and measuring the intensity at each individual wavelength. Typically, a spectrophotometer is a scanning instrument that measure nanometres, it has a monochromator which separates the individual wavelengths of light. For example, it will scan from something like 400 nm to 700 nm at 100 nm per minute.
Marijuana likes light in the spectrum from approximately 420nm through to 750nm
Marijuana in nanometres – What light does marijuana need?
The simple answer is P.A.R or Photosynthetically Active Radiation which covers the spectral range (wave band) of solar radiation from 400 to 700 nanometers.
This is the range of light that photosynthetic organisms can use for photosynthesis. Some light outside of this band can also be useful to mimic more ‘Spring’ or ‘Autumn’ light which plants do generate growth from. Chlorophyll is a pigment in plants and the most common, it is most efficient at utilizing red and blue light. There are other pigments in plants known as carotenes and xanthophylls which utilise green light within the photosynthetic process but you can see that plants do not use much of this light as leaves are green meaning they are reflecting that colorlight.
What we perceive as visible light is comprised of seven spectral colors. Look at a rainbow and you’ll see them in their pure form. All the more subtle colors we see like pinks, browns, and beiges are just mixtures of other colors. As we already know, wavelengths of light are expressed in nanometers (nm) or billionths of a meter. Visible light occurs within the 400 to 700nm range. Below 400nm, we move towards ultra violet (then into x-ray and gamma rays); above 700nm is infra red, then microwave and then radio waves.
As with visible light as a whole, each of the spectral colors has its own wavelength band, as shown in the chart below.
380nm – 430nm Violet
430nm – 500nm Blue
500nm – 520nm Cyan
520nm – 560nm Green
560nm – 590nm Yellow
590nm – 625nm Orange
625nm – 780nm Red
Humans recognize these wavelengths as different colors starting at approximately 400 nm as the shortest wavelength within violet light and 700 nm as the longest wavelength within red light. Human eyes are particularly sensitive to wavelengths between 500nm and 600nm, so we perceive light with a high proportion of yellow and green as being particularly bright.
For plants, however, the green and yellow ranges are slightly less useful because more light (only about 3% more) in these wavelengths is reflected by leaves. This is why we perceive most plants as green!
Almost all plants can use light across the entire visible range for photosynthesis. Some wavelengths are even more useful at certain stages of a plant’s growth, such as the blue, orange and red ranges. Light with a high proportion in the blue range enhances vegetative growth, while the orange and red ranges more strongly influence flowering and fruiting, as well as the swelling of bulbs and tubers. You can observe this in nature – in spring and early summer, when days are lengthening and plants are bursting into growth, the light is more blue. In the shortening days of late summer and early autumn, the light is characteristically warmer due to the greater proportions of orange and red. Landscapes by the great painters such as Van Gogh and Turner illustrate this changing character of light very clearly.
Marijuana plants (Cannabis) need light from the right part of the spectrum with the right amount of intensity to encourage proper yet fast growth. Cannabis plants respond principally to light from the blue end of the spectrum for vegetative growth (420 to 550) and from the yellow, amber, red end for root growth and flowering (550 to 750).
The image above outlines the entire spectrum of visible light in nanometres (nm);
Point (A) in the blue spectrum indicates is the light spectrum that marijuana plants require for vegetative growth;
Point (B) outlines the yellow, amber and red spectrum which are required for pre-flowering and flowering (budding).
Growing marijuana will use the light from approximately 400 through to 700 nanometres (nm). This will cover ‘chlorophyll a’, ‘chlorophyll b’ and photosynthesis production which is known as the Photosynthetically Active Range, or P.A.R for short. You may have heard of P.A.R meters?
Luminous flux is the brightness or the amount of light a lamp puts out frequently measured as lumens although PAR is preferred. Lumens are the least reliable way to measure grow lights, PAR is the second least reliable. PAR is preferred to lumens, but is certainly not the best, so no argument here. To determine the best one would have to have a universally agreed upon optimal spectrum (there is no such thing) and do a quantum or weighted reading on each frequency or group of frequencies so all we have are approximations. We are currently working on the Light Spectrum article where we get into this. Unfortunately, it may not clarify, as there is tons more work to be done by botanists in the area.
PAR Example: PAR of our noonday sun = 2010MicroMol; PAR from a 3D cell flashlight at 10cm = 1100MicroMol – you surely wouldn’t grow a plant with a hand torch? 200W of just green at 550nm will read the same as 200W of red at 660nm, or 200W of full spectrum light that covers everything in between.
What do grow lights offer then?
HPS lights offer light in the spectrum from approximately 540 through to 700 nm, typically yellow, amber and red light so they are very good for flowering/budding marijuana.
MH lights offer light from approximately the 350 to 550 nanometres (nm) in the blue, green to yellow range. However it always depends on what brand lamp your purchase as some HPS lamps are ‘cooler’ and some MH lamps a bit ‘warmer’. MH lights are good for the vegetative stage but not good at the end flowering/bud stage.
LED grow lights are able to have a mix of LED diodes together. The manufactueres choose exactly what LEDs they wan from ‘bins’ and the best mimic sunshine, a more ‘white light. The best LEDs are full spectrum and cover P.A.R with some blues, some reds and most importantly a very stable white to create good broad (full) spectrum LED grow light. These are the new breed of grow light that cover the full grow range from around 420 through 750 nm and can be used for growing marijuana from the seed through to good solid buds. Read our full LED lighting guide.
The colors of light and Kelvin (°K)
The formula between Kevin and nanometres; 2,897,768 / K = nanometres (nm) however the two do not relate perfectly and this can only be used as a guide as to where a light source might actually be peaking in a precise nanometer (nm). Remember each light source will offer a spectrum measured in nanometres (nm), say 400 to 700 nm.
We can see from the scale below how different light sources hit the Kelvin (°K) scale at different places. The lower the degree K, the more “warm”, or red the light appears. The higher the degree K, the bluer, or “cooler” the lamp appears. Kelvin is a scientific term; Kelvin temperature measures the color of a light source relative to a black background.
In simpler terms, it is the average degree of warmth or coolness of a light source, not with regards to the physical temperature, rather to the visual temperature of the light.
Depending on which metal halide (MH) grow light you use they have an average of 3,200 to 5,500 °K while high pressure sodium (HPS) grow lights have an average of 2,200 °K and full spectrum LEDs use a number of different LEDs in a mix which cover the blues, reds and also whites and far red. You often see LEDs that just look like a mix of blue and red light these can work for the vegetation side of growing but the better versions look like ‘white’ light and emulate the sun very well.
So how does this all relate to grow lights?
Metal Halide (MH) lamps typically produce a high proportion of light in the blue wavelength, so are used early in the growth cycle to promote compact, bushy plants. High Pressure Sodium (HPS) lamps produce high proportions of light in the orange and red wavelengths but relatively small amounts in the blue range. This is why HPS lights are used to trigger bud formation and enhance flowering and fruiting. In practice, plants can be grown under HPS lamps for their full cycle, however, they are likely to grow taller with longer joints between the leaves. To overcome this, some growers use blue Compact Fluorescent Lamps (CFLs) together with HPS to supplement the blue light during the vegetative stage. CFL grow lights run much cooler than MH or HPS, so can be placed closer to the plants. Several manufacturers are now producing CFL more in tune with growing plants, including producing light into the orange end of the spectrum. Some growers routinely use CFL for all of their seed/clone/early vegetative stages of growth.
We have used the HPS lights for a long time but have migrated to LED grow lights these days as they offer a lot more value for your money in the long term. If you purchase full spectrum LED grow lights you will be able to grow successfully from seed to harvest with one light with amazing results above 1 gram per Watt. LED grow lights run on around 50% of the electricity when compared to the same HPS grow light so a 500 Watt (true draw) LED grow light will offer better output in terms of yield than a 1000 Watt HPS unit. The LEDs just don’t waste electricity by creating heat and running ballasts. Also the LED light has a far lower heat signature (output), no ballast and amazing bulb length of 50,000+ hours.
Should I buy HID or Full Spectrum LED?
Making a choice between traditional High Intensity Discharge (HID) grow lights and full spectrum LED grow lights used to be a difficult decision because the LED market contained a lot of dubious traders out to make a quick buck with websites full of misleading claims. These days there are some really solid performing LEDs and we know numerous pro growers (a couple of the biggest (!!) name seed breeders too in the Netherlands) who prefer them. The LEDs save them money on electricity, produce better results, reduce fire hazards, heat issues and bulb replacement. To help you make a more informed decision, we look at the pros and cons of the different types of grow lights and guide you through the science behind the manufacturer’s specifications. There is a good comparison within our LED lighting guide.
HID (High intensity discharge) marijuana lighting, HPS and MH
There are two types of HID grow lights used for growing marijuana, metal halide (MH) and high pressure sodium (HPS). Although high quality LEDs are a better bet as they save you money in the long term and eliminate heating issues, they do cost more for an initial setup when compared to HPS lighting. So if you just want to have a bash at growing some marijuana at home then maybe a cheap HID (HPS/MH) setup is more your style. More detail on HID grow lights.
HID = High Pressure Sodium (HPS) or Metal Halide (MH)
Metal halide bulbs produce an abundance of light in the blue spectrum by an electric arc through the high pressure gaseous mixture of metal halides (bromine and iodine) and argon, xnon and mercury. This color light promotes plant growth and is excellent for green leafy growth and keeping plants compact.
High pressure sodium (HPS) bulbs emit an orange-red glow and are deficient in the blue spectrum. This band of light triggers hormones in plants to increase flowering/ budding in plants. They are one of the best lights available for secondary or supplemental lighting (used in conjunction with natural sunlight or MHs). Using this as a sole point of light is only recommend for greenhouse growing applications. More detail on HPS grow lights.
Other than LEDs we feel HID’s are still one of the best types of light to be used as a primary light source for vegetation (if no or little natural sunlight is available) but depending on the brand you may lack important amber and red light required for flowering. Metal Halide (MH) and High Pressure Sodium (HPS) lamps have been around for a long time, and over the years, we have seen slow but steady improvements in bulb life and light output. However, their drawbacks remain. More detail on MH grow lights.
Drawbacks of HPS and MH grow lights
- HID bulbs get VERY hot. HID lamps are poor at turning electricity into light, with a great deal being wasted producing unwanted heat. This means you generally need to install cooling fans or ventilation systems in your grow room. If the grow room is large enough, air conditioning will be necessary to keep plants from burning. You also need to be wary of positioning lights too close to the plants, especially in the early stages of growth as they will get burned;
- Due to the heat that is emitted from these intense types of fixtures, HID fixtures need to be hung high above the plants. Smaller wattage systems (100W or 250W) should be hung about 2 to 3 feet from the top of the plants. Medium Wattage systems (400W or 600W) should be hung around 4 feet from the top of the plants. High Watt systems like the 1000 Watt HPS lights should be placed at least 4 to 6 feet from the plant tops to avoid burning;
- MH and HPS bulb performance deteriorates progressively with use and bulbs need to be replaced at a bare minimum every 18 months to two years or so – not left until they burn out. However, most growers replace bulbs every grow cycle or two, meaning, 2-4 times per year;
- The spectral range of MH and HPS lamps is limited, so you need more than one light to provide ideal conditions for both vegetative growth and flowering (so-called dual spectrum lights with a MH and an HPS lamp in the same fitting are at a best a poor compromise). MH lamps produce a high proportion of light in the blue part of the spectrum, which promotes strong vegetative growth, while HPS are heavy at the red end of the spectrum, but lacking in blue light, so are used during the flowering stage.
Perhaps the biggest drawbacks of all are the high electricity consumption and running costs. A lot of energy is used to power the electromagnetic ballasts – around 60W to 120Watts – or to produce light at wavelengths that are of limited use to plants. Having said all this, any competent grower can be reasonably assured of good yields with these lights, being the standard of home and commercial users for decades.
Estimates for crop yields depend on many variables, such as type of plant, specific strains, and growing techniques. Using marijuana as an example, 0.5 grams per Watt based on 45-70 days in flower should be achievable. Some growers claim to get yields of over 1 gram per Watt in ideal conditions, but this is an exception. We find with LED grow lights it is relatively easy to achieve 1 gram per Watt with the better full spectrum solid state models.
Full spectrum LED
LEDs were first introduced into electronics in 1962 and it should be no surprise that after 50+ years those science boffins have had enough time to tweak them so that they are now commonplace in car headlights, traffic lights and hydroponic growing. High intensity discharge (HID) lamps such as metal halide (MH) and high pressure sodium (HPS) also work well but they are extremely wasteful in turning electricity into light with a vast amount of electricity turned into unwanted heat which usually requires solid ventilation systems to move air and keep the environment cool. LEDs are so efficient they use up to 60% less power than an equivalent HID because they create pure light instead of wasting electricity on heat.
As the name suggests, full spectrum LED grow lights can provide the right kind of light for both vegetative growth and for flowering. Full spectrum, however, is not a scientific term and doesn’t have a fixed definition. The concept of full spectrum lighting is credited to photographer John Ott who was employed by Disney during 1950s to make time-lapse documentaries of plants in growth. His idea was that full spectrum lighting should mimic natural daylight.
The problem with natural daylight is that it is constantly changing, which is not what you want from an artificial light, so a theoretical notion of daylight has to be used. In photography, a daylight lamp is rated at a Correlated Color Temperature (CCT) of 5500º Kelvin. At 5500ºK, the color approximates daylight and this is the CCT that John Ott sought to achieve with his full spectrum lighting. In contrast, a typical metal halide lamp will have a CCT of 6500K, creating a cool (blue) light, while a typical HPS has a CCT of 2700K, giving it a warm (orange-red) glow. If you compare CCT with spectral distribution, or, the actual colors that make up the light, MH lamps produce spikes in the 480-600nm range (blue, yellow and green) while HPS lamps peak in the 560-640nm range (yellow, orange, and red). Theoretical natural daylight (5500ºK) produces a much smoother curve across the whole visible spectrum (400-700nm) but is highest in the 420-600nm range, falling off as it goes towards both violet and red. Read our full LED lighting guide
However, there is more to a full spectrum lamp than just emulating daylight
HID lights produce distinct spikes at specific wavelengths caused by the metal compounds used in lamp. Natural daylight, on the other hand, has a smooth distribution of light across the entire visible spectrum, without any major spikes or troughs. Some manufacturers of LED grow lights aim to create a broad (full) spectrum that closely follows natural light. Other manufacturers have a looser interpretation of the term and advertise lights targeted at bands within the blue and red ranges as being full (or broad) spectrum. Lights described at full spectrum also may emit small amounts at the infra red and UV ends of the spectrum. John Ott believed that this was necessary for the health of both plants and humans. There is also speculative evidence that small amounts of UV and infra red increases yields more resins and essential oils. Too much UV, however, will damage plants. Further, it has been proven that light in certain frequencies above 700nm serve to regulate a plant’s photocycle.
You may note the Super Grow LED banners on this site, we only recommend these lights because they are the best on the market beating the likes of Advanced, HydroGrow and Prosource. Why? To start with they beat those other companies hands down on a $ per sq ft of flowering basis and deliver higher yields per Watt than HPS lights, we find most average users achieve 1 gram per Watt and the PROS get closer to 1.5 grams per Watt. Super Grows LEDs are proper full spectrum using white LEDs with light from 420 to 750 nm and no gaps. They are built beautifully with high quality materials such as aluminium anodized heat sink and chassis, run with NO FAN and you get UV stabilized TIR lenses to maximize the LED power. Super Grow LED have incredible service with in-house lighting engineers and their product is backed by an industry leading 5 year warranty. Their new Spectrum King (SK450) has 450 Watts of LEDs (3W x 150) but runs at 250 Watts (true draw) and two of them are equal to a 1000 Watt HID. Read more at Super Grow LED.
How much light
So far, we have looked at the quality of light. Equally important is the quantity of light your marijuana will receive. HPS and MH are used to light streets and stadiums and are visibly very bright. However, as we have learned, the important thing for grow lights is to provide enough light in the right wavelengths. Although variable, as little as 15% of the light from MH and HPS may actually be utilized by plants. LED grow lights have the potential to be much more efficient by accurately targeting the relative photosynthesis absorption at each wavelength, but some do this much better than others. LED rigs with bright red and blue lamps require deeper investigation into actual spectrum delivery. Manufacturers should, at the very least, describe the spectral distribution of their lights and provide full relative absorption data across the spectrum. In true full-spectrum lighting, what you want to see from your LED lights is a very ‘white’ or ‘pink’ looking bright light from the LED lamps, not purple, made up of reds and blues. In fact some LED lamps may look like they are not even turned on, that is because they are emitting light not visible by the human eye.
The question is, how can you decide which grow light meets your needs based on the manufacturers’ specifications? There are various ways of measuring light. A lot of manufacturers quote figures in lumens – units for measuring the visible light energy emitted from a source. You may also come across lux, which is a measurement of light falling on a surface (lux are lumens per sq m). However, lux is rarely quoted as it varies according to the distance of the light from the surface, the angle of the beams, and, with HID lights, the angle and efficiency of reflectors.
For growers, the big problem is that more lumens or more lux don’t necessarily mean more light for your plants. Lumens are based on human vision and are biased towards the green-yellow area of the spectrum. If you take a standard 400W MH lamp it will produce around 140 Watts of light energy. In comparison, a 400W HPS produces around 120 Watts of light energy, but will have a higher lumen rating because it produces more yellow light.
The figures for light energy in the above example are based on Photosynthetically Active Radiation or PAR. This is how photobiologists and agricultural scientists measure the amount of light in the 400-700nm wavelength falling on the leaves. Readings are taken with a quantum meter, which counts the number of photons (tiny light particles) falling on a square meter of surface area per second. PAR readings can be expressed in several ways. The simplest and most commonly used is the Photosynthetic Photon Flux Density (PPFD) method which counts the photons within the 400-700nm wavelength range. The measurements are in micromoles or μmol (μmol =16 x 1017 photons) per m2 per second.
Some LED manufactures have now started to publish PAR readings for grow lights but there is no real consistency. Statements such as “High PAR readings, higher than HPS”, as quoted by one manufacturer, are next to useless. A single figure for peak photon delivery is also not very helpful without anything to compare it with. What we need to know is how it relates to the spectral distribution and the optimal requirements of the specific crops we are interested in.
While PAR readings are a positive step forward to help growers understand and compare lighting systems, accurate PAR readings can be notoriously difficult to achieve. Researchers have also shown that quantum meters from different manufacturers measure in different ranges and can produce different readings under the same conditions. In addition, PAR utilization varies between different plant species and strains, and according to the growth stage of the plant. Until there is an agreed methodology adopted as a standard amongst all LED suppliers, PAR readings are best treated as a guide rather than strict comparative data.
The bottom-line is the more light, the bigger the yield. Just make sure you are not going to burn your plants.
What about Watts? 3W x 150 LEDs = 450 Watts = Not true!
In the days when we just had HID lights, a good general rule of thumb was to aim for around 50W per sq ft. Clearly, we can’t apply this formula to LED grow lights, as they deliver more useful light for less Watts. So how can we tell whether or not they give out enough light? Wattage for LED lights is commonly misquoted. For example, although you may expect 120 x 3W LEDs to deliver 360W, by reading the full specifications, you may discover the system is only powering the 3W LEDs at a maximum of 2W or 2.2W per unit. You can work this out using the formula Watts = Amps x Volts. Say, for example, the system runs at 110V and is rated at 2 Amps The maximum Wattage available is therefore 240 Watts (110 x 2). If there are 120 LEDs, that leaves no more than 2W available for each one. While stating LED potential rather than actual power draw may be misleading, the fact that LEDs are not powered to their full potential is a design point meant to reduce heat and therefore, extend an LED grow light’s lifespan. A few manufacturers openly distinguish between true Watts and LED Watts, but many don’t, so, do the math before you buy.
What a lot of the LED grow light specifications do include is a flowering and vegetative footprint. These are the recommended coverage areas for the lights. In theory, we should be able to calculate and compare Watts per sq ft from these figures. In practice, it’s not that simple because there are so many variables such as spectral distribution of the lights, affected by ‘beam’ angle of the LEDs, lenses and reflectors used in the design (lenses and angled beams produce a more intense light over a smaller growing area with less wastage). Because LED grow light systems can be configured in so many different ways, there is not necessarily any direct correlation between Watts per sq ft and the amount of usable light when comparing different models from different manufacturers.
Marijuana lighting system costs to run
To calculate your daily electricity cost you need to add up the total Watts for each part of the grow system; lights, ballast, fans and pumps etc. Take the grow lights and other electrical equipment’s combined Wattage, and divide it by 1,000 to find the number of kilowatts (kWh) used per hour. Then multiply that number by the amount your electric company charges per kilowatt hour (kWh).
Example; HID lights will use the number of Watts it emits per hour, i.e.; 1000 Watt system will use 1000 Watts per hour (regardless of spectrum) + around 150 Watts for the ballast, that equates to 1.15 kilowatts. So if your electric company charges you $0.20 per kilowatt hour this lamp will cost 1.15 X $0.20 = $0.23 per hour to run. Multiply that out for a daily or monthly cost per lamp, at 18 hours on per day it will cost $124 per month to run. LED grow lights run at about 50% to 60% of these rates.
LED grow light manufacturers typically claim that their systems are equivalent to an HPS lamp of twice the wattage. Based on this, the running cost of LED lighting would be up to 60% cheaper than HPS, when power used by the ballast is taken into account.
With HPS and MH, you also need to account for extras like fans, vents, active cooling (air conditioning) and extra reflectors that you may not need with LEDs. However, you can buy a good 1000W HPS lamp with ballast and reflector for around $250, + ventilation system + bigger grow tent to cover height between lights and plants so call it $500. Prices vary a lot, but consider an equivalent (high end) set up with two x 250W LED rigs cost around $1,700. That’s an extra $1,200 on your set up costs. Read on…
Electricity costs for the HPS are likely to run at something like $1,000 a year for 5,000 hours of use and replacement bulbs cost around $40. The two 250W LEDs will use half the electricity, so our bill for LED will be $500. In the first year, the LED system has cost us $2,200 compared with $1,540 ($500 kit and extra fan tent size + $1000 electricity + $40 bulb) for HPS. However, from year two onwards the LED will be saving us $540 ($500 in electricity + $40 in bulbs) a year in running costs. After five years, that’s a total saving of nearly $3,000. With HPS, it’s also likely that your ballast will need replacing after 20,000 hours, which could cost you another $120 or more.
This is just a very rough calculation but it does illustrate the potential to make big savings in the long term by switching to LED including less electricity for ventilation, less space required, higher yields, limited chance of fires and far lower heat signatures.
Conclusions LED vs HPS /MH
Having a basic understanding of how LEDs differ from MH and HPS grow lights will help you make a more informed decision. But comparing LED grow lights in terms of their light output and spectral distribution is not as straightforward as it could be. The more information manufacturers supply the better and the more honest they are, the more they will win our trust. If you study the websites, you should be able to distinguish between the fly-by-night suppliers making extravagant claims and those who are in the business for the long term.
Fluorescent and compact fluorescent (CFL) grow lights
This type of light is perfect for starts and seedlings. They are also popular for growing low-light plants like herbs and African violets. Fluorescent lights are low intensity and need to be placed within 8” (up to 15” for shade loving plants) of the plants to be effective. They are generally a poor light source for flowering and budding primarily because of their low lumen output. Read about the new CFL grow lights. Like LEDs and CFLs incandescent lights are also good for starting germination and seedlings They are not very good at all for the vegetative growth or flowering stages because of their low lumen output and limited range. In more recent years LEDs have over taken CFLs even in small grow areas due to the vast improvement in the cost of individual powerful LEDs.
Optimal light height (OLH) and the ‘inverse square law’
Light is governed by the inverse square law. Basically this means that if the light is twice as far away from your plants it will only receive a quarter the amount of light. Placing the lights closer to the plants increases the intensity but decreases the growing area. With MH and HPS, placing the bulbs too close will scorch the leaves. With LEDs, though a mounting height of 12”-24” above the plants is generally recommended, it usually won’t hurt if the lights are even closer.
LEDs – around 12 to 24 inches from the top of the plants;
400W HPS – 18 inches;
600W HPS – 24 inches;
1,000W HPS: 24 to 36 inches.
Vegetative grow light timing
After seed germination your plant will move into the vegetative growth phase known as the ‘photoperiod’ which is the direct relationship that your plants have with the hours of light and darkness offered to them.
With control over the amount of light your plants receive you can effectively dictate their grow cycle. You can replicate vegetative growth by using a timer to have your lights on between 18 and 24 hours and off with darkness for the remaining period; between 6 and 0 hours every day. More information on pruning during vegetative growth.
HPS lights are not as active in the blue / green light spectrum required for active vegging and although MH lights are they can be quite harsh to the young seedlings primarily due to their heat generation. If you plan to use HPS lights later on then we suggest starting out with CFL’s or LEDs to let your seedlings build up to stark heat given off by MH and HPS lights. Think about nature where the seeds would germinate in spring when the sun is not as strong as it will be in summer, it is best to try and replicate that.
An alternative to growing under HPS or MH during vegetation is to set up a vegetative area, and a flowering area. The vegetative area would use a cloner or something similar that would allow seeds or clones to grow under lights 24 hours a day.
Induce marijuana flowering (budding) timing
In their natural environment plants begin to flower as the days shorten at the end of summer and start of fall. To induce flowering (budding) you need to switch the light cycle to 12 hours of light and 12 hours of darkness for during every 24 hours.
When you make this change is up to you but it’s best after the plant has had some time to get from 12 to 20 inches tall. Forcing plants to flower when they are quite small increases crop rotation and overall yield. By doing this you will be able to fit more plants under your lamps which is what the Dutch concepts ‘Sea of Green’ or ‘Screen of Green’ replicate. If you are limited by the number of marijuana seeds, clones or viable cuttings that you have you may wish to wait until the plants are bigger so that you get more buds per individual plant.
Once you have induced flowering you will see changes in your plant within one to two weeks, it is imperative to remove the male plants. Read how to do that and more about marijuana flowering here. While the flowering plants are going through their cycle with the hydroponic set up, you can germinate other seeds or clones and get them ready for the next cycle. New seedlings or clones can then be started in the cloner. Since the flowering plants need absolute darkness during the dark phase, the light from the vegetative area can’t reach the flowering plants. So they need to be isolated, either by using curtains or put in separate rooms.
Technical lighting basics and grow light setups
Growing your marijuana indoors means that you will be in charge of meeting all their light requirements. There are a variety of artificial grow lights on the market which work well, albeit to different degrees.
- Method 1 – Either MH or a HPS light through both stages of growth (good);
- Method 2 – Run a MH light through the vegetative phase of growth followed by HPS light through flowering (very good);
- Method 3 – Run both MH and HPS light through all phases of growth (very good);
- Method 4 – Run a full spectrum LED system from seed to flower and they run cool and save electricity versus ‘Method 3’ (excellent).
Let’s get into the more technical side of lighting as it is useful to know what you are actually doing. There is no point in buying the wrong lights. All light has a wavelength, frequency and speed. Some light is visible and some like infrared invisible to the naked eye, below are a couple of terms that get thrown around while you look for your next grow light, it’s worth knowing them.
Candela – This is the unit of luminous intensity with one candela.
Efficacy of a light source – This measure lets us know just how efficient a light source is and is simple maths to work out. Total light output divided by the power input (Watts) = lumens per Watt.
Foot-candle – The Imperial measure of how intense your light is ie how much light is received by 1 square foot of any surface situated 1 foot from the light source of 1 candle.
Lux – The metric unit of measure for luminance of a surface. Therefore one lux is = to one lumen per sq meter or 0.0929 foot-candles.
Lumen – This is the unit of light flow which is also known as luminous flux. All lights can be measured to a total lumen output. Light fixtures can be expressed in lumens and usually the lamp will lose intensity (lumens) as it gets older.
P.A.R – Light (or photons) in the wavelengths of 400 to 700 nanometers (nm) is the energy source for photosynthesis and is named Photosynthetically Active Radiation (PAR) or Quantum. It is often expressed in Photosynthetic Photon Flux (PPF).
The more right light marijuana plants receive, the faster and stronger they will grow. Please follow these links for more detail and reviews on LED grow lights, HPS grow lights and CFL grow lights.