Plants need light to grow. The light must have the proper spectrum and intensity to ensure strong and healthy growth. Light is made up of a variety of spectral colors, each of which provide your plants a signal for a particular kind of growth. Outside, the sun’s position in the sky changes throughout the course of the year and as a result the color of it’s light changes. In the summer the light is more in the red spectrum and in the winter it is more in the blue spectrum. Plants have evolved with the earth and their growth patterns will change in relation to this. Growing indoors under artificial lighting is pretty much the same.
The bulbs that you will use in your fixture(s) also come in different spectrums. Generally HPS (High Pressure Sodium) bulbs produce an orange light, if you live in the city you see these everyday. MH (Metal Halide) bulbs produce a light more in the blue spectrum. However, indoor growing has evolved greatly over the years and these general rules don’t always apply. You can buy bulbs today that range from 2k to 10k that will cover every stage of growth. To learn about indoor lighting in more detail select from the menu below, where we have provided a lot of very specific information to help you calculate how much light you will need based on the square footage of your space.
The main portion of light that is used by plants is between 400 and 700 nanometers (nm). This region is known as the Photosynthetically Active Region (PAR) zone. PAR watts is the measure of the amount of photons that a plant needs in order to grow. Photons are a measure of light energy. Photosynthesis is the chemical process that plants use for growth and is a result of the plants ability to assimilate these photons.
Bulbs used in the grow room are marketed by their color or their K rating. K, or Kelvin generally refers to the color of the light that gets emitted by any particular bulb. More specifically, K is the SI (Standard International) base unit of thermodynamic temperature, equal in magnitude to the degree Celsius.
The different colors of light activate different functions within the plant. For example, the ability of a plant to orient its leaves toward a light source is controlled by the light spectrum. Optimum vegetative growth can be achieved with bulbs that emit light in the 2k to 6k range. And bulbs that emit light in the 10k area are used during the last few weeks of flowering to encourage ripening. While not all of a plant’s spectral needs can be met using synthetic light, most can. See the chart below to get an idea of what the color spectrum looks like as represented by the Kelvin rating.
How is PAR Measured?
PAR is measured in two ways:
PPF = Photosynthetic Photon Flux – This measures the light’s intensity by determining how many photons per second are emitted from the light. This is measured as micromoles per second (μmol/s).
PPFD = Photosynthetic Photon Flux Density – This measurement is even more important because it considers both the intensity of the light and the amount of that light that actually reaches the plant. It is measured as micromoles per square foot per second (μmol/m²/s)
If you want to measure the exact PAR output of your grow light, then you need to purchase a PAR meter.
What is a good PAR output?
Your plants require a lower PAR output during the vegetative stage and a higher PAR output during the flowering stage; this is because flowering plants use more energy and therefore need more food.
As a general rule of thumb, here are the PAR levels you should consider for each growth stage:
200-400 PPFD – for new seeds or clones and parent plants.
400-600 PPFD – for plants in the vegetative stage.
600-900 PPFD – for plants in the flowering stage.
Generally speaking, the higher wattage your grow light is, the higher the PAR output will be. However, that’s not always the case.
Why you need a PAR meter
Cannabis Light Needs
High intensity discharge lamps are very bright. Managing this intensity effectively will result in healthier and stronger plants. Light intensity is the magnitude of light energy per unit of area. The light intensity is greatest near the bulb and diminishes rapidly as the distance from the source increases. For example, plants that are just 2 feet from the light source will receive only 1/4th the amount of light compared to plants that are 1 foot away. A 1000 watt HID light source that emits 100,000 lumens will only provide around 25,000 lumens at the 2 foot distance. That same 1000 watt light source will provide only around 11,000 lumens at a distance of 3 feet. Poorly designed reflective hoods will deteriorate these lumen outputs even further.
So the closer that a plant is to the light source, the more PAR watts it will receive. However, don’t go putting your plants right up against the light source as this will be disastrous and burn the foliage. The wattage and type of lamp that you will be using will determine the minimum amount of distance that is required between the light source and the top of your plants. The scientific approach to finding this distance is called the Inverse Square Law. The Inverse Square Law states that the intensity of light changes inversely to the proportion of the distance squared. I am not going to blow your mind with mathematics at this point. See the table in the section Light Spacing & Distance for a basic guideline when using 400, 600, and 1000 watt bulbs.
Light intensity doubles for every six inches closer that an HID light is placed to the top of the plants in the garden. When the intensity of the light is low due to an old lamp or too great of a distance between the light source and the top of the garden the plants will stretch, lengthening the internodal spacing of the branches.
The following information will help you determine the distance and spacing of your light source(s) for maximum efficiency.
- A 400 watt HID lamp will deliver enough light to illuminate a 4 x 4 foot area. Hang your fixture from 12 to 24 inches above your plants.
- A 600 watt HID lamp will deliver enough light to illuminate a 4 x 4 foot area. Hang your fixture from 18 to 24 inches above your plants.
- A 1000 watt HID lamp will deliver enough light to illuminate a 6 x 6 foot area. You will need to hang your fixture at least 24 inches above your plants.
* When using double ended fixtures and bulbs you will want to add an additional 12 inches to the fixture hanging distance above your plants. Unless you have a space with 10 foot ceilings or better you will probably want to stay with a regular single ended bulb setup.
You should always use a hygrometer in the garden to measure the temperature under your light source(s). Our distance numbers are a general guideline, using a hygrometer to check the temperature under your fixture will give you a better idea of the distance that will be needed to not burn your plants. When using forced air cooled fixtures or light movers you will be able to adjust these numbers for maximum efficiency.
There are some benefits of using lower wattage bulbs:
- More point sources of light
- A more even light distribution
- Able to place fixtures closer to the garden
Using (3) 400 watt bulbs can cover about 35% more growing area than a single 1000 watt bulb and can be hung closer to the top of the garden. Using (3) 600 watt bulbs will deliver more light to plants than (2) 1000 watt bulbs and again will provide more points of light at less distance than the more powerful bulbs.
On the other hand there are some benefits of using higher wattage bulbs as well:
- Less equipment to maintain
- Easier to set up in smaller spaces
- Less overall cost
A ballast regulates the specific starting requirements and voltage for HID (High Intensity Discharge) lamps. There are 2 options available when dealing with ballasts, the old reliable magnetic coil setup and the newer somewhat less reliable digital ballasts. The most common characteristic that these 2 types of ballasts have in common is heat, the number one destroyer of its internal components. All ballasts have a metal encasement of some sort. The magnetic coil ballasts almost always have vents on the side which may or may not have an integrated heatsink to help disburse heat away from the unit. All of the top performing digital ballasts have a casing with integrated heatsink fins.
Types of Ballasts
The magnetic coil ballasts have been around for a long time and were the only real option for powering HID systems until the digital ballasts came along. Magnetic ballasts use a large spool of wire wrapped around a set of steel sheets to generate the high voltage and wattage required by HID lamps. Most magnetic ballasts use a metal or plastic canister known as a capacitor as part of the regulating system. These ballasts are heavy, rugged, and built-to-last for many years in harsh weather conditions.
Magnetic ballasts are certified by the Canadian Standards Association (CSA) for electrical safety and fire protection standards. They are designed to meet lamp operating standards set by the American National Standards Institute (ANSI). These standards ensure the lamp and ballast will operate correctly together. Metal Halide and High Pressure Sodium (HPS) lamps typically require their own separate ballasts for operation, however there are switchable magnetic ballasts that can operate both types of lamps. The user must flip a switch prior to operation. Switchable ballasts are a compromise because they typically operate the HPS lamp to specification, but operate the Metal Halide lamp out of specification. But you can also run what are called conversion bulbs that are MH bulbs that use the same lighting technology as the HPS bulbs and operate in the ballast when it is set to the HPS setting. These conversion bulbs use a technology called Pulse Start as opposed to Probe Start like the regular MH bulb. We will talk more about these bulbs in the Bulbs – HPS/MH/Conversion/Double Ended section.
Today there are more efficient electronic ballasts on the scene that use semiconductors and microchips to provide the high voltage and wattage required by HID lamps. High wattage electronic ballasts (anything above 250W) are not manufactured to meet lamp operating standards set by ANSI. Standards have not yet been written by ANSI for electronic ballast compatibility with HID lamps. Because there is no ANSI standard, all electronic ballasts are made differently. This has resulted in inconsistent lamp performance on electronic ballasts. Many of these electronic ballasts are not properly ETL or UL listed for fire and electrical safety.
Electronic ballasts have been around for a while now but originally they were only available in less than 100 watt setups. As the technology evolved we started getting higher wattage units that today go as high as 1100 watts. The history of the electronic ballast has not been favorable and the failure rate was extremely high. Today some electronic ballast manufacturers like Solis Tek claim a return rate of less than 1%. The Phantom ballast made by Hydrofarm has also had a good reputation for reliability. Most manufacturers provide a 3 year warranty, so keep your receipts just in case.
Physically speaking, one of the nice features of the electronic ballast over the magnetic coil ballasts is the unit’s size and weight. Electronic ballasts are pretty light at around 10-15 lbs, conversely the magnetic coil ballasts can be really heavy at around 40+ pounds (there are some newer magnetic ballasts that have a smaller footprint and are a bit lighter in weight). The electronic ballasts are also really quiet, where the magnetic ballasts can sometimes be noisy.
Most, if not all, electronic ballasts on the market today are not even digital though they may be marketed that way. Digital would imply that the ballasts have a microprocessor inside. Today when people refer to digital ballasts they are lumping electronic and digital units together, but digital will become the standard as the manufacturers add more features to the ballasts that require an onboard microprocessor. We will just refer to them as electronic ballasts for now since that terminology covers all of them.
When it comes to features the electronic ballasts win hands down, and the list of features keeps growing. Some of the newer units have features like a life/usage odometer, built in timer, dimming and boost capability, remote controls, staggered startup, end of lamp life indicator, etc . Solis Tek also sells a splitter that will allow you to run (2) 600 watt fixtures with the 1000 watt ballast at a slightly diminished output at around 550 watts each, that’s a pretty nice option.
About the dimming capabilities of these new digital ballasts hitting the market…I am not sure yet if this is really beneficial or if it is potentially damaging. For example, I doubt that you will get the same light output dimming your 1000 watt bulb down to 600 watts compared to running an actual 600 watt bulb and ballast. And not only that but I am hearing from some hydroponic and gardening shops that running bulbs at a dimmed setting isn’t good for a bulbs life expectancy. I guess we will need to do some testing and revisit this topic, so manufacturers please send us your dimmable ballasts and we will get right to it.
Pulse Start vs. Probe Start
Some of the new digital ballasts can also run both HPS and MH bulbs. However, only Pulse-Start lamps should be used with digital ballasts. Probe-Start lamps are designed with an igniter in the loop, which is improper for the circuitry as the system was not designed this way. If you notice, most Pulse-Start lamps have only 2 electrodes entering the glass envelope. Probe-Start style lamps are equipped with a third electrode, which is not needed or required in digital ballasts (Click on the picture at the right for a more detailed view). Using a Probe-Start lamp in a digital ballast is not recommended and causes stress on the lamp and ballast.
For example, if you run a Pulse-Start metal halide lamp in a switchable magnetic core and coil ballast, you would switch the magnetic ballast to HPS mode (even though it’s a MH) because this includes the igniter in the loop, as the igniter is not designed into the Pulse-Start style lamp. Probe-Start lamps have an igniter built in, which is not proper for digital ballasts.
Total Harmonic Distortion (THD)
When looking at digital ballasts the one specification that you should be mindful of is the units THD or Total Harmonic Distortion. THD is a very complex subject that we can go into a lot of detail about. In short, the measurement of THD measures how clean the output signal is going to the lamp. A lower THD% will allow for the components in a digital lighting system to produce a higher quality electrical signal due to reduced harmonic distortions coming from the ballast. The lamp in a digital/electronic ballast setup broadcasts the frequency and any signal noise generated by the ballast. Electronic ballasts have varying THD% ratings, poor performing electronic ballasts with a high THD% will cause problems for both the lamp and the ballast due to high amounts of signal noise, which translates into diminished light quality and shorter life expectancy.
The electronic ballasts on the market today typically have THD% ratings that range from 4% to 10%. At the time of this writing the Hortilux Platinum series electronic ballast has a THD rating at 4% and you will pay a hefty $600 MSRP for it. The Hydrofarm Phantom 2 and Solis Tek Matrix are rated at < 8% and most of the rest are rated at < 10%. We will get more into the different ballasts and their performance in the Equipment section where we will put some of these units head to head and see who comes out on top.
There are many sizes and shapes of reflective hoods and some reflect light more evenly than others. A well designed reflector that distributes light evenly can be placed closer to the top of the garden without stressing your plants. A poorly designed reflector can cause hot spots and burn foliage if hung too close to your plants. Efficient reflectors in combination with reflective wall surfaces can just about double the growing area and increase the harvest yield.
There are a few different kinds of reflective hoods to choose from and you can find some really good deals on ebay purchasing from wholesale hydroponics retailers. One thing to keep in mind when shopping for your new light, ballast, and reflector is that some companies like Hydrofarm use a proprietary plug on some of their ballasts that only work with their reflectors.
Air Cooled Reflectors
Air cooled reflective hoods are very popular for obvious reasons. HID lamps get very very hot and you will need to manage this or you will have nothing but problems especially in smaller spaces with multiple lights. In larger spaces with high ceilings this heat management can be done more effectively with powerful ventilation systems. Nice air cooled hoods are closed systems that utilize a hinged glass door that will allow you to service the bulb. When closed the heat is pushed and pulled away from the unit by using inline centrifugal fans that will bring air to the fixture to cool the bulb and then another to help pull it away to evacuate the warm air from the space or to keep the airflow moving to the next fixture.
Parabolic & Cone Reflectors
Parabolic and cone style reflectors with vertically mounted bulbs are less efficient than reflectors with horizontally mounted bulbs. Like horizontally mounted bulbs, vertically mounted bulbs emit light from the sides of the arc tube. This light must strike the side of the reflector before it gets reflected downward towards the garden. This reflected light is going to be less intense than light coming down directly from the bulb because the light needs to travel a greater distance. So if this is the type of reflector that you prefer, go with the more efficient horizontally mounted units.
Some of the nice features of parabolic and cone style reflectors:
- Ability to dissipate heat quickly
- They reflect light relatively evenly over a large area
- Are usually lightweight compared to the metal enclosures used by forced air cooled fixtures
People love the wing reflectors because they deliver the most amount of light. There is a deflector under the bulb that allows this type of reflector to be hung nice and close to the plants.
A few of the benefits of the wing reflector include:
- They offer twice the power and efficiency of most other types of reflector
- Increase the growing area under each lamp by up to 75%
- Increase yield per lamp by up to 50%
- Typically they will utilize a pebble or hammertone surface that offers good diffusion of light and a larger surface area for reflecting
Combination Ballast & Reflector
And finally there are the combo units that combine both the reflector and the ballast into a single unit. You don’t really see this too often in the indoor grow room but they are nice and compact and in theory since there is a very short or no cable running from the ballast to the light, there would be less of an issue with the units Total Harmonic Distortion (THD) output creating radio interference. To learn more about digital ballasts and THD checkout the Ballasts section in the Lighting category.
HID (High Intensity Discharge) bulbs are high-powered light sources. They include Metal Halide lamps, High Pressure Sodium lamps and Conversion Lamps. Unlike incandescent light bulbs, HID lamps do not have a filament. HID lamps are electrical discharge lamps. This electrical discharge is similar to the arcing of a lightning strike, only with much lower power and much longer duration. These amps must be used in their specified operating positions in order to maximize lamp performance and longevity. Incorrect operating positions may also create the possibility of a lamp rupture.
Be careful when handling bulbs. You do not want the oil from your hands getting onto the bulb because this residue will weaken the bulb as it gets baked onto the glass surface. Only use a clean cloth if you need to buff out any fingerprints that get onto the bulb.
Bulbs will lose brilliance over the course of their life. The optimal lifespan for a bulb is 12 to 18 months. As a bulb loses brilliance you can move it closer to the garden, but I would suggest replacing old bulbs to maximize efficiency and harvest yield.
HPS Bulbs (High Pressure Sodium)
The HPS bulb is the most efficient HID lamp to use during the flowering stage of growth. Typically used for flowering and fruit production, HPS lamps offer great lighting that can be used through all phases of the plant’s growing cycle. The HPS lamp emits an orange color in the light spectrum at around 2000K. Light from the red end of the spectrum stimulates floral hormones in plants, promoting flower production. According to some growers, flower volume and weight increase when using HPS lights. But with the variety of bulbs available today you can get MH bulbs that emit light in the red spectrum as well at around 3200K.
HPS lamps have the longest life and best lumen maintenance of all HID’s. Eventually the sodium bleeds out through the arc tube. Over a long period of daily use, the sodium to mercury ratio changes, causing the voltage in the arc to rise. Finally, the arc tube’s operating voltage will rise higher than the ballast is able to sustain. When this happens, the lamp will start, warm up to full intensity, and then go out. This sequence will repeat over and over, signaling the end of the lamps life. The life of a 1000 watt HPS lamp will be about 24,000 hours, or 5 years when operating at 12 hours per day. Replace HPS bulbs after 18-24 months to keep the garden bright.
MH Bulbs (Metal Halide)
The MH lamp is the most efficient source of artificial white light available to growers for the vegetative stage of growth. Studies have shown the most productive method for indoor growing is by starting plants with Metal Halide and then switching to High Pressure Sodium grow lights during the flowering phase of growth. Metal Halide lamps provide optimal spectral energy required for vegetative growth and they are ideal for starting and growing plants up to the flowering and fruit production stages of plant growth.
Metal Halide lamps produce light by passing or arcing electricity through vaporized gases (argon, mercury, thorium iodide, sodium iodide, & scandium iodide) within the quartz arc tube. After they are in their proper concentrations in the arc tube, the characteristic bright white light is emitted. This process can take about 3 to 5 minutes.
The average life of a MH bulb is about 12,000 hours, almost 2 years of daily operation at 18 hours per day. Many will last even longer. The lamp reaches the end of its life when it fails to start or come up to full brilliance. This is usually caused by deterioration of the lamp electrodes over time, loss of transmission of the arc tube from blackening, or shifts in the chemical balance of the metals in the arc tube. Replace bulbs every 10-12 months or 5,000 hours.
DE Bulbs (Double Ended)
Double Ended (DE) bulbs and fixtures are the latest development in indoor horticulture. These lamps come in both HPS and MH configurations and are offered in a broad spectrum of colors used in the vegetative, flowering, and ripening stages of growth. These lamps are known for supplying the highest PAR output in the industry.
Due to enhanced UV, DE lamps promote healthier vegetative growth and increased oil production in the flower, while still supplying the intensity that DE lamps are known for to maximize the flower size and weight. Always limit your exposure to lamps that produce UV light while they are on and always wear eye protection when in the grow room.
As with all HID bulbs, oil from your hands can damage these lamps, take extra care when handling. A quality Double Ended lamp will have an average depreciation of around 5% at 10,000 hours of regular usage.
If you have a ballast that works with only one kind of bulb or the other, then conversion bulbs are for you. These bulbs emit light in all of the desirable areas of the spectrum which will allow you to maximize your system for each of the different stages of growth.
Initially, HPS magnetic core and coil ballasts were designed to only operate HPS lamps. Metal Halide core and coil ballasts were designed to only operate Metal Halide lamps. Conversion lamps give you the freedom to use your equipment with more flexibility. Conversion lamps give you the ability to operate a HPS lamp on a Metal Halide ballast or Metal Halide lamp on an HPS ballast.
Many gardeners have great success using conversion bulbs. If you have a Metal Halide system but want the extra red and yellow light of an HPS lamp to promote flowering, or if you have an HPS system but want to run a bulb during the vegetative stage that produces the extra blue and white of a MH system, simply buy a conversion bulb.
* If you are running an electronic or digital ballast, make sure that you only utilize bulbs that use the Pulse-Start ignition and not the Probe-Start. For more on this topic please checkout out the Pulse Start vs. Probe Start section within the Ballasts subsection of the Grow Room Lighting category.
The photoperiod is the relationship between the duration of the light and dark periods. Most strains of cannabis will stay in the vegetative growth stage as long as the light period lasts 16 to 24 hours. There are some autoflowering varieties on the market today that will enter the flowering stage of its life cycle regardless of the photoperiod. For the most part, flowering can be induced with a 12 hours of light and 12 hours of dark regimen. When the plants are at least 2 months old, after they have developed the sexual characteristics, altering the photoperiod to the 12 light and 12 dark cycle will induce visible signs of flowering usually within 1 to 3 weeks. Older plants usually show signs of flowering sooner. These are only general guidelines as today there are many different strains and hybrids available with a multitude of genetic variations.
One thing for certain is that bouncing the day and night periods around will have negative results for some genetically unstable strains. Plants have been known to display hermaphroditic tendencies when the photoperiod is unstable, so keep the cycle consistent to avoid creating problems. The photoperiod signals plants to start flowering, but it can also signal them to remain in or revert to vegetative growth. Cannabis must have 12 hours of uninterrupted darkness to flower properly. Any stray or dim light during the dark period will prevent the plants from blooming since interruptions in the dark period will confuse plants. You can however visit the garden during the night cycle by using a green light. Cannabis does not respond to the green portion of the light spectrum so using a green light in your grow room will not have any negative effects on the plants.
Some growers have reported that they have had success getting their plants to flower by adjusting the photoperiod to 12 hours of light and 6 hours of darkness. They also claim that this expedited 18 hour photoperiod regimen yields 25% more in the same 8 week flowering period. Sounds like an interesting experiment worth investigating further.