Choosing Grow Lights

With the wave of legalization hitting the US, many cannabis users now have the ability to legally grow their own cannabis for the first time. With new growers questions about how to get started. The first and quite possibly most important decision to be made is what kind of lights to use. By now you’ve probably seen LED lights like these starting to take over the grow light market. LEDs have a good number of advantages that make them the best lights for new growers in our opinion, but are they always the best light for everything? There are definitely advantages to other types of lights that are worth considering, so depending on your needs, you may want to choose a different light, or multiple types of lights.


Before we dive into the lights, you need to understand the photosynthetic action spectrum. Seen to the left, the photosynthetic action spectrum denotes the frequencies of the electromagnetic spectrum that plants actually use. The light that is visible to us makes up only a tiny portion of the electromagnetic spectrum and plants only use a small portion of that visible light for photosynthesis. As you can see from the image provided, plants primarily use blue (~400-450nm) and red (~640-700nm) light. When comparing the spectrum outputs of different grow lights, we want the light that matches the photosynthetic action spectrum the best. This is why LED lights, which can provide very specific light wavelengths, can be as effective as much brighter lights with broader spectrums. This is also why LED light manufacturers have started boasting their light’s units of photosynthetically active radiation (PAR) instead of lumens (brightness), since PAR is what your plants actually use.


High-Intensity Discharge (HID) Lights

HID is a broad term that covers all electrical gas-discharge lamps, including High Pressure Sodium (HPS), Metal Halide (MH), and Ceramic Metal Halide (CMH aka LEC). These are some of the most efficient lights available, especially for larger lights. They have been the work horse of the cannabis horticultural industry for a long time, particularly HPS lights. Depending on how you measure it, they are arguably the most efficient lights on the market in terms of grams per watt yields, even with the rise of LEDs. Before LEDs came along, these were without question the light of choice for commercial growers. They are usually mounted in a fixture with a reflective hood (like this one) meant to reflect light emitted upwards back down onto the plants. They require a ballast (example) to control the changing levels of power required to operate and regulate the lamp. Unlike LED and fluorescent lights, they can generate a lot of excess heat, which is why larger HID fixtures have spaces for fans and ducting to be mounted and carry the hot air away.


Metal Halide lamps tend to have more of a bluish tint, similar to the summer sun, which plants prefer while in vegetative growth. HPS lamps tend to have more of a reddish tint, similar to the fall sun, which flowering plants love. Both are very efficient, however HPS are even more efficient. This is convenient since flowering plants also tend to need more light overall. It’s also worth mentioning that only two-sided HPS fixtures achieve such high efficiencies, so if you’re starting a grow with HID lights, we strongly recommend that you use two-sided bulbs and fixtures, although they are a bit more expensive. Another reason HID lights remain popular is because they’re very standardized. Each 600W HPS lamp is going to be nearly the exact same, barring a few minor differences in ballasts, fixtures and bulbs. This means that if you have questions about how to use your light (how high to hang it, etc), you can just go online and find many other growers with the same exact light. Furthermore, you can easily replace parts at your local hydroponics store as soon as a problem arises.


Although there are many advantages to HID lights, there are plenty of reasons to not choose them too. The biggest disadvantage, and the root of many other disadvantages, is the extra heat they generate. Higher temperatures can be bad for plant growth, and if put close enough can even burn the plants over time. Dealing with this extra heat can not only be annoying, but it can also be dangerous. Poor management of HID lights can easily lead to a fire, which your plants probably won’t like. This is one reason why HID lights have so many parts. Not only do you need the lighting fixture and the bulb, but you also need a ballast, a way to hang it, a mounted fan, and ducting too. Not only is this confusing and difficult for novice growers to setup, it also drives up the starting cost significantly, not to mention the increased electricity costs. The ballast used to control these lights also consumes some electricity and generates heat, but not much compared to the light itself. Fortunately HID lights are often sold with all necessary parts included, such as this set. If managing all the extra odds and ends isn’t a problem for you, you may want to consider using HID lights for your next grow.


LED Lights


The new name in the grow light game is the LED light. LEDs are lights made from semiconductors. For a long time they were far too expensive to be used for anything more than indicator lights and seven segment displays (aka digital clock numbers). Luckily Moore’s law and advances in optics technology have led to an exponential increase in the power and efficiency of LEDs, as well as a sharp decrease in manufacturing costs. This has made them viable replacements for general lighting, including grow lights. The color of LED lights is dependent upon the material properties of the semiconductor used to make the LED. For example, the gallium arsenide used to make the first LEDs produced red light only.


Manufacturers can create a very specific wavelength by choosing the semiconductor with the right properties. This is important because we know plants use red and blue light more than other colors of light. By using almost all red and blue LEDs we can give the plant exactly what it wants without wasting any power. While older LED lights did use only red and blue, current LED lights give a bit more of a broad spectrum, but still focus on red and blue. This is because the wavelengths of light that were left out are important for the plants metabolism of various nutrients. Fortunately these other colors aren’t required in large quantities so LED lights can still use primarily red and blue light. If you are going to buy an LED light, make sure it has more than just two colors of LEDs.


Another big reason LEDs have become so popular is that their efficiency means that they emit very little waste heat compared to HID lights. They do emit some heat, but fortunately almost all LED lights come with built in cooling which conveniently vents all the heat up and away from your plants. Unless you use a large number of powerful LED lights in a very confined space and/or live in a hot climate, you probably wont need to vent your grow room like you would with HID lights. Furthermore, since you don’t have to worry about a ballast, fans, ducting, etc, set-up time and costs are decreased. Even still LED lights are still generally considered to be more expensive. Although there haven’t been any conclusive scientific studies on the matter yet, a lot of anecdotal evidence shows that LED lights produce bud that is generally more rich in trichomes, cannabinoids, and terpenes. Many choose LEDs for this reason alone because for some people bud quality is more important than yields when it comes to choosing a light.


While there are plenty of reasons to love LEDs, there are also reasons they haven’t totally taken over yet. One reason is that when they fail, the whole panel is usually considered broken, so you have to pay to replace the whole unit, not just a bulb or fixture. If you have some understanding of how they’re built, you can fix most broken lights to work at nearly full capacity, but a lot of growers don’t have those skills. Each individual LED on a panel can be set with a lens that focuses the light for maximum canopy penetration, but better canopy penetration means that most LEDs must be placed a safe vertical distance from your plants, which can be a problem if you have limited vertical space. You may have heard that LEDs actually have lower yields per watt than HID lights, which can be true, but often it’s all down to how you measure it. We’ve seen growers yield over 1.5 grams per watt with LEDs, which is far and above the best yields we’ve seen with HID lights. The most common complaint about LEDs is usually their cost. While many LED grow lights are still prohibitively expensive, a savvy buyer can find LED grow lights with a cost competitive with most HID setups. The reason it’s so difficult to find a good deal on LED lights is because the LED grow light business is filled with unscrupulous vendors who try to take advantage of the confusion of various specifications of LED grow lights. This confusion is definitely the biggest barrier to entry to the LED market for new growers. Fortunately we give a much more thorough explanation of how to avoid this in our LED buyers guide, but if you want to skip that and just choose the lights we recommend, we recommend any lights by Mars Hydro on Amazon such as this one as a cheaper option, or this one if you prefer higher quality Cree LEDs. Just make sure you know how much LED power you need for your grow, based on actual power consumption, or as Mars Hydro calls it: True Watt. Mars Hydro is a more reputable Chinese manufacturer, so you can get a low cost light and expect to get exactly what they tell you, and since you’re buying from Amazon you can return it if you encounter any issues.


Fluorescent Lights


Although fluorescents are often dismissed, they still have their place. As you can see from the frequencies of light they emit in the image on the left, their frequencies of light are pretty similar to what plants like aside from the high levels of green. Fluorescent lights come primarily in two forms. First is CFL bulbs, which are the curly bulbs that fit into regular light bulb sockets that you probably use throughout your house. If you choose to go with CFL lights, make sure you get bulbs that are made for growing, such as this $10 60W CFL grow light. The other type of fluorescent lights that are frequently used to grow cannabis are T5 fluorescents (example), which come in long tubes and fit into fixtures that often hold multiple bulbs.


The main reason to keep fluorescents around is for vegetative growth. In the vegetative stage, plants like the spectrum of light that CFLs provide, and they require far less light overall. CFLs can provide that light at a much cheaper cost than almost all other lights. Another big reason fluorescents are so useful is because they’re so readily available. You can simply go down to your local hardware store and pick up a few fixtures and bulbs and you’ve got enough start a grow. Fluorescents also emit very little heat, much like LEDs. Furthermore, the light from fluorescent lights is fully diffused and not focused in any one direction. Coupled with their low heat emission, this means that you can keep fluorescents within inches of your plants without causing any problems. It also means that you can put fluorescents into spaces that most other lights can’t fit into, such as in small cabinets. We like to utilize this advantage by placing fluorescents under the canopy, which can do a lot to improve the size and quality of the buds on the bottom of the plant. Additionally, their efficiency makes them useful for saving on electricity costs. They’re also useful to keep around in case of a light failure that takes more than a day to fix, such as a broken LED panel.


There are also plenty of reasons why fluorescents aren’t used hardly ever for flowering. Because fluorescents must be placed so close to plants to be used efficiently, this means that using them effectively requires almost daily management of light positions, whereas LEDs and HIDs can easily sit safely 18″-36″ above the plant for weeks without needing to be moved. Because fluorescents have poor canopy penetration, to use them for flowering you need to train your plants to have a flat and even canopy, much like what’s done with the ScrOG method of growing. Unfortunately, even with effective training, fluorescents can really only offer about 20-50% of the yield in grams per watt that LEDs and HIDs can get. This is the biggest reason they aren’t used more often. While their flexibility and low cost means that pretty much anyone can set up a decent grow at a low cost within just about any constraints, you’re almost always going to be better off with a more high powered grow light providing you have the space for it.