Proper lighting is essential for healthy plant growth, but which option should you choose: high-pressure sodium? Metal halide? LED? Why not mix and match until you’ve created the best possible system given the limits of your space?


Story & photos by Erik Biksa


What’s the best light source for an indoor medical cannabis garden?


Most experienced growers have likely drawn their own conclusions already. However, in doing so, they may have missed the forest for the trees. This is to say that perhaps a single light source or type of light isn’t the best way to illuminate an indoor garden and get healthy growth and great yields of high-quality buds. Perhaps it’s really all about combining two or more sources – different in wattage, spectrum, and position.


Sure, this isn’t a new idea; for example, closet and grow-chamber cultivators have been using fluorescent light tubes to decrease shadows and increase bud production in the corners of their gardens for decades, while using smaller-wattage high-intensity discharge (HID) lights overhead. Luckily, today’s indoor growers have a huge range of grow lights to choose from in order to dial in the perfect lighting system for their individual setup: magnetic or digital ballasts (HID lighting), plasma lamps (LEP), light-emitting diodes (LEDs), high-output (HO) fluorescent lights, and an ever-growing range of horticultural HID lamps available in various wattages and spectral ratings (i.e., the “color” of the light, which promotes different plant-growth responses).


Show Me, Don’t Tell Me: A Working Model

Medical growers who work within strict plant-count limits typically try to maximize their yields and growth rates per plant by providing strong lighting – sometimes in excess of 1,000 watts of HID lighting per large mature specimen. However, the growers who reap the greatest rewards do so by combining light sources rather than just relying on a single type of light or lighting application.


In our working model, each large plant is finished in a 20-gallon pot, with one 1,000-watt high-pressure sodium (HPS) bulb in a horizontal light reflector overhead and two 400-watt metal halide (MH) lights in the center, between the rows, equidistant from the plants and hung vertically as a bare bulb with no reflector. So, for six plants, we have six 1,000-watt HPS lamps and two 400-watt MH lamps. That’s 6,000 watts over the tops and 800 watts on the sides of six plants!


As a result, the plants yield an ample quantity of bud per watt consumed and have amazing crop quality from top to bottom; as a bonus, they seem to finish the bud cycle almost a full week earlier than the same genetics grown using more conventional lighting methods. There are several elements contributing to the success rate in this model. Granted, you may require fewer lights and smaller plants (or perhaps more lights and larger plants), but let’s identify and isolate the key factors.


First, we have a considerable amount of light quantity here: over 65 watts of HID lighting per square foot of linear growing space. And very importantly, the heat generated by this much HID lighting in a confined space is well managed – there’s no sense adding more light if you can’t control the temperature. If the temps exceed 85°F in the grow area with the lights running, you may end up doing more harm than good and spend more money to do it, further compounding the loss.


Second, light is being delivered from more than one source and traveling in more than one direction. The plants are getting lots of intense lumens from above via the HPS light shining straight down into the plant canopy. While there may be some reflection coming off the white walls, the sides of the plant go largely unlit by the HPS bulb. This type and placement of lighting concentrates most of the production in the tops only and causes the plants to stretch straight up into the light, making less use of the horizontal space available to increase yields and bud production.


However, the smaller 400-watt MH lamps suspended vertically broadside the lower parts of the plants with light. Because the wattage is less and the bare bulbs can be kept fairly cool, they can be maintained very close to the plants (within a few inches). The delivery of this intense light from the sides signals the plants to put effort into production from top to bottom rather than the tops only. That means bigger yields and more consistent quality from your harvests of medical buds.


Third – and also very important – the spectrum is blended for the growing phase (i.e., budding). This has been accomplished by using two different types of horticultural HID lamps: in this instance, a Red Diamond HPS, which is an engineered “red”-type lamp, and a PlantMax MH lamp, which is an “extreme blue”–type lamp rated at 7,200°K (Kelvin).


To understand the benefits here, you must first recognize the influence of the light spectrum on plant responses in the budding phase. The quick-and-dirty version is this: 1) Blue light tends to promote shorter, tighter, sturdier growth as well as better bud quality and more leaves. 2) Red light decreases maturation times and promotes floral/bud formation, resulting in bigger yields of buds in most cannabis strains versus those grown with blue light only.


The point here is that spectrum counts. You’ll hear claims about “enhanced blue spectrum” in horticultural HPS lamps, and while it’s true that there may be an increased percentage of blue light available from these lamps, the difference is small in comparison to using an independent secondary source of light that is blue-rich in spectrum, such as MH lighting. With the configuration described above, the intense blue light is being delivered where it will be most beneficial to yields and quality: the lower parts of the plant, which are typically “looser” and don’t finish as nicely as the tops. The extra dose of blue light in the lower and side portions of the plant canopy can make a big difference here, getting more production from top to bottom throughout the entire garden.


Putting It All to Work

So from this model for cultivating mid-sized but productive medical cannabis plants indoors, we can examine how the principles might factor into other styles and types of indoor gardens. Let’s take a look at just a few of the possibilities:


Type I

Connoisseur Closet Cultivation

With closet cultivation, the desired finishing height is small to moderate (let’s say 1 to 2 feet tall at maturity). A bushy growth pattern with plenty of chunky “satellite” buds is preferred for getting good yields without requiring lots of plants. Of course, yield matters, but there is also a strong emphasis on quality here.


High-output LED grow lights rich in reds and oranges (but with some blue diodes as well) are a good choice. Higher operating wattages, typically from 300 to 600 watts per panel, usually provide the best results. Not all LED grow lights are created equal, so do your research beforehand and be prepared to spend some money if you want something that works well.


For the sides and in between individual plants, HO fluorescent lamps rated at 5,500°K or more may be suspended vertically. These 45-watt lamps are easy to find and can be purchased at a reasonable price. (Though make sure your garden’s environment can handle the added heat, as the ballast system is built right into the lamps.) The warm-running HO fluorescents will be balanced by the cooler-running LED fixture overhead in a heat-unit-per-watts-of-lighting comparison – a critical consideration in tighter spaces.


In our Type I example, there may be 300 watts in one LED fixture hung above horizontally, and four 45-watt fluorescent lights – or 180 watts total – hung vertically in an area of about 1 square meter (3.3 feet by 3.3 feet). This setup borrows from the principles that make the larger medical garden discussed earlier work so well.


Type II

Grow Big or Go Home

Here, maximum production from a given space is the goal, with initial startup costs being less relevant; it’s more about sustaining big yields a long time after setting up. Quality matters here too, especially for the medical market. In this example, Sea of Green (SOG)–style high-density planting methods employ wider beds for the plants (for example, 5 feet wide – or, if the crop will be composed of larger specimens, a 5’ by 5’ area allotted to each plant, with plenty of space at the roots).


Overhead, we have one 1,000-watt HPS in an XXL horizontal reflector (preferably air-cooled or with an adjustable light spread) per plant or per 4’ x 5’ area in the growroom.


On the sides, we have a 600-watt HID lamp running on a digital ballast, one beside each large plant. The lamps are suspended vertically, about one-third of the way from the top of the plant canopy, in order to broadside the plants with lumens, achieving all of the benefits of side-lighting described earlier (although here we went with one side lamp in between every other plant).


With the SOG style of growing, the 600-watt HID lamps are suspended vertically in between the extra-wide beds, side-lighting the plants on the outer edges for better harvests. (Otherwise, these plants would be shaded out, producing lighter weight and lower-quality buds.) By using a digital ballast, the grower can switch from a 600-watt MH retro lamp (blue-enhanced, with a higher Kelvin rating) early in the flowering cycle to an HPS horticultural lamp (red-enhanced, with a lower Kelvin rating) in the latter stages – i.e., once the plants have stopped vertical growth and are approaching the peak bud phase – to achieve maximum production.


Plants given a balanced nutrient regimen, good air quality and lots of space for roots can get very big very quickly in this type of setup, so plant spacing and veg times should be adjusted accordingly (i.e., don’t get caught off-guard by amped-up monster plants basking in all that light energy). Good temperature control is an absolute must with this type of wattage and investment. A high-temperature kill-switch is recommended as a backup precaution for cooling-system failures.



Type III

Maximize Spare Room for a Yield of Dreams

Here, we want lots of production but have moderate space constraints. The level of quality we’re seeking is largely dependent on strain selection; however, using high-output LEDs for our overhead light as opposed to HIDs puts the grower in the driver’s seat on the road to achieving exceptional potency.


Note: A pre-made, high-quality grow tent (i.e., one with a nontoxic covering) is a great choice, especially if you don’t want to renovate the room for spills, light leaks, reflectivity, etc. The fact that the plants are completely surrounded by highly reflective material helps to promote more efficient light use and provides a tight environment if you choose to use carbon-dioxide enrichment to further boost medical bud production (though keep in mind that good ventilation is crucial in tightly sealed tent grows).


On the sides and vertically suspended by chains between the plants, higher-wattage HO compact fluorescent lamps (CFLs) – anywhere from 95 to 250 watts – help add more blue light, while distributing lumens from more angles throughout the lower portions of the garden. Though CFLs still operate hot, they’re safer to use than bare HID lamps in prefab growing enclosures. You can also select and combine different wattages and Kelvin ratings for more “blue” or “red” light with CFLs, as there are plenty to choose from, allowing you to really tailor your light regimen for maximum production or quality.


The End: Always a New Beginning

The horticultural community is only beginning to scratch the surface of what we can achieve in our crops with advanced lighting techniques and technologies, but we’re already leaps and bounds ahead of what was available to light up indoor medical cannabis crops only a decade or two ago. By combining and experimenting with different wattages, delivery angles and spectrums from a variety of lights, we can unlock a virtual treasure chest of tools that can further raise the bar in terms of medical cannabis quality and crop production.