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hps for growing weed

Hps for growing weed

HPS grow lights are not as well suited for grow areas that have…

High Pressure Sodium grow lights, or HPS grow lights, are the golden standard in growing cannabis. HPS lights can be used for the entire cannabis grow from seed to harvest (though some growers like to start with Metal Halide or florescent grow lights before the cannabis hits the flowering stage). HPS grow lights also seem to help growers produce the best cannabis yields. However, HPS lights use a lot of energy and produce a lot of heat. These issues can prevent a beginning grower from deciding to use HPS grow lights.

Some growers choose to use a grow tent to be able to better contain and control the heat. You can set a fan to expel hot air directly out of the tent to a window or other vent. Make sure to allow enough vertical (up-and-down) room for the HPS, the plant, and 1-2 feet of extra space that will need to be between the light and the top of the plant when it’s at full height. Remember 60 inches = 5 feet.

Grow Tents

Hps for growing weed

HPS is used to mimic the natural response in cannabis plants to react to the suns’ position when it’s on the horizon. This red wavelength triggers an internal response to begin flowering due to shorter days ahead.

The types of plants that you decide to grow are also going to impact your lighting choice.

HIDs, such as HPS and MH are both not as ideal for seedlings as they are during the later stages of growth.

Metal Halide (MH)

LEDs also offer dimming options and include a “night mode” that mimics the light wavelengths of the moon. Some even come equipped with green diodes so you can check on your plants during the night cycle without doing any significant harm.

When growing in an already hot environment, a 1,000-watt HPS grow lamp just isn’t feasible. Nor is a set of 600-watt MH. What is reasonable is a low heat-emitting light such as CFLs and LEDs.

The Stages of Growth

Each light has a pro and con for these different stages.

Cannabis undergoes various stages between its initial germination and future harvest.

Hps for growing weed

This is congruent with other findings that some added blue in flower will cause this. But the reason we see these amounts vary is because every strain reacts differently to different spectrums. Keep in mind that the cannabis plant grew in nature all over the world at different longitudes with different spectrums. With all of the variations of cannabis strains in today’s market made from these ancestral plants from all over the world, each will perform differently under different spectrums.

When comparing both technologies, always compare light levels of the entire grow and not just fixture to fixture. I cannot stress enough that there is no such thing as a 1-for-1 replacement of HPS by LED! That simply does not exist in the marketplace. Yes, there are LED fixtures that exist when measured directly beneath the fixture. The output looks equal to or even greater than an HPS. But, when you move the PAR sensor any direction from centered beneath the LED the light level decreases much more under the LED than any HPS fixture!

No manufacturer , supplier, or customer can state they know the optimum LED recipe for cannabis (or tomato’s, or cucumbers, or orchids, etc.) as nobody has tested and tried multiple, different spectrums with each one being tuned to a specific strain or cultivar of each plant type to be able to render a quantifiable opinion.

After veg and moving to flower, most growers also agree that switching to a light source with more of the red spectrum is better for flowering. The old standard has been to use either CMH or fluorescents in veg and switch to HPS into flower. This is the tried and true lighting recipe for indoor cannabis growers. By far the majority of cannabis produced in North America today is grown in this way. With the onslaught of LED lighting manufacturers jumping into this new “green rush” of opportunities we are seeing this standard lighting recipe being challenged more and more.

This is a big consideration to consider when comparing these technologies. There is no easy math to calculate an ROI for this but when comparing light plans from one source of light to another be sure to take into consideration the distance of the light source from the canopy.


There is a lot of information out there on the internet from various growers and lighting suppliers. You can find a contradiction to almost every point you find! Most growers agree that for young plants through the veg stage of growth you will want to have a light source with a good amount of blue light or even white (full spectrum) lighting. The main reason for this is to reduce internode stretching to produce a stockier, healthier plant.