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Growing cannabis with LED grow lights. Differences between growing cannabis with LED and HPS.


Cannabis home growers are slowly transitioning away from HPS, with many upgrading to LED. LED grow lights offer many advantages over HPS, but learning how to get the best from LED can take some time. Here are some of the most important things to know about growing cannabis with LED.

LED grow lights produce less heat than HPS.

HPS is the cheapest way to illuminate your grow room, but HPS produces large amounts of heat. LED grow lights produce a fraction of the heat. During summer months many HPS growers find grow room temperatures may exceed the optimum target (~25ºC), this restricts plant quality and THC levels. The best LED lights convert over 50% of the electricity into narrow wavelength light with less heat production. In summer that’s a bonus, in winter it may mean an LED grow room requires additional heating. Many growers cultivate their autos with 24 hour light throughout winter, it’s a good way to keep your grow room warm and productive during the cold season.

Watering frequency may be reduced with LED grow lights compared to HPS.

When you upgrade from HPS to LED one comment reported by some, but not all, LED growers is that they see a reduced need for watering the plants. With less IR heat in the grow room, plant transpiration rates (moisture loss) are often lower with LED. That means less watering may be required. Some growers feel they can use less nutrients too, but there isn’t universal agreement about this amongst LED growers. One useful tip to growers is to consider a moisture meter, such as a ‘Blumat Tensiometer’, this gives an accurate measurement of the water content allowing you know exactly what the moisture content is at the plant root level.


LED Light Burn / LED light stress

Above, LED light burn / LED light stress on cannabis leaves

LED light burn / LED light stress.

Get your LED grow lights too close to your cannabis plants and you may see light burn on the leaves. This is a yellowing of the leaves caused by excessive light intensity. But because the best LED grow lights produce limited heat its possible to get the LED lights close enough to cause light burn to the leaves even with cool air temperatures. This means the leaves will not experience any heat stress, but they are overloaded with light. This causes the leaves to fade, with yellow appearance and green veins. The leaves may well have brown tips which looks like nutrient burn. If uncorrected, light stress will stunt the growth of your plant.The best solution to LED light burn is to move the LED further away from your plants, and if you have adjustable light power then reduce light the intensity as well. With LED it’s important to keep the correct distance between the light and your plant. Bring your light too close and your plant will suffer light stress which can cause a variety of issues which will slow your plant growth and limit both yield and quality. LED light burn / LED light stress is a relatively new issue for cannabis home growers, but its important that the issues is well understood if you are to get the very best performance from your LED grow light.


Calcium & Magnesium nutrients and LED grow lights.

Some LED growers add extra Calcium and Magnesium to their plant feed. This is partly related to grow room temperatures and heat emission. Good quality LED grow lights produce less heat than HPS, with less plant transpiration (moisture loss) as a result. This can mean reduced mobility for Calcium and Magnesium ions in your cannabis plant. One way to avoid potential Calcium/Magnesium deficiency is simply to add extra in with your normal plant feed. Most grow shops sell Calcium/Magnesium supplements. Calcium/Magnesium deficiencies tended to be most common with some of the older LED grow lights, and especially some of the lower quality versions. This suggests that the cal/mag deficiency may be related to e.g. an incomplete light spectrum. With good quality modern LED grow lights the cal/mag deficiencies have not been as obvious.

Understand the correct distance to hang your LED grow lights above your cannabis plants.

Some LED systems, such as the powerful California Lightworks SolarSystem 550 should be hung well above the plant canopy, 50cm+. But other ‘bar-style’ LED grow lights such as SANlight should be positioned much closer, 15-30cm above the cannabis plant canopy. Ask the supplier of your LED growlight for their recommendations. The reason that the bar-style LED grow lights can be positioned closer to the plants is that the light bar distributes the light intensity evenly across the full length of the bar. This makes a LED light bar a good solution for SCROG growers with a level plant canopy hight. The SolarSystem 550 has all 400W of LED power coming from concentrated central point, this means the light should be hung well above the plant surface to allow the light to distribute evenly. It is also worth noting that some cannabis varieties can tolerate higher light intensity levels than other varieties. Cannabis plants can tolerate more intense light the older they become. Remember, if you see your cannabis plants suffering from light stress then raise the lights further from your plants and perhaps reduce power level too. Yellow leaves with green veins are a sign of light stress.

HPS heat stress causes lower THC levels, reduced taste and less aroma.

Keeping the grow room around 25ºC is the best way to optimise quality levels and THC content. Upgrading from HPS to LED is the best way to permanently reduce heat levels, and heat stress. And with a cooler grow room you will notice more of the terpenes stay in the plant tissue rather than evaporate in the heat and disappear in your carbon filter.  As any home grower already knows, the best cannabis comes with a rich taste, deep aroma and high levels of THC and other cannabinoids. 


If you are thinking of upgrading to LED, then please take a look at Dutch Passion’s sister company, Crazy LEDs & More.  Crazy LED’s select the best grow lights from the most experienced and professional LED growlight manufacturers. All the LED grow lights are professionally tested and approved by our team before we sell them.

November 3rd 2017
Easy Indoor
Grow Guide

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Dutch Passion advise their customers to reassure themselves of local applicable laws and regulations before germination. Dutch Passion cannot be held responsible for the actions of those who act against laws and regulations that apply in their locality. Cannabis seeds should be kept as collectible souvenirs by anyone in an area where cultivation of cannabis is not legal.