From HPS to LED Growing Cannabis

Cannabis home growers are slowly transitioning from HPS 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

LED light burn / LED light stress

On the image above you can see LED light burn / LED light stress on cannabis leaves.

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 with 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.

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, LED By Passion.  LED By Passion select the best grow lights from the most experienced and professional LED growlight manufacturers.

All LED grow lights are professionally tested and approved by our team before we sell them.

 

11 Comments. Leave new

  • With regards to LED lighting, what do you feel is the optimal colour temperature for growing autoflowering cannabis? I currently use 3500K with 660nm, UV & IR thrown in, with good results, but I’d appreciate your opinion as I’m due to put down a crop of auto glueberry soon.

    Reply
    • Hello,

      From our own experience we notice that many spectrums work, there is not 1 holy grail. UV can be tricky if you use this from the beginning.
      The most important advice we can give you is that a good quality LED light is the way to have the best yields for your grow 🙂

      Good luck on your Auto Glueberry OG grow!

      Reply
  • Thomas Henderson
    16/02/2021 23:08

    Using a Mars hydro 4800 are they any good new to led’s plz 👍🏻👍🏻

    Reply
  • Handsome Pete
    05/03/2021 19:19

    I’ve left my 150W LED on 24 hours. Plants are still in vegetation stage (about 4 weeks in). They look good, maybe need a little calcium. Will Auto critical orange flower well in these conditions? Should I change the schedule now or just leave it and see what happens. I’m not too bother if there is lower THC levels and I do not need a massive yield.

    Reply
    • For all of our autoflowers we advice a light schedule of 18 hours light and 6 hours darkness during the complete grow :). However, they will flower automatically, also if you leave your lights on 24hours.

      Greetings!
      Joe
      Dutch Passion

      Reply
  • Jimbo slice
    02/04/2021 12:44

    I live in a hard water area (e.c .4-.5) I’m using a spider farmer sf4000 on 8 sour chiesel plants, will the hard water cause a cal mag issue and because the EC is so high coming from the tap do I now need to go and spend more money on a reverse osmosis machine? Thank you in advance

    Reply
  • THCdoctor
    06/07/2021 19:51

    Just switched to LED after years with HPS but im having major deficiencies and drooping plants i have a daylight led 660w turned down to 50% hanging 90cm day temps air 28 degrees plant surface temp 26 degrees night air temp 23 dergrees Celsius. ive got some brown speckles and spots and a couple of leaf tip burns im feeding the same as i did with HPS just adding nearly double the amount of cal/mag there growing slow with new growth a touch on the lime green side of things. Im starting to think i added to much cal/mag after hearing they need more they started under MH as ive always done but swapped to LED instead of going HPS like normal. Never seen so many deficiencies happen so fast in the space of a week

    Reply
    • Having same issues. Light green leaves. Have same clones with same temps/soil/vent/feeding
      One under 600w Lumatek Proled and others under 660w LEC.
      LED plants are all light green and showing some burnt leaves whilst LEC plants are doing great.

      Had same issue last grow.

      Think I´m going to add 30% more nutrients in the water I´m giving to the plants under LEDS.
      They use less water so I imagine they need more nutrients.

      Reply
    • Have the plants recovering from last post.
      Was a bit desperate and added 130% of recommended growing fertilizier. I knew they were not over fed because the clones in the LEC system were doing great.

      Two days after feeding with 60ml/L of Terra Vega (50ml/L is max recommended and I have never given my LEC plants that much, usually 35ml/L) the plants started getting dark green and showing fast growth.

      My conclusion is the following: The plants under LEDS use less water and need higher nute values.

      Ill try the same during the flowering stage.

      Reply

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