Making The Switch from HID to LED

Cultivators all over are making the switch from high intensity discharge (HID) to light emitting diode (LED) grow lights. While we can attribute some of the attention to technological advancements in creating LED lighting, growers have also come a long way in learning how to adapt their plants and environment to grow using modern LED fixtures.

When it comes to growing cannabis, light is vital to the health of the plants and eventual yield. Cannabis grown under LEDs can significantly exceed those produced under traditional HID lights in yield and quality. But, this result can only be achieved with the careful manipulation of several environmental factors that growers must be aware of if they want to make the switch from HID to LED.

Temperature

The biggest and most notable factor to take into consideration when switching from HID to LED is temperature. Typically, LED grow lights produce far less radiant heat than traditional HID lighting. A large portion of HID lighting’s output is infrared heat, which raises the temperature on the leaf’s surface. In HID grow rooms, leaf surface temperature is often 10-20o hotter than the ambient air temperature, so HID owners must keep their air conditioning on at around 70-75o.

Cannabis performs photosynthesis at optimal efficiency within a specific temperature range, a goldilocks zone from 25-30oC (77oF-86oF). While HID grow rooms have to turn their air conditioning up high to keep their plants from overheating, LED grow room operators can actually lower their AC and let the ambient temperature rise.?

When it comes to LED, higher ambient temperatures are needed for the plants to stay within the goldilocks zone.?

“Every LED room that I’ve run has been within the 82-85 degree temperature range for the majority of the timeframe.” says Mike Howard, Grower Liaison at Fohse. “This actually gives plants the added benefit of a much more even, gentle temperature gradient throughout the canopy, since extreme heat no longer has to be combated with extreme air conditioning.”

Humidity

As ambient room temperature increases, so too must humidity in order to maintain an optimal vapor-pressure deficit (VDP). VDP is intrinsically tied to many facets of plant growth.?

Leaves are fully saturated with water, so when they’re in an environment that is less saturated, they transfer their moisture into the atmosphere at a rate dictated by the VPD. VDP is calculated by finding the difference between how much water the air can hold and how much is actually in it. A high VPD draws moisture out of the plant, because the difference between the plant and the environment is larger.

Higher temperatures allow the air to hold more water, which is why muggy environments often feel “heavy” with humidity. That’s because, technically speaking, the air is heavier, because it is holding a larger amount of water vapor and exerting more pressure on the environment. An LED room run at 82-83o will need a higher humidity than an HID room run in the 70s in order for it to maintain an adequate VPD.?

Sometimes you will see negative feedback loops with HID created from the abundance of heat put off by the fixtures. This excess heat creates the need for more cooling, which in turn cools and dries the air too quickly, removing the humidity. The dryness in the air draws more water from not only the plants, but directly from the soil as well. This is why HID rooms typically need a humidifier, while LED rooms may require a dehumidifier.

Transpiration

Higher humidity and higher temperatures can draw out more water from plants, this leads to an increase in transpiration. Transpiration is a process that occurs within a plant, where water and nutrients in the soil are absorbed through the roots and sweated out through pores in their leaves known as stomata.?

Plants need to expel excess water in order to keep cool and grow strong. 90% of plant water is transpired, while only 10% is used by the plant for growth. As transpiration increases, so will the amount of water and nutrients the plant will need. Increasing the frequency of watering or the volume of water being supplied is something that needs to be done incrementally as the intensity of the light is increased.?

The result will be plants that use more water and nutrients, but also grow at a faster rate. You may also find that your plants need more nutrients, so keep an eye on any deficiencies that may arise and adjust your feeding regiment accordingly.

Intensity

Switching from HID to LED lights allows users to exercise more control over the quality of the light plants receive. Most LEDs are equipped with a control that allows you to increase or decrease intensity so you can slowly acclimate your plants to higher light levels.

“I recommend going up in intensity slowly, while increasing CO2 and EC [Electrical Conductivity] in conjunction, as the plant is being pushed to its limits,” said Howard. “If any stress factors are noticed while increasing intensity, scale back for a day or two and reevaluate if you need to increase your EC or if your cultivar is at the max light levels it can handle.”

Most cultivars can take PPFD levels over 1,200, and some upwards of 1,500 if acclimated correctly. However, some cultivars are a bit finicky and will not take higher light levels. The ideal grow set up would have the ability to increase or decrease intensity to each cultivar. If you only have one controller per room, it’s recommended to group plants that are able to take higher light levels together, allowing you to push them to their max potential without negatively impacting other cultivars that can’t handle such intensity.

Spectrum

Being able to change the spectrum is something new for those stuck under a high pressure sodium (HPS) bulb the last few decades. LEDs have the ability to control the spectrum of light plants receive, and thus manipulate plant morphology.

Typically, higher end LED fixtures like the Fohse A3i will give you the ability to change spectrums that simulate seasons like spring, summer, and autumn. The spring spectrum mimics the natural light levels of the season by supplying the plants with a higher concentration of blue light, and a smaller amount of red and far red light. This triggers a photomorphogenic response within the plant, encouraging it to vegetate and grow.

After the first 2-3 weeks in flower, switching to the summer spectrum provides a higher concentration of red spectrum light, which tells the plant to not only continue flowering, but when mixed with far red, sends signals to the plant to grow even taller.

Increasing the amount of red spectrum during the first 2 weeks of flower will trigger the plant’s shade response, signaling it to stretch and increase internodal spacing. While growers can switch over to autumn anywhere between weeks 5-6, in the right hands, the summer spectrum can be used all the way until the end of harvest due to the natural light-hungry genetics of Cannabis. Cultivators should be weary, though ,as too much far red can lead to skinny, brittle crops.

LED spectrum control allows for growers to experiment and see what works best with the cultivars they grow or the product they want to make. Some growers will even switch back to the spring spectrum for the last week or so in an attempt to boost terpene and cannabinoid levels. It all depends on the desired result, and the light’s full spectrum capabilities.

Plant Height

In a traditional single tier HID grow facility, plants can only be grown to a certain height, which is determined by the height of the fixtures. As plants grow taller, they get closer to their light source. The excess heat produced by HID lighting negatively impacts the terpenes and cannabinoids if the plant is grown too close to the fixture.?

Under LED grow lights the plants can be grown taller, as the heat emitted is far less intense, and the intensity can be adjusted as the plant grows to meet its needs. Alternatively, with less radiative heat, LED grow lights can be hung in multi-tier setups to maximize indoor space and crop yields.?

This style of farming has permeated herbaceous crops and cannabis rapidly. Some vertical farms boost production by 10-100 times more per acre while using up to 90% less water.?

Phenotypes

When switching from HID to LED, it’s vital to reassess your operation, from top to bottom, and that includes the plant phenotype itself. As an HID user, you may have already ‘pheno hunted’ your genetics under HID lighting, selecting varieties that grow well under that spectrum and environment.?

Adjusting humidity, ambient temperature, water and nutrient levels to match the new light intensity, spectrum, and heat will bring about a stronger, healthier plant with more varied characteristics. Whether it’s the plant morphology while growing or the final product, there will be differences, some indistinguishable, and some more pronounced in the product you had under HID lighting. This may force you to re-evaluate the effects of the strain due to the presence of more terpenes and cannabinoids.?

Yield

A big difference in HID to LED lighting comes in the final product. Under high intensity lighting, some growers reported increases to yield that their drying facilities weren’t equipped for. “Our typical harvest was 80-90 pounds with our old LEDs,” said Green Life Productions founder Steve Cantwell. “Now our harvests are 160-190 pounds. The last one was 197 pounds! We didn’t plan for this much weight [in this facility]. We’re having to rethink how we dry and store the increase.”?

It may sound like a humble-brag, but it’s important for growers switching from HID to LED to prepare for an increase in yield, or else they will be forfeiting the very gain that will outweigh the initial cost of upgrading. If higher yields, larger plants, and better quality sound like something you want for your crops, visit FOHSE.COM and request a free light plan now.

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