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Vegetative vs. flowering growth.

Cannabis is an annual plant, it reaches the end of its lifecycle in the fall, sheds seeds, then dies. During its life, it goes through two distinct phases of growth: vegetative and flowering. The first phase is vegetative when the plant grows branches and leaves. The second stage is flowering when the plant grows buds.

Flowering and vegetative growth phases are regulated by a hormone called florigen. Florigen accumulates in cannabis plants only when they aren’t exposed to long periods of light everyday. When daily light exposure is low, florigen accumulates . After surpassing a threshold florigan causes plants to flower.  

The amount of light a cannabis plant receives each day determines its phase of growth. More than 12 hours of light exposure each day and it will remain in a state of vegetative growth. Less than 12 hours light exposure each day and it switches to its flowering growth phase. 

Outdoors, growth phases are determined by the number of hours the sun is up each day. Vegetative growth happens in the spring when the days are longer. As the days grow shorter in the fall, daily light exposure drops, florin accumulates, and cannabis plants shift to their flowering phase.

Flowering plants with early stage bud growth.

Vegetative growth is typically maintained indoors by attaching lights to a timer device and exposing plants to 16 – 24 hours of artificial light per day. 

Growers can control when their plants flower indoors. By maintaining daily light exposure above 12 hours plants will stay in vegetative growth, theoretically forever. Or they can be forced to switch to flowering growth, even only after a short period of vegetative growth, by setting their daily light exposure to 12 hours a day or less.

Indoors, setting the light schedule to 12 hours of darkness per day causes cannabis plants to enter their flowering growth phase. To sustain flowering, plants must be keep in total, uninterrupted darkness. Light leaks can cause a reversal back to vegetative growth.

Darkness is typically achieved inside a tent that can be zipped up, shielding plants from light.

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About LED lighting hardware.

LED lighting is a newer technology for horticultural lighting that generates light by passing electricity through semiconductor materials. It replaces traditional HID lighting that works by superheating bulbs filled with gas.

Simple LED fixture.

An LED light is made from light emitting diodes powered by a DC power supply. Some LED lights also have a heatsink, a fan, and sometimes a protective casing.

LED lights are available in different shapes, rectangles, squares, long strip lights, or mounted in tubes. 

A worthwhile feature available on some LED lights is a dimmer switch. Plants in early stages of growth need less light than plants in late flowering. A dimmer switch allows the light intensity of a panel to be turned down so it can use less power during the plants early growth stages and then turned up for later stages when plants require more light. This feature can save on electricity costs and reduce heat. It also allows for gradual light increases. Plants prefer gradual increases to intensity, not abrupt change to a higher light level.

Most LED lights are directional, meaning they emit light at 180 degrees, directly down at the plants. Directional light is efficient, saving costs compared to other types of lighting. For example, HID lights emit light over 360 degrees, requiring reflectors to bounce light back at plants. Light that needs to be reflected back is less efficient because whenever light gets reflected some energy is absorbed by the reflector. A straight path to the plant canopy is better.

The chart below shows the light distribution for a 20 x 26”  light panel. The numbers indicate light readings taken under the light. It demonstrates how light becomes less intense the further away from the centre point directly under the light fixture.

Reflectors positioned at the sides of the canopy, such as the silver reflective walls of a 4’ square grow tent, will reflect some light, boosting light levels at the sides.

Silver reflective walls of a grow room.

Light panels positioned beside each other can be arranged so that the beams overlap, creating a spread of even intensity over a large coverage area. This is called a grid pattern.

4 light panels arranged on a single board for large coverage area.

LEDs create light by electroluminescence, which is the phenomenon of a semi conductor material emitting light when an electric field is passed through it. The type of semi conductor material used in LEDs determines the colours of light it emits. For example, indium gallium nitride is a material used for blue and green LEDs, and aluminum gallium arsenide is used for red LEDs. Different semiconductor materials can produce specific wavelengths of light. LED panels that house a mix of different LEDs can output customized light recipes by combining wavelengths. 

White full spectrum + red LEDs

LED panels are very efficient because they are designed to produce only the wavelengths of light that plants use. In comparison, HID light systems emit a byproduct of infrared light which is not useful for plants and also adds a lot of heat to a grow room. 

LED systems also generate heat, but not from a bulb with superheated gasses, as is the case with HID lights. The heat created by LEDs is created by its semiconductor processes. 

LED systems generate less heat overall than HID systems. HID systems turn 20% – 40% of power into usable light. LED systems convert about 40% – 60% of power into usable light.

Heat generated by LED semiconductors is removed using passive and active cooling methods. For passive cooling, heat is conducted away from the semiconductors, either through the board the LEDs are mounted on, or a finned heatsink. For active cooling, fans blow heat off the board and heatsink. 

LED tube light positioned very close to canopy.

Because heat is dissipated away using active and passive cooling methods the front of an LED panel is cool to the touch. Plants can be positioned very close to a light without a risk of starting a fire. Having plants positioned close to the light also saves energy. The further light has to travel from the panel to reach the plants the less intense it gets. The further away the plants are from the light source, the more power a fixture has to draw to generate enough strong light for the plants. Lights don’t have to work as hard when plants are closer to them.

A good LED light panel should last a very long time if properly cared for. Although it varies from light to light, an LED grow light lasts on average 60,000 hours. In comparison, HID light systems last only 20,000 hours.

The output of an LED light will degrade over its lifespan. Over 60,000 hours, a loss of 3-5% can be expected.

LED light boards and heatsinks are solid state. There are no moving parts that can become detached or gasses to leak so they aren’t prone to breaking down over time. 

Excessive ambient heat will shorten the lifespan of LED lights and degrade their performance. To prevent heat damage, the ambient temperature in the grow room should stay under 30°C. 

To prevent damage from static electricity, anti static gloves should be worn when handling LED boards. Devices and equipment in the production area should be well grounded.

Some LED boards are housed in a protective casing. The casing is sometimes rated for water and dust resistance with the Ingress-Protection code, or IP. It is expressed as IP with two numbers afterwards. For example, IP64. The first digit indicates dust resistance and the 2nd digit indicates water resistance. The symbol X in place of either the 1st or 2nd number indicates that there is no data available. 

Dust resistance rating (1st digit):

less than 5 = poor dust resistance

5 = some dust resistance. 

6 = completely protected from dust.

Water resistance rating (2nd digit):

less than 3 = resistant to some drips, but vulnerable to sprays.

3 = resistant to damage from spraying water.

6 = resistant to powerful jets of water projected at enclosure.

LED lights can be run without a protective casing as long as they aren’t sprayed directly with water and are cleaned periodically using isopropyl alcohol to remove dust.

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How to measure light output.

There are several measurement types that get used to describe energy output. Most common are LUMENS, LUX, PPF, PPFD. They don’t all measure light output the same way.

LUMENS and LUX measure visible light only. Because plants use some light that is invisible to humans, LUMENS and LUX don’t always account for all the light available to plants. 

PPF and PPFD measure all the light plants use, including visible light and invisible light, making them the most accurate measurements for horticultural lighting.

LUMENS and PPF are measurements taken at the fixture.  They measure raw output. They are good measurements to use when comparing power draw vs. output to determine how efficient a fixture is.

LUX and PPFD are measurements taken at a distance from the fixture, accounting for light falloff and reflection.  They are highly accurate and useful measurements because they indicate exactly how much light a plant’s leaves are receiving.

PPFD measurements are captured by lighting manufacturers using special equipment in a lab environment. Light manufacturers include PPFD as part of their published specifications for their lights, usually.

The best way to present PPFD measurements is in a chart that illustrates the intensity and coverage of the light at a stated distance from the canopy. To create these charts, a tester hangs a light above a graph that represents a coverage area. With the light turned on, a quantum sensor takes spot PPFD readings at the intersections of the graph. The data is compiled into a PPFD chart that makes it easy to visualize the light’s performance.

PPFD chart taken 18″ over canopy.
PPFD chart taken 24″ over canopy.

A good PPFD chart includes these features:

• Readings expressed in micro moles per square metre per second (μmol/m2/s)

• Distance the readings were taken from the light. For example, 12″ from light, 24″ from light.

• Multiple readings taken in a grid pattern over a wide coverage area, not just a single reading.

• Indicates if the readings were taken with or without reflection at the sides to reflect back light and boost the light levels. It’s better if the readings are taken without reflection because reflection could be used as a way of exaggerating the capabilities of a light.

Some manufacturers publish only a single PPFD measurement, not a chart that includes multiple readings. This can be misleading if the measurement was taken directly under the fixture where light is strongest.  If a single PPFD reading is given, it should be an average of several readings. For example, an average PPFD over a 4′ x 4′ coverage area.

The amount of PPFD required for cannabis depends on its stage of growth and the strain. The recommended light levels listed below will work for most strains. Keep in mind that hybrid strains with more southern genetics (sativa) require more light than northern varieties (indica). The measurements can be modified up or down 10-20% depending on the strain.

Also consider the depth of the canopy. As the charts above demonstrate, light becomes less intense the further it has to travel. All buds should receive adequate illumination, not just the ones at the top. If the canopy is a foot deep, consider if the amount of light reaching the lower half is adequate.

Below are the recommended PPFD levels for each stage of growth: 

Seedlings, clones: 100 PPFD 

Vegetative, moderate growth: 310 PPFD

Vegetative, heavy growth: 620 PPFD 

Flowering, moderate growth: 620 PPFD 

Flowering, heavy growth: 925 PPFD

PPFD is sometimes alternatively expressed as μmol/m2/s.

LUX is another useful measurement because although it only measures visible light, it can be measured using a commercially available handheld device. This gives LUX a distinct convenience advantage over PPFD which can only be measured in a lab. A LUX metre is a fantastic tool for taking spot measurements that every grower should own. 

Below are the recommended Lux readings for each growth stage:

Seedlings, clones – minimum range: 5,000 LUX

Seedlings, clones – maximum range: 7,000 LUX

Vegetative – minimum range: 15,000 LUX

Vegetative – mid range: 40,000 LUX

Vegetative – maximum range: 70,000 LUX

Flowering – minimum range: 35,000 LUX

Flowering – mid range: 60,000 LUX

Flowering – maximum range: 85,000 LUX

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Measuring lighting system efficiency.

The efficiency of a lighting system is calculated by dividing input power (WATTS) vs. output (LUMENS or PPF).

Watts is an international measurement representing 1 unit of input power. It is the most important measurement for determining how expensive a light is to run because it’s the one used by electrical utilities for calculating the cost of electricity.

Electrical utilities calculate costs based on kilowatt hours (kWh). A kilowatt hour is how much energy gets used by a 1000w appliance in 1 hour. For example, a 2000 watt appliance would use up 1 kWh in 1/2 an hour. A 500 watt appliance would take 2 hours to use up 1 kWh.

The best light is the one that outputs the most power for the least amount of money.

Divide WATTS with an output measurement, either LUMENS or PPF. This results in a number you can use to compare the efficiency of different light systems. The lower the number, the cheaper a light costs to run.

Sometimes efficiency measurements are already calculated by light manufacturers and published with their product specifications. Efficiency measurements are expressed as PPF/W (PPF per watt), or lm/W (LUMENS per watt). PPF/W is sometimes alternatively expressed as µmoles/joule.

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What are light wavelengths?

The wavelengths of electromagnetic radiation span from the very short wavelengths of cosmic and gamma rays, to the very long waves that are used for radio broadcasting. The wavelengths plants use for photosynthesis fall within a very small portion of the total electromagnetic spectrum called the visible and near visible light spectrum. 

The wavelengths of the visible and near visible light spectrum is measured in nanometers, 100 nm – 1000 nm, which refers to the length of the wave. At the low end of the spectrum is short wave ultraviolet light, 100 nm. At the high end of the spectrum is long wave infrared, 1000 nm. In the middle of the spectrum are the wavelengths that plants use for photosynthesis, 400 nm – 700 nm.  

This range of 400 nm – 700 nm is referred to as PAR. PAR is an abbreviation of photosynthetically active radiation.

Across the range of PAR, there are different wavelengths of light. We see these wavelengths of PAR as colours like this:

400 nm – 500 nm /  blue – green

500 nm – 600 nm / green – orange

600 nm – 700 nm / orange – red

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How to optimize wavelengths for different growth stages

Wavelength patterns are not the same for every light. For example, a hospital light emits a bluish hue because it outputs more blue wavelengths than red, while a street lamp emits an orange hue because it emits more red wavelengths than blue. 

Horticultural lights output unique wavelength patterns that can target a plant’s photoreceptors, stimulating optimal growth, and also influence if they direct their energies towards root growth or top growth.

The chlorophylls in cannabis that power photosynthesis capture light at specific wavelengths. The chart below shows the wavelengths where chlorophyl gets captured. Maximum absorption of chlorophylls a and b occurs between 430 – 453 nm (blue) and 642 – 662 nm (red). Horticultural lights often feature high output of these specific wavelengths to target the chlorophylls.

Chlorophyl light absorption peaks at 430 – 453 nm and 642 – 662 nm. To cater to the chlorophylls, some horticultural lights output almost all their power on these wavelengths. Their partial output pattern is represented in the chart below. The light output by these fixtures looks purple due to very high output of red and blue wavelengths, combined with very low output of green wavelengths. 

Partial spectrum wave light distribution.
Part spectrum light emits a purplish hue.

Although partial spectrum lights can be used to successfully grow cannabis, evidence suggests better results are achieved with full spectrum light that includes red,  blue and green light. 

Green light has important special properties. Green light can penetrate the surface of the leaf, helping deliver energy to leaves under the canopy, and to the inner portion the leaves. Without green light, shaded leaves get deprived of energy.

Cannabis grows best under full spectrum lighting that includes green light, with extra output between 430 – 453 nm (blue), and 642 – 662 nm (red) wavelengths that target chlorophylls. 

For production plants in their bloom phase, a full spectrum light that targets the chlorophylls and also has intense output on the red end of the spectrum works best. 

Flowering growth wavelength pattern.

When there is more intense red light, plants respond with greater top growth and stem elongation. These are the desired plant behaviours during budding.

Production plants in vegetative growth also grow best with a full spectrum pattern, but with extra intensity on the blue end of the spectrum. Blue wavelengths suppress the top growth promoting effect of the red wavelengths, allowing plants to focus greater energy on lateral root growth and branching. 

Vegetative growth flowering pattern.

When a blue biased wavelength pattern is used for vegetative growth, you can gain shorter, bushier plants with more roots.

Blue wavelengths encourage root growth.

A blue bias is also helpful for nursery plants because seedlings and mother plants should not be encouraged to grow tall. Root development and lateral branching are the most important objectives for them.

Nursery wavelength pattern.

Nursery lights should be full spectrum with a heavy bias away from red. 

It’s not absolutely necessary to have separate wavelengths for both production growth stages. Any full spectrum light will work for both stages of growth provided it can deliver suitable intensity for the size of plant. Custom wavelength patterns are a performance optimization, not a requirement of life. If budget does not permit separate fixtures for vegetative growth and flowering growth, it’s best to use the full spectrum bloom wavelength pattern with the red bias for both growth stages. The effect of strong red light to boost budding is the most important benefit to be gained from wavelength optimization.