Promoting Photosynthesis

Photosynthesis: How it Works

The energy of light (usually from the sun) is harnessed to fix nutrient minerals from the environment.

  • Plants use phosphorous to help harness this energy.

The energy from light helps the plants to oxidize water (H2O) in to free hydrogen
The same energy is then used to fix carbon directly from atmospheric carbon dioxide (CO2), and with the hydrogen, synthesize carbohydrates (Calvin-Benson cycle).

  • Carbohydrates are then used for energy and to form other carbon based molecules (e.g. cellulose, proteins, THC)

Simply put:
Light + CO2 + H2O = C6H12O6 (glucose, a simple carbohydrate)

  • An important bi-product is atmospheric oxygen (O2)

To promote optimal growth, indoor cannabis requires special attention to the artificial lighting provided.

Water (H2O), a key ingredient for Photosynthesis

  • Water is critical to all plant processes, especially transpiration and photosynthesis. During photosynthesis, water (H2O) is oxidized into free hydrogen. This hydrogen is fixed along with carbon (from atmospheric CO2) to form carbohydrates and other forms of cellular energy (e.g. ATP). Additionally, water keeps many nutrients in their available form and transports them to where they are needed for plant growth.
  • Water is absorbed by the roots and should be readily available at all times. This ensures that nutrients never become unavailable and transpiration is always supplying photosynthesis.
  • Watering demands can vary greatly, and are determined by the environment, grow medium, pot size/material, the plant’s root system and canopy size.
  • A good watering schedule can ago a long way in preventing setbacks and maximizing yields. This starts with choosing the right soil.
  • When creating a watering schedule for your room, start by observing your plant. Allow for your very topsoil to lose moisture before watering again. (It will turn from dark brown, to a lighter brown.)
  • Do not allow the soil to become dry

Maintaining this level of moisture:

  • Exposes root hairs to additional oxygen
  • Prevents stem-rot in young plants
  • Combats common pests (fungus gnats)
  • Promotes a healthy rate of transpiration
  • Ensures nutrients are available

Water Quality

Not all tap water is equal. Many municipal water supplies have high amounts of chlorine and other solids that may be detrimental to plant health.

If your tap-water is known to be “hard” or harshly treated, it may be necessary to invest in a filtration system. If you are unsure, check with your local government to see if a water quality report is available.

  • Practical filtration options include Reverse Osmosis, and Carbon Filters
  • Many growers test and track the total dissolved solids (TDS) in the water, both before and after any supplemental nutrients are added. TDS is measured by an electronic meter in parts per million (ppm), and gives an idea of the quality of the tap water, and strength of added nutrients.
  • Average tap water usually measures between 200 – 300 ppm of TDS.
  • Reverse osmosis filtered and distilled water should measure between 0 – 20 ppm of TDS.
  • Carbon filtered water should measured between 50 – 75 ppm TDS

TDS and EC to measure nutrient strength

TDS can also be a measurement of nutrient strength in the water.
For soil, 700 – 1000 ppm is normal for water with added nutrients.

  • remember, start low and go slow!

Above 1600 ppm starts nearing toxic levels

Feeding and Runoff water can be measured to prevent salinity build up and potential to harm a plant.

The electrical conductivity or EC is another way to measure the amount of solids in a solution and uses the same data as TDS, but on a different scale.

  • To get an EC value, multiply the ppm reading by 2 and divide by 1000. Thus, if your EC is 1: 1*1000/2= 500 ppm.

Measurements limits depends on the cultivar, environment, and watering schedule. In general:

  • For soil, try to keep TDS below 1200 PPM
  • EC should be below 3

Carbon Dioxide (CO2) for Photosynthesis

  • Nearly all life on earth is carbon based. Nearly all organic molecules on earth are bound and stabilized by a carbon skeleton. All organic carbon (including the carbon you are made of) was at one point fixed from atmospheric CO2 via photosynthesis. Plants are autotrophic in the sense that they pull their carbon (and other elements) directly from the environment. Animals are heterotrophic because they derive carbon (and other elements) from other life forms (like plants).
  • During photosynthesis plants intake CO2 and expel O2.
  • It’s easy to see why an indoor grow-space must maintain CO2 in the atmosphere.
  • When in an enclosed grow space, plants can quickly deplete the amount of CO2 and enrich the amount of oxygen (O2). Because CO2 is required for photosynthesis and the formation of carbohydrates, depleted levels can greatly limit growth.
  • It is important for a indoor garden to account for depleted CO2 levels by either completely exchanging the atmosphere with fresh-air, or by supplementing the grow space with generated or compressed CO2 gas. It is recommended that new growers utilize the fresh air exchange strategy for their first few grows (covered in the previous section).
  • Generally speaking, using a fresh-air intake fan to control humidity and heat are enough to also freshen CO2 back up to ambient levels.


Understanding Photosynthetic Light

Light is a form of electromagnetic radiation, which travels in wavesand is made of photons.

  • These waves lengths carry different levels of energy. (Higher frequency = higher energy)

Animals (including humans) use only a small portion of these waves (visible light) to see.

  • Animals see different visible light waves as different colors

Plants and some bacteria also use a similarly small portion of electromagnetic radiation, known as photo-synthetically active radiation (or PAR).

  • Most, but not all of PAR is in the visible light spectrum.
  • Plants use different PAR wavelength at different photosynthetic efficiencies.

The Importance of Light “Spectrum”

Red and Blue light provide the greatest photosynthetic efficiency and in turn, provide the most growth and yield.

  • Other “colors” boost this efficiency synergistically. (The “Emerson effect”)

Plants use colors and the ratios between colors as information to determine how to grow.

  • For example, the lower the ratio between red light and far red light the less “stretching” occurs during growth (Stretching leaves more stalk and less bud-sites)
  • Blue light intensity also helps control stretching

Some wavelengths are absorbed better than others, and it’s not always the apparent “brightest” light that has the best photosynthetic efficiency.

Color Wavelength
violet: 390–450 nm
blue: 450–495 nm
green: 495–570 nm
yellow: 570–590 nm
orange: 590–620 nm
red: 620–750 nm

Measuring Light

  • Good horticulture lighting companies make lights that provide a lot of energy at a spectrum that more closely matches this PAR graph. Thus, electricity is spent meeting the demanding requirements of photosynthesis without wasting watts on light the plant can’t use as efficiently.
  • Artificial lights created to appear “bright” to the human eye are usually measured in Lumens.
  • Artificial lights created to promote photosynthesis are usually measured in PAR.
  • Remember: Through photosynthesis, plants use PAR, H2O, CO2, and inorganic nutrients to create carbohydrates.

Indoor Photosynthesis: Lighting

Photosynthesis cannot occur without light. The more light (photons) provided, the more photosynthesis and growth can occur. Light quality, intensity, and exposure time, all impact growth rates. Special attention should be paid to the kind of lighting used in your grow space.
There are three major contenders when it comes to indoor horticulture lighting for growing cannabis. Each type comes with its pro and cons. Choosing the best one can be specific to your budget and grow space.
Most bulbs, like Fluorescent, Metal Halide (both ceramic and traditional), and High Pressure Sodium are the result of electronically excited pressurized gases. Due to the process, the intensity and spectrum are inherently limited by gas compositions and electrical supply and frequency.

    • Fluorescent
    • High-Intensity Discharge (HID)
    • Light Emitting Diodes (LED)

Color Blue Green Red Far Red
Type 400-500 nm) (500-600 nm) (600-700 nm) 700-800 nm) R/FR Ratio
Direct sun 23% 26% 26% 25% 1.1
Fluorescent 21% 52% 24% 2% 10.7
HPS 5% 51% 38% 6% 6
Incandescent 2% 12% 34% 52% 0.7
Metal halide 18% 49% 25% 8% 3
LED Any Any Any Any Any


CFL/T5 Panels

  • Inexpensive
  • Energy-efficient
  • Low-heat
  • Good spectrum


  • Low yield per watt
  • Not good for flowering

Great for:

  • Young plants and clones that cannot yet withstand more powerful lights.
  • Vegetative Growth
  • Small/cramped grow spaces.

High Intensity Discharge (HID)


  • Good yield per watt
  • Decent Spectrum
  • HPS R/FR
  • MH V/FV


  • High energy consumption
  • Limited spectrum
  • Very high heat output
  • Requires use of additional venting
  • Higher setup cost
  • Easier to light burn/heat stress plants
  • Requires ballasts
  • Potentially Hazardous
  • Grower Implication: HID lighting required a lot of energy, but give great results if you can keep them vented.

HID (High Intensity Discharge) lighting has been the tried-and-true standard for growing high-demand indoor plants like Cannabis for many years. These lights provide a large amount of high-energy light at a decent spectrum. These pressurized bulbs contain elements that when excited with high-frequency electricity, emit a very bright light. This altered electrical current is provided by an external ballast.

Light Emitting Diode (LED)


    • Low energy cost
    • Low heat output
    • Customizable spectrum
    • Plug & play (no ballast)
    • Can achieve very high yield per watt
    • Very long bulb (diode) life


    • Wide claim disparities across the market
    • Expensive
    • Due high energy and low heat it can be easier to “light bleach” canopy
    • Keep at least 2ft from plant

Grow Implication: If you’ve found a brand you trust, and know what you’re looking for, LED lighting can be a great option.