Vertical Farming 101 – What makes a plant grow?

PAR – Photosynthetic Active Radiation

PAR or Photosynthetic Active Radiation refers to the wavelengths that plants can see and use to grow. This is a small range of wavelengths (400 – 700nm) that are within the visible light range on the electromagnetic spectrum.

The electromagnetic spectrum is the entire distribution of all electromagnetic radiation that is based on frequency or wavelength.

PPFD — Photosynthetic Photon Flux Density (umol/m2s)

Photosynthetic photon flux density is the intensity of PAR light that you measure right at your plant canopy. The units of PPFD are umol/m2s. This measures the number of umols of PAR light that are hitting your grow area every second. This is important because all plants need different intensities of light for optimal growth.

Photoreceptors

Photoreceptors are cells in a plant that detect and respond to light in order to initiate different cellular processes of a plant. Types of photoreceptors include phytocromes, phototropins, and cryptocromes. Each type of photoreceptor responds to different wavelengths of light and is responsible for different processses. Phytocromes detect red and far-red light and help regulate seed germination and flowering. (Therefore if you want to grow a flowering plant, you will need to make sure red and far red are in the light spectrum!) Phototropins detect blue-light and help regulate stomata opening, as well as growth towards light. Cryptocromes detect blue light and help regulate the cicardian rhythms of a plant which helps with timing its photosynthetic activity.

Stomata

Stomata are pores (or tiny openings) on the surface of the plant that regulates water and gas exchange between the plant and surrounding environment. A plant needs to transpire (release water) as it creates a pressure differential between the roots and leaf of the plant. This drives water and nutrient uptake in the plant which is required for the plant to grow. This process also allows the plant to cool itself down, while keeping its structure rigid. Lastly, this allows the plant to exchange gases (CO2 and Oxygen) with the environment so that photosynthesis can occur.

Vapor Pressure Deficit

Vapor Pressure Deficit (also known as VPD) is a measure of the difference of water the air can hold and the amount it is currently holding at a set temperature. (It is a combination of humidity and temperature conditions). From the moisture in the air, a certain pressure will be exerted on the leaf of a plant which will cause the stomata in the plant to either open or close. For each life cycle stage of the plant there is an optimal amount of pressure that should be exerted on the leaf to drive reasonable transpiration rates in the plant. At different temperatures different humidity levels will create the optimal pressure conditions you are looking for. It’s best to figure out what VPD you require and consult a VPD chart to see what combinations of temperature and humidity will get you those values!

Carbon Dioxide

Carbon dioxide is a requirement for photosynthesis. Often if CO2 is not being supplemented it will be the limiting factor for how fast your plants will grow. Ambient CO2 concentrations are around 400 ppm. Past research studies have showed however that the saturation point for carbon dioxide uptake is between 1000 – 1200 ppm. Therefore, if you supplement CO2 your plants are likely to grow faster!

Oxygen

Oxygen is required for cellular respiration in the root zone of the plants (a process which creates chemical energy for the plant). This allows nutrient and water uptake in the plant which is critical for its growth. In traditional agriculture, oxygen is absorbed by the soil through the atmosphere. Therefore in some types of indoor growing systems (such as deep water culture hydroponic setups) it can be critical to provide a way to oxygenate your nutrient solution — as oxygen may be depleted quickly and not restocked at the rate it is required.

Electrical Conductivity

Electrical conductivity is a term used to measure the amount of electrical current that a solution can carry. Water is made of hydrogen (+) and oxygen (-) ions that naturally create an electrical current. When nutrients are added to water they dissociate into their ionic forms and increase the electrical current of the solution. Therefore, electrical conductivity is a way to measure overall nutrient concentration of a solution.

Xylem & Pholem – How the plant transports nutrients

This is a fun fact on plant physiology! The xylem and pholem of the plant are two tissues that are used to transport nutrients and water within a plant. The xylem is responsible for transporting water and nutrients from the roots to the rest of the plant. The pholem is responsible for transporting sugars and other nutrients from the leafs to the rest of the plant.

Airflow

As plants typically grow outside, naturally they are accustomed to wind. To mimic these conditions you should use a fan for your indoor growing setup. A fan can help plants develop stronger stems (making them more resilient). Additionally it can help move the air around the plant canopy, which prevents moisture build up . This can help with preventing mold growth, ensuring adequate transpiration rates, and adequate gas exchange between the leaf of the plant and the surrounding environment.