Each year, new genetics arrive for cultivators, and so does new terminology. My favorite concept recently has been “crop steering”. Technically, this concept refers to manipulating inputs that drive hormonal responses in the plant. The variables or “inputs” relevant to this concept can be found into two cultivation sections: fertigation and canopy.
Fertigation involves the controlled delivery of water, nutrients, and O2 to the plant root system. Both the bare minimal and optimal quantities of each input will change depending on the plant genetics, biomass, and environmental factors. Root zone pH is also an important variable that directly relates to plant health and productivity.
Most plant species are around 90% water by weight. Water acts as a chain during photosynthesis to drive nutrients from the root system up through the plant foliage, and eventually out through pores on the leaf surface to the surrounding environment. Light has photons which use energy to split water molecules and form chemical compounds ATP and NADPH.
Plants primarily photosynthesize using CO2 and light where chlorophyll is present, but the root zone operates through O2 and cellular respiration. The capacity for dissolved oxygen in water is affected by the temperature of water. Cold water can hold more dissolved oxygen than warm water which is one reason water temperature should be closely monitored.
Plant tissue temperature will also directly impact the speed of plant metabolism and thus rate of photosynthesis. Limiting any of these cultivation inputs, including plant temperature, can slow down plant growth. It is important to note that the style of cultivation lighting will directly impact the relationship between room temperature and plant surface/tissue temperature.
Flowering rooms with Light Emitting Diodes (LED) lighting typically need to run at higher room temperatures to achieve the same plant tissue temperature as High-Pressure Sodium (HPS) flowering rooms. It is not uncommon for LED flowering rooms to have temperatures over 80 degrees Fahrenheit while “crop-steering” Light Intensity Is often measured with a quantum sensor that measures Photosynthetically Active Radiation (PAR). This measurement is called Photosynthetic Photon Flux Density, and the typical unit of measurement in cultivation is micromols per second or umol/s. The total amount of time the plants receive light multiplied by the light intensity is referred to as Daily Light Integral (DLI) and is typically referred to as Mol/day.
During the life cycle of a cannabis plant, biomass will continue to increase. As biomass increases, DLI should continue to increase. During the flowering stage the maximum light intensity with the addition of supplemental CO2 should be 1200 micromol/s. A typical flowering cycle is 12 hours of lights on, and 12 hours of lights off.
(1200 micromol/s * 3600 second/hr. *12 hours) / 1,000,000 micromol per Mol = 51.8 Mol/Day
CO2 should be supplemented between 1000-1500ppm for this stage. Outdoor atmospheric levels of CO2 are closer to 400ppm.
Nutrient concentration measured as Electrical Conductivity or EC should also be increased in unison. Plant genetics, vegetation time/size, and cultivation styles will determine what the maximum nutrient concentration should be for this stage. This often requires trial, error, research, development, and time. A nutrient analysis of plant tissue can be a great solution to ensure that a crop is not limited by any deficiency or toxicity. Learn more about this with our nutrient analysis testing.
Adequate humidity is also important, especially in a situation with elevated light and temperature. The relationship between leaf surface temperature and relative humidity of the immediate surrounding area is called Vapor Pressure Deficit (VPD). More information can be found in our previous blog post on VPD.
Optimizing all Inputs in Crop Steering
Optimizing all of these inputs with the right timing can lead to optimal rates of photosynthesis and ultimately synthesis of beneficial plant compounds (I.e., cannabinoids and terpenes). Be aware, increasing all of these inputs will lead to a decrease in margin of error. Therefore, it is critical to understand why, when, how and how much to increase these inputs.