Promoting Transpiration


Transpiration is the process plants use to move water up a the stalk to the demanding canopy.
Like the heartbeat of a plant, continuous transpiration is directly responsible for:

    • Pulling nutrient and water up from the soil to the demanding canopy
    • Turgor pressure
    • Keeping the plant upright and functional
    • Facilitating critical plant processes

E.g. photosynthesis

  • Plant growth rate and yield
  • high transpiration can mean high yields in the right environment

How Transpiration Works

Stomatal pores expel water vapor at the canopy

  • Negative space is created at the top of the stalk
  • Additional water rises to fill the space using the assistance of capillary action.

Within the stalk are tube-shaped cells called xylem. Xylem cells, like the cells of humans hair, are dead upon maturation.

  • They wick nutrient-carrying water up to the canopy.


Transpiration Disruptions

“Wilting” is a very common symptom of a transpiration imbalance (loss of turgor pressure).

  • Turgidity is highly dependent on a balance between water uptake at the roots and water vapor
    escaping from the canopy.

Total canopy surface area and health
Stomatal health

  • Covered or clogged stomata can drastically reduce transpiration rates. Special care should be taken to ensure the leaf surface is not covered by dirt, dust, oils, or other leaves.
  • Abrupt environment changes can lead to a lag in the closing or opening of the pores.


  • Pest and disease damage can greatly reduce leaf surface area, or clog/destroy/cover stomatal pores


  • Saturated air makes it impossible for leaves to expel additional water, stopping the flow of water/nutrients

Total root area and health

Damaged or “Scored” Roots/Transplant shock

  • Rough transplant techniques often lead to damaged and disrupted root systems.
  • Root disease
  • Harmful bacteria or root pests can also greatly disrupt water and nutrient uptake.

Compact/dry soil

  • Improperly structured soil and over/under watering reduce the roots uptake efficiency, and in the worst case, roots will even suffocate.


Creating an Environment for Transpiration

> The rate at which a plant transpires has a direct influence on the amount of water and nutrients it can uptake, photosynthetic rates, growth rates, and overall yields at harvest time.
Influences on Transpiration

    • Transpiration rates are influenced by:
    • the difference between the soil moisture and air humidity
    • dry air & wet soil increase transpiration
    • air circulation
    • temperature
    • air composition (access to fresh air/CO2)
    • slow with high O2 / low CO2
    • To achieve high yielding medicinally valuable flowers, indoor grow space environments must be manipulated to promote optimal transpiration. This means giving each plant, branch, and bud site access to airflow, light, proper humidity, and carbon dioxide levels.
  • Fortunately this can be easily achieved by a home gardener, and these concepts are scaleable for larger operations.


Air Circulation

Proper air circulation is often overlooked, but is a critical component to a healthy garden. A light breeze should persist over the canopy, enough for all the plant’s leaves to very gently sway. Each shoot and bud site should be exposed to open air and regular circulation. This will promote transpiration at each of those surfaces, and in turn will also promote growth.
Proper canopy circulation:
Strengthens shoots

  • Swaying causes micro tears which heal and strengthen over time.

Regulates room/canopy temperature

  • disperses hot zones directly under lights

Regulates humidity

  • moves otherwise stagnant water vapor away from leaf surface
  • increases transpiration/nutrient uptake

Prevents mold and mildew
Too strong or too persistent of a “breeze” will cause wind burn (drying and death of tissue) on your plants. Fans should be kept far enough back to create a very gentle breeze. If possible, fans should be kept oscillating.

Grow Space Design: Don’t forget to allow space for oscillating fans, and account for a few feet between the fan and the plants. Also allow for distance between each plant and the wall. This will improve airflow and allow yourself necessary work-space around each plant (for watering, pruning, etc.).

Oscillating fans, either clip-on, floor standing or wall mounted, are standard in most grow rooms.



Air humidity has a major influence on the rate of transpiration. Transpiration rates are highest when the air is dry and the soil is moist. This allows water to be easily wicked up the plant’s stalk and dispersed into the air.An overly humid grow is a common mistake for new growers. Overly humid environments can cause very slow growth, mold and mildew on the plants and even the grow space walls. This should be evident as soon as you enter the room. If it feels to humid for you, then it is likely too humid for your plants. Severely humid environments are evident when moisture is evident on leaf surfaces or when condensation appear on any surface in the room. This issue should be corrected immediately to avoid crop loss.

Humidity may be managed by:

  • A good hygrometer (usually combined with a thermometer)
  • Fresh air exchange (covered later in this section)
  • A dehumidifier
  • Air flow (to disperse areas of concentrated humidity, usually above and within the canopy)

Digital Hygrometer/Thermometer combos help easily measuring both humidity and temperature in one place.


Canopy temperature should be kept between 70-86F for optimal transpiration.
Cannabis growth is slow in extreme hot or cold conditions.

  • Signs of stress also become evident. Heat stress is often expressed through “canoed” or “rolled” leaves and burnt tips. [Pictured right]
  • Cold stress is evident by purple tissue and very slow growth. [Pictured right]Heat may be managed a few ways:
      • A good thermometer and/or IR Thermometer
      • Venting lights (covered in the lighting section of this course)
      • Fresh air exchange (covered later in this section)
      • Air conditioning
      • Air circulation (for dispersing hot zones)
      • Placing as many electrical devices outside the grow space as possible (e.g. ballasts).

IR Thermometer Guns like this are a great way to get an idea of the canopy temperature.

Fresh-Air Exchange – The Open Loop System

  • Exchanging grow space air can be an important strategy in battling heat and humidity as well as depleted levels of CO2. Grow rooms that exchange grow space air are often referred to as open loop systems.
  • Grow rooms that do not exchange air, and manage the air quality by CO2 supplementation and dehumidification alone are considered closed loop systems.
  • Open loops systems are recommended for your first few grows and are the focus of this beginner course.
  • In an open loop system there is an intake fan [right] in the wall that draws fresh air into the grow-space through a filter. At the same time, air is vented out the opposite wall into a space outside the grow area.
  • The intake fan(s) should be mounted securely and plugged into a thermostatic controller or a simple wall-plug timer. On demand, it should blow air into the room long enough for a total exchange to take place. The exact amount of time depends on the size of the room and fan. Fans are conveniently measured in Cubic Feet per Minute (CFM) which should be compared to the cubic feet of the grow space (W x L x H) and an appropriate amount of time calculated from there (Fan CFM x Minutes > Room CFM).
  • Note: It may be necessary to filter intake/exhaust.

Thermostatic controllers like this one can be used to effectively control fresh-air intake fans like the one pictured below. This will not only help control the grow room temperature, but will also help refresh CO2 levels.