Plant Physiology

Is Transpiration Really Such a Good Thing?

Amazingly, a plant only holds on to around 10% of the water it uptakes via its root system. The rest evaporates or "transpires," mainly through tiny holes in the leaves called stomata. It's probably most accurate to think of water as "flowing through" a plant, rather than being drank up and used. So what is transpiration for and is it a good thing?

Transpiration is the term used to describe the loss of moisture from a plant. More than 90% of water escapes from plants through their leaves via tiny openings called stomata. 

The rate of transiration is influenced by environmental factors: temperature, humidity and air movement.

Humidity: If we were to look at the microclimate round the stomata of a healthy leaf we would find a very moist environment with relative humidity approaching 100%. So, if the surrounding air (i.e. the air in your garden) has a lower relative humidity it will act like a "moisture sucker" (as water vapor always diffuses to an equilibrium.)

Thai basil leaves curling due to localized low humidity stress caused by T5 fluorescent lighting being too low

Temperature: Temperature affects relative humidity because warmer air can hold more moisture. So, in other words, the higher the temperature in your garden, the more your plants will want to transpire. Furthermore, if your grow lights are a little too close and are heating up the surface temperature of your plants' leaves (so that your leaves are warmer than your garden) this will cause even more transpiration!

Air Movement: Increased air movement will contribute to increased transpiration rates as you are effectively moving water vapor away from plants and replacing it with drier air, speeding up the cycle of transpiration.

So ... is transpiration a good thing or a bad thing?

Scientists argue that transpiration plays a key part in nutrient absorption and distribution and also aids significantly in keeping leaves cool (a little bit like humans sweating on a hot day.) However, experiments using radio-tagged mineral elements have demonstrated that minerals continue to move around inside the plant even when it is not transpiring.

It's just as likely that the function of stomata is to let carbon dioxide into the plant rather than letting water vapor escape. Of course, a plant opening up (even microscopically) leads to a drying effect in all but the most humid environments, so transpiration could be seen as an evolutionary method of dealing with that.

In Summary

  • Healthy growing plants give out a lot of water into the atmosphere!
  • This moisture lost to transpiration must be replaced.
  • Transpiration is essentially driven by differences between the vapor pressure around the stomata and the ambient air in your garden.
  • The water vapor that a plant emits into an indoor garden must be captured (in a dehumidifier) or vented out of the garden to avoid issues caused by excessive humidity.