Transpiration Vs Evapotranspiration – Greenhouse Management
Transpiration and evapotranspiration are two very similar concepts. Both describe water evaporation, but despite being so similar, there’s an important distinction.
Understanding your water cycle can help prevent diseases and molds, improve growing conditions, and save energy.
Transpiration and evapotranspiration are important factors in greenhouse and indoor growing. They have a major impact on cultivation and managing them effectively can help increase yields and improve produce quality.
This article explains what transpiration and evapotranspiration are, how they affect greenhouse and indoor growing, and how to efficiently manage them.
What Is Transpiration?
Transpiration is the plant’s process of releasing water through the stomata, which are small pores in the plant’s leaves.
When water evaporates from the leaf surface, it creates a pull, often called transpiration pull, or xylem pull. This pull is what causes the plant’s roots to absorb water from the soil, and it’s a major part of the plants’ development process. It’s what drives the transport of nutrient rich water throughout the plant and delivering it to the different plant cells.
So, transpiration is a critical factor in plants’ development and growth process. Without transpiration, there’s no movement of water in the plant, and no new nutrients being taken in, resulting in slower development.
What Determines Transpiration Rates?
There are several environmental factors that affect transpiration in greenhouses and grow rooms:
- Humidity
- Temperature
- Light intensity
- Wind speed and air movement
How Humidity Affects Transpiration
Humidity is the leading driving force behind transpiration. In order to release water vapor, the air must be able to carry more water molecules. So, if relative humidity is 100%, meaning the air’s completely saturated, the plants won’t be able to transpire.
On the other hand, if relative humidity is too low, plants shut down their stomata, to conserve water and avoid water loss.
So, in order to maximize growth, humidity needs to be within a comfortable range.
How Temperature Affects Transpiration
Temperature also has an effect on transpiration.
Diffusion and evaporation are higher in higher temperatures. This is true in the stomata as well. So higher temperatures increase transpiration, and lower temperatures reduce it.
How Light Affects Transpiration
Light, or radiation, affects transpiration in two ways – directly and indirectly.
First, as light usually emits heat, it increases the plant’s transpiration rate, indirectly, through temperature.
Second, is the direct impact light has on the opening and closing of the stomata. As part of the photosynthetic process, when there’s more light, the stomata open wider. They do so in order to allow more carbon dioxide (CO2) in, to improve photosynthesis. The wider opening creates a larger surface area, from which water evaporates quicker.
So as light or solar radiation increases, transpiration increases as well.
How Airflow Affects Transpiration
Airflow, or wind, moves water vapor, carrying it away from its source. As most moisture in the greenhouse comes from the plants, it helps to reduce humidity levels in their surrounding air.
So airflow, in effect, speeds up diffusion from the leaf, promoting more transpiration.
What Is Evapotranspiration and Why Does It Matter?
Evapotranspiration is the sum of all water vapor sources in the greenhouse. It includes both transpiration, which comes from the plants, and evaporation from other sources, such as soil moisture.
As mentioned before, high humidity interferes with nutrient transport, harming the plants’ development and leading to smaller, lower quality yields. It also leads to disease and mold development, like botrytis or powdery mildew.
So, understanding your evapotranspiration rate provides the big picture. It’s the best gauge for the amount of humidity that gets generated in the growing space. This, in turn, determines the necessary amount of dehumidification.
How to Reduce Greenhouse or Indoor Humidity
The most efficient way to remove water vapor from the greenhouse is using dehumidifiers.
However, to get the best results, it’s important to match the dehumidification capacity with the amount of evapotranspiration in your growing space.
If your dehumidifier’s capacity is too low, it won’t be able to handle the amount of water vapor, and humidity will stay high.
On the other hand, matching your dehumidification capacity with the maximum amount of water vapor in the space provides many major benefits.
Maintaining a dry environment stimulates growth and keeps plants healthy by preventing the conditions necessary for disease development. It also saves a lot of energy, when compared to alternatives, such as heating and venting, or HVAC.
Using a condensation-based dehumidifier, like DryGair, also provides a source of clean reusable water. It extracts as much as 45 liters per hour from the air, which growers can collect and use.
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