Growers around the world use lights to increase their production. Indoor grow rooms require artificial lights as a substitute to sunlight. But many greenhouse growers use artificial lights as well, to lengthen the day, increase radiation intensity or to increase the amount of light on cloudy or rainy days.
There’s good reason to add lights to your greenhouse. Increasing the amount of light during the day directly increases the rate photosynthesis, contributing to plant growth and health. Lighting in the early morning and during the evening lengthens the plant’s day as well, adding valuable hours of photosynthesis. A combination of these lighting methods dramatically increases yields and improve crop quality.
But for the extra radiation to actually improve growth, there are a few additional factors to consider.
Greenhouse Lighting Impacts Humidity
Increasing light duration and intensity is a great way to increase production. But there are side effects to increased radiation, especially when it comes to humidity.
Increased lighting can lead to high humidity, which slows down growth, rather than accelerating it. High humidity levels also contribute to disease outbreaks such as downy mildew, botrytis and alternaria.
When increasing the amount or duration of light, it’s important to control the humidity. The most efficient way to avoid high humidity is by using dehumidifiers.
Though no type of supplementary lighting is exempt from increasing humidity, the impact of different types of lights differs.
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Greenhouse Light Types
There are two major types of greenhouse lighting – HPS (high-pressure sodium) and LED.
Until recently, HPS lights led the greenhouse lighting market. But HPS lights’ narrower spectrum, combined with their high heat emission and energy consumption drove many newer greenhouse installations to choose LED over HPS.
LED lights are much more efficient in terms of energy consumption and their heat emission is minimal compared to HPS lights. But the cost of LED lights can be as high as 5 times that of HPS lights, requiring a larger initial investment.
How Lights Increase Greenhouse Humidity
There are two ways in which lights increase plant evapotranspiration – short-wave radiation and long-wave radiation.
Both types of energy are emitted from the sun or supplemental lights and both affect the energy balance of the plant.
Short-wave (solar) radiation
This is the plant’s primary source of energy. It’s transferred without affecting the greenhouse air, being absorbed directly by the leaves. This input of energy causes a reactionary output – evaporation.
Increased short-wave radiation therefore accelerates evapotranspiration, increasing greenhouse humidity.
Long-wave (thermal) radiation
Long-wave radiation refers to heat. Though it plays a more minor role in the plant’s energy balance, it still has a major impact.
Heat is physically transferred through convection; it flows from warm objects (or air) to cooler objects (or air).
When the air is warmer than the plant’s leaves, the plant will absorb the heat, increasing its own temperature. This too is an input of energy, resulting in an output – evaporation, raising humidity.
It’s important to note that though LED lights emit much less heat than HPS, their long-wave radiation is still substantial and shouldn’t be ignored.
Humidity Control is Necessary When Using Supplemental Greenhouse Lights
The increased evapotranspiration caused by the additional lighting increases greenhouse humidity. Without proper humidity control, high humidity may cause disease outbreaks.
Besides causing diseases, high humidity has an effect on the plant’s physiological processes. When the air is saturated with water vapor, plants can’t transpire, causing them to stop taking in water and nutrients through the roots. This is a stressor, slowing down plant activity and impeding development.
The attempt to increase photosynthesis with greenhouse lighting will backfire in this case, leading to unsatisfactory results.
Using supplemental greenhouse lights to increase the duration of the day comes with additional dangers.
During dusk and dawn the weather changes rapidly. In the early morning, plants receive their first light, which suddenly stimulates activity, causing enhanced transpiration.
In the late afternoon and evening, on the other hand, temperatures drop rapidly, leading to an increase in relative humidity which may cause dangerous dew point condensation.
Additional lighting during these times will increase humidity further, leading to higher risk of disease and stagnant growth.
Dehumidifiers are complementary to greenhouse lights
The most efficient method to control and reduce humidity is with the use of dehumidifiers.
When adding lighting to a greenhouse, or designing a new facility, it’s critical to factor in these humidity raising processes.
Dehumidification is an all-around growing tool that provides growers with more than just increased lighting efficacy; it has multiple benefits and enhances the use of various greenhouse equipment:
- Allows growers to keep shade, blackout and thermal screens closed
- Increases the energy efficiency of heating or cooling systems such as HVAC
- Provides large quantities of distilled water which can be re-used in chillers, cooling pads, etc. (water removed from the air)
- Reduces the need to ventilate, retaining energy invested in the greenhouse
- Promotes climate uniformity and increased climate control
- Increases control over CO2 concentration by allowing the greenhouse to remain closed
- Decreases the need for pesticides and fungicides
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