Choosing the Right Greenhouse Covering to Maximize DLI

Posted in Agriculture
Choosing the Right Greenhouse Covering to Maximize DLI
By Mark Weaver, Technical Manager

When choosing the optimal cover for a commercial greenhouse, there are a multitude of materials and configurations to consider. Different materials provide different properties, and understanding the impact of those properties on the energy balance of the greenhouse is vital for making the right choice! 

While most growers strive to get as much light into their greenhouses as possible, there are many factors involved in determining light measurements. In the following example, we will assume maximum light into the greenhouse is the goal, as using natural daylight cuts down on the cost of artificial light sources. The light measurement most growers rely on is Daily Light Integral (DLI) which indicates the daily amount of PAR accumulated. This is basically a rain gauge for light.

PAR (Photosynthetic Active Radiation) measures the intensity of the portion of the light spectrum between 400 nm and 700 nm. This is the light a plant needs for photosynthesis to occur, allowing plants to grow.

PPFD (Photosynthetic Photon Flux Density) is a measurement unit for the amount of photons that reach the plant leaf over a 1 square meter area every second, and is indicated in micromoles (μmol) per square meter (m-2) per second (s-1), or: μmol·m-2·s-1 of PAR.


DLI is the measurement of PAR in μmol·m-2·s-1 accumulated over the course of a 24 hour period

There are many factors that affect the amount of light being seen by the plants down at the leaf level, including:

  • Greenhouse structural members
  • Internal obstructions
  • Angle of the sun
  • Directional orientation of the greenhouse
  • Geographic location and time of year. 

To simplify things, we will focus primarily on the factors involved in creating optimal sunlight penetration of the greenhouse covering. The angle of the sun to the earth’s surface plays a very large part in greenhouse DLI measurements. This angle varies throughout the day and throughout the year as the earth rotates around itself and around the sun.

For example, a greenhouse in Pennsylvania, in July, can receive an average of 40-45 mol·m-2·d-1. The exact same greenhouse in January will receive 10-15 mol·m-2·d-1. With this kind of variance, it is imperative that in the winter months, every effort is made to get as much of the available PAR light into the greenhouse. There are many plants that need as much as 22 mol·m-2·d-1 to thrive and flourish. 

A simple solution might be to put clear glass on the greenhouse. While this sounds logical, it isn’t this simple. If we were only measuring light at 12:00 noon every day, it could be assumed that glass would be the optimal covering.

However, for an accurate DLI measurement, we need to measure light over the course of the entire day from sunrise to sunset. This poses challenges for all flat covering materials. The angle of incidence of the sun to the greenhouse canopy surface plays a very large part of the overall PAR light transmission into the greenhouse.

When the sun is at its lowest point in the sky at sunrise and sunset, flat greenhouse canopies reflect more light than they transmit. So while glass may transmit an extra 1% of light while the sun is at its highest, it will reflect most of the light as the sun sets lower. Corrugated polycarbonate, on the other hand, refracts light at low angles, thereby increasing its average light transmittance into the greenhouse.


Light in the Greenhouse

The graph below is based on data from Wageningen UR light lab (The Netherlands), and compares 0.8mm corrugated polycarbonate to standard 4mm greenhouse glass, the two most common long-term greenhouse covering materials. Readings clearly indicate that while at direct impact angle the glass and the polycarbonate panels show similar light transmittance; at low angles of incidence the 0.8mm corrugated polycarbonate transmits up to 20% more light.

The corrugation profile is able to capture the sunlight at lower angles and redirect it inwards as opposed to reflecting it outwards. At latitudes of 45º-55º this advantage is even more significant during the five critical months between November and March, as the sun shines most of the day at a 15º-25º angle with the horizon. 

Corrugated polycarbonate refracts light at low angles of incidence, reduces reflection, and facilitates more light penetration into the greenhouse than any other material. Of course light transmission is not the only attribute to be taken into account. Structural elements create shadows and clear glazing creates direct light impact that may damage plants. 

Polycarbonate manufacturers are now able to maintain the same high light transmission with corrugated polycarbonate, but add diffusing agents or create embossed light diffusing surfaces which results in light that is evenly dispersed throughout the plant canopy. This makes for an ideal growing environment without the increased risk of plant burn due to an intense light beam. 

Diffused light also reduces heat buildup in the greenhouse, resulting in a pleasant work environment and reduced possibility of bug infestation.

Wrapping Up

If you consider the information above, along with corrugated polycarbonate’s light weight, high impact resistance, and durability, corrugated polycarbonate clearly stands out as the champion of greenhouse coverings!

About Palram Americas

Palram Americas is an end-to-end, state-of-the-art thermoplastics manufacturer based in the Lehigh Valley, PA. For 60 years, Palram has provided high-quality polycarbonate & PVC solutions used in countless markets spanning construction and architecture, sign and display, agriculture, safety & security, transportation, DIY and outdoor living.

Whether you’re a contractor, fabricator, designer, architect, printer, or even a homeowner interested in DIY projects, Palram’s range of polycarbonate and PVC solutions will help transform your vision into reality. Together, we will build on.

To learn more about Palram’s commercial greenhouse coverings, click here

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