What is Supplemental Lighting?

Most greenhouse plants need at least six hours of photosynthetically active light per day. If day time has gotten seasonally short or there is fog or cloud coverage, supplemental lighting can be used to replace the sunlight. Supplemental lighting has to be "photosynthetically active." This means it needs to be the type of light that a plant can use to make energy. Metal halide lamps, high-pressure sodium lamps (HPS), or LED plant lights are three types of greenhouse lighting that can be used to help the plants grow when nature is not giving them enough of what they need. Long-day plants, short-day plants, and neutral plants can all utilize supplemental lighting in the greenhouse.

When Is Supplemental Lighting Needed?

On overcast days and days during the winter months, there may not be enough sunlight for crops to maintain uniform growth rates. Supplemental lighting promotes even crop growth and development.

Frequent cloudy weather
Growing crops through winter
Lack of natural light

Photoperiod Lighting vs. Supplemental Lighting

Lighting for photoperiod control differs from supplemental lighting in greenhouses. Photoperiod lighting is used to induce flowering in long-day plants (or prevent flowering in short-day plants), where supplemental lighting is used to increase light exposure to maintain crop productivity on days where there is little sunlight. Photoperiod lighting can be achieved with low-intensity lighting that does not promote crop growth, but most growers will opt for photosynthetically active lighting.

How Much Supplemental Lighting Is Needed?

High-intensity light that measures 400-600 ft-c/50 to 75 µmol∙m−2∙s−1 is needed to increase daylight integrals (DLI). DLI is the unit used to measure lighting versus day length. It can be influenced by season, weather, greenhouse glazing, greenhouse structure, and shade cloth use. Supplemental lighting needs to have these high-intensity wavelengths or it will not successfully mock natural sunlight.

To find out how much supplemental lighting is needed in a greenhouse, growers must calculate the DLI specific to their facility. This can be done using an online calculator. Especially with young plants, ensuring a DLI of 10-12 mol∙m−2∙d−1 will reduce cropping time and maintain crop quality.

Supplemental Lighting Options

There are three main types of supplemental lighting to consider. Those are high-pressure sodium (HPS) grow lights, ceramic metal halide (CMH) grow lights, and LED grow lights.

High-Pressure Sodium Lamps

More orange and red-colored light
High-intensity light output
Photosynthetically active light
Some growers prefer to use HPS for flowering and metal halide lamps for vegetative growth with short-day plants

Ceramic Metal Halide Lamps

More blue-colored light
High-intensity light output
Photosynthetically active light
Some growers prefer to use HPS for flowering and metal halide lamps for vegetative growth with short-day plants

LED Grow Lights

The best LED grow lights have full-spectrum color
Many have changeable color modes for vegetative growth and flowering growth stages
High-intensity light output
Lower energy consumption than HPS and metal halide lamps
Runs cooler than HPS and metal halide lamps so can be placed closer to the plant canopy