Garden & Greenhouse


Supplemental Lighting for the Greenhouse

Posted June 16th, 2015 by Eric Hopper in , ,


Due to the time of year or the geographical location of a greenhouse, some gardeners are installing artificial lighting systems to supplement the sunlight. Commercial greenhouse growers view supplemental lighting as a way to increase growth rates and yield and, in turn, increase profits. With careful calculations, supplemental lighting can increase growth rates and keep light levels of a greenhouse at the ideal intensity for optimal results. For hobby greenhouse growers, supplemental lighting is generally used to maintain a certain level of growth and to extend the growing season.

There are a few different types of lighting technologies used for supplemental lightning and each has its own advantages and disadvantages. To determine the best type of lighting technology to use, a grower should first assess what he or she is trying to accomplish with the supplemental lighting. In other words, a supplemental lighting system should be chosen based on the crops being grown, the time of year and the amount of natural sunlight available.

When to Use Supplemental Lighting

Many hobbyists install supplemental lighting in their greenhouses to extend the growing season. This is especially true for gardeners in northern climates where temperatures are lower on average and the sunlight hours in the spring and fall are far less than other regions. Other hobbyists, who wish to garden year round, utilize supplemental lighting to create an artificial environment conducive to plant growth in the winter months. By supplying the plant with enough light hours in a 24 hour period, gardeners can still produce fresh vegetables and aromatic flowers even in the dead of winter.

Generally speaking, growers use supplemental lighting to extend the daylight hours. Gardeners turn the supplemental lights on before the sun rises and/or after the sun sets. This creates a continuous light cycle for the plant in the greenhouse by combining the artificial light cycle with the natural light period. Although some commercial greenhouses will operate the supplemental lighting during non-peak hours (usually sometime between 10 PM and 6 AM) to save money on the electric bill, this is not always advisable for hobbyists. Two separate light cycles in the same 24 hour period can “confuse” particular plant species and actually hinder their growth and/or production. Because of this, it is best to leave the split light cycles to the professionals.

A 24 hour timer and a lighting system is usually all a hobbyist needs to get a supplemental lighting system up and running. The timer needs to be adjustable (preferably by fifteen minute increments) as the photoperiod will need to be altered as the length of natural sunlight increases or decreases depending on the season. Most annual plants will maintain vigorous growth rates at a 14-18 hour light cycle per day. To calculate how many hours to operate a supplemental lighting system, a grower must first determine the total amount of light hours desired and then subtract the amount of natural light hours that will be received at that particular time of the year.

Types of Supplemental Lighting Systems

There is more than one option to choose from in horticultural lighting technologies. Each technology has its own advantages and disadvantages. The most commonly used lighting systems for supplemental lighting are high intensity discharge (HID), fluorescents, LEDs, and sulfur plasma.

High Intensity Discharge (HID)

High intensity discharge (HID) lighting systems include both metal halide (MH) and high pressure sodium (HPS). These systems are still the most commonly used horticultural systems by both greenhouse growers and indoor horticulturists. They are relatively inexpensive and produce a spectrum of light suitable for maintaining vigorous plant growth.

The biggest advantage of HID lighting systems for supplemental lighting is they can cover a large area without taking up a lot of space. Space is such an important issue because the larger the lighting system, the more it will block the natural sunlight when it is not operating. HID lighting produces significant amounts of heat which can be an advantage during winter months but can also be a disadvantage during warmer times of the year. Growers in cooler climates who wish to extend the growing season should consider HID lights for the added heat they generate. In some cases, the heat produced by the HID lighting will be sufficient so that another heat source, e.g. an electric heater, will be unnecessary.


Another commonly used horticultural lighting technology for supplemental lighting is fluorescents. Fluorescent lighting can also cover a large area and is usually a little more energy efficient than HID lighting. The advantages of fluorescents are they are relatively inexpensive and available practically anywhere. Fluorescents run a little cooler because they disperse the heat over the entire length of the bulb (usually four feet in length).

The length of the bulbs can be a disadvantage though as the light intensity is decreased due to the fact that it is emitted across the entire length of the bulb. This makes fluorescents a great choice for plants with lower light requirements. The biggest disadvantage of using fluorescent lighting in a greenhouse is that the fixtures are usually large which can block the natural sunlight.


Over the last ten years LEDs have made great strides within the horticultural sector. LEDs have a few advantages including longevity (they usually last ten years), being specifically tailored to the light spectrums of plants, and they operate much cooler than HID or fluorescent lighting. LEDs do not use hoods or ballasts, making them better for smaller areas. Also, LEDs are used to supplement lost PAR light from cloudy days or greenhouse glass ceilings. Moreover, LEDs do not require any acclimation time when switching from LED to natural sunlight as their spectrum is so close to mimicking natural sunlight. LEDs that contain UVA light are able to fuel certain carotenoids that HID do not as HPS fixtures do not contain UV light.

Due to their cool operation, LEDs are more energy efficient than other horticultural lighting technologies. This works well for gardeners in warm climates who are looking to extend the growing season. However, growers who require additional heat may want to choose HID lighting over LEDs. The largest disadvantage of LEDs is initial cost. However, due to their longevity and lower energy costs, LEDs can be viewed as a long term investment.

Sulfur Plasma

Sulfur plasma light systems are relatively new to the horticultural sector. These lights produce a spectrum closer to that of the sun than any other artificial lighting technology. Because of this, they make excellent supplementary lighting systems. Although plants do well under any type of supplementary lighting system, they can adjust most easily to sulfur plasma because of its unique spectral output. In other words, there is less acclimation for the plant when switching from sulfur plasma to natural sunlight than with other artificial light technologies. Like LEDs, sulfur plasma lighting lasts close to ten years. Sulfur plasma fixtures run cooler than HIDs but not as cool as LEDs. The biggest disadvantage of sulfur plasma is the initial cost of the system.

How Much Supplemental Light?

There are many detailed light studies on various crops which are used by commercial growers to determine the optimal amount of light energy required for a greenhouse. However, most hobbyist gardeners are growing more than one type of plant in the greenhouse and are not necessarily looking for the largest return on investment.

Most hobby growers will just need to follow general guidelines when setting up a supplemental lighting system. If using HID lighting, a hobbyist should plan on having 5 watts per square foot. In other words, a 1000 watt lighting system would cover 200 square feet. If using fluorescent lighting, a grower should plan on roughly 5-10 watts per square foot. LEDs are more difficult to determine because the quality of the bulb, the type of reflective lens and the actual wattage vs the rated wattage will all come into play. In most cases, a grower who wants to use LEDs should plan on 3-5 watts (of actual wattage) per square foot of garden space. A grower using sulfur plasma for supplemental lighting should plan on 2-5 watts per square foot of garden space.

Some growers will use light movers or light spinners as a way to cover more area with less wattage. These devices are great for supplemental lighting systems as they extend the light coverage area. Please remember these recommendations are for supplemental lighting. Greenhouse hobbyists or indoor horticulturists who wish to use artificial lighting as the primary lighting source will need much more wattage per square foot of garden space. Supplemental lighting should be viewed as lighting that simply complements natural sunlight. The majority of the light energy for plant growth comes from the sun.

Automation of Supplemental Lighting

Most small greenhouse growers will use no more than a timer and a lighting system to create the supplemental lighting. However, there are specific light sensors and light controllers designed to automate and make an artificial lighting system as efficient as possible. A more elaborate supplemental lighting system will include a light controller. Light controllers can turn lights on, off, or reduce the intensity (for some light technologies) as necessary.

A light controller is generally used in conjunction with a light sensor. The light sensor measures the light intensity within the greenhouse and sends data to the light controller which controls the lights as necessary. In some cases, the system will turn on the supplemental lighting during sunlight hours when the light level drops below the set point. For example, on a cloudy day, the sensor can tell the light controller that the plants are not receiving the optimal amount of light. This will trigger the supplemental lighting system to operate until the sun comes out again and increases the light levels within the greenhouse. Elaborate lighting controller systems are fairly expensive and are generally only used by commercial or professional growers.

Regardless of which type of lighting technology a greenhouse grower chooses to use, providing a greenhouse with supplemental lighting can give the plants the extra light they need to continue growing even when sunlight is not available. Supplemental lighting also gives a grower the ability to extend the growing season or to continue gardening year round. Commercial growers who use supplemental lighting must take into account light intensity, light distribution, light uniformity, the cost of operation and the system’s maintenance costs.

For most hobbyists, these are important considerations but supplemental lighting is used more as a way to continue a hobby instead of increasing profits. This isn’t to say that careful planning and consideration of electrical costs and light distribution should be ignored. A good starting point for any hobbyist who wishes to experiment with supplemental lighting is to begin by using the general guidelines regarding light energy per area (watts per square foot) and provide light to a section of the greenhouse. From there, a gardener can quickly determine how much light will be needed to adequately provide supplementary light to the entire greenhouse. Be forewarned, as soon as you begin to incorporate supplemental lighting to extend the growing season by a few weeks, you will quickly realize how easy it is to turn your hobby of gardening into a year round activity.

Photos provided by Urban-Gro

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at

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