Hartley Magazine

All the latest news, hints, tips and advice from our experts

Supplemental Light and Your Greenhouse

As the air turns crisp and winter casts a shadow on these early fall days, who can ignore the fact that everyday becomes shorter too? Will there be enough light on winter’s shortest days for your indoor plants?

Although the quantity of solar radiation the earth receives just outside the atmosphere is nearly constant,  the amount reaching the earth’s surface depends on the composition of the atmosphere, our location on earth, and the season. Clemson University researchers have drawn upon thirty years of light data to create maps that illustrate the average daily light across the U.S. through the year,


Compared with conditions in even the sunniest corner inside our homes, greenhouses appear bathed in unfettered light. But is this really the case? Although greenhouses are designed to maximize the transmission of natural light to the plants inside, a remarkably diminished amount actually does so. Greenhouse glazing materials can contribute to the problem—single layers of glass are definitely best. Pay attention to dust or shade paint that remains on the greenhouse exterior and interferes with light transmission. Greenhouse framing also blocks light because it is opaque– absorbing or reflecting light, reducing available light by as much as 15%. Overhead clutter, ranging from hanging baskets to pipes and conduits can also be a problem. All these factors together insure that greenhouse light transmission is generally in the range of 35-70% of what’s available outdoors! While not such an issue during brighter times of year, this assumes new importance in winter, when the sun’s elevation is low, and the hours of daylight shorter.

That’s why supplemental light can make a difference in your greenhouse. When you choose lights, you’ll want to consider the above factors– and an additional very important one—the plants you’re growing. If fruit or flowers matter, decide whether you’re growing long day/short night plants, like rudbeckia, lettuce, or potatoes, or short day/long night plants such as chrysanthemums, poinsettia, and Christmas Cacti. The flower and fruit of day neutral plants—tomatoes, cucumbers, and roses—are unaffected by the period-length of light they’re exposed to, although plant quality might be.

For greatest efficiency, commercial growers use computer-directed systems that monitor greenhouse conditions and control both supplemental lighting and heat. These systems negotiate those times when no one is available to make adjustments.  With or without such systems, supplemental lights can help you create optimal growing conditions for your plants.

Alice McGowan