Supplemental lighting and Partially Sunny Windows

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Supplemental Lighting

Supplemental lighting can extend the area that the plants are grown in, and be used to control flowering in plants subject to photoperiodism.

While it is nice when there is a full sun window available to light indoor plants with, sometimes that isn’t available, and supplemental lighting can make up the difference.

When planning a window lit garden area, it is helpful to find out how many hours of direct sunlight it receives in a day. Write down the time the area first receives sunlight, and then the time when it is no longer directly lit. The difference between the two determines the hours of natural sunlight available to the area.

Light Requirements

Plants are often characterized in part by how many hours of sunlight (or the equivalent) they need each day:

Full shade plants can be grown in an area that only receives an hour or two of direct sunlight. Many popular indoor decorative plants can be grown in full shade.

Partial sun (or partial shade) plants need 4-6 hours of direct sun.

Full sun plants require a minimum of 6 hours of direct sunlight.

For a hypothetical example: a windowsill gets 2 hours of sunlight a day. With this amount of light full shade plants could be used without requiring supplemental lighting, but it would need at least 2 additional hours of light for partial sun, or 4 hours of additional light for full sun. Only use the additional lighting when the area is not in full sun or there may be issues with heat and bleaching.

There are LEDs (Light emitting diodes) and T5s (fluorescents) in form factors well suited for long narrow spaces. If a larger space is desired, there is a plethora of indoor gardening lighting fixtures on the market to choose from.

Supplemental lighting can make the difference between partial shade and full sun

Supplemental lighting can be used to manipulate flowering in plants influenced by photoperiodism. While it can’t make the nights longer than the season (light deprivation), it can make the nights shorter by extending lit hours. This is important because photoperiodism in plants refers to the length of the dark period being the determining factor in triggering flowering. The terms describing photoperiodism were chosen to describe plants that are photo dependent for flowering because the behavior was observed before the mechanism involved was understood. So while it would be more accurate to describe them as “long night” and “short night” plants, by convention the terms “short day” and “long day” are used instead. Some greenhouses use artificial lighting to manipulate flowering in much the same way, just on a larger scale.

Supplemental lighting can turn otherwise unused underlit areas into productive growing spaces. From a single bulb T5 giving a windowsill an extra boost to full sized lighting used in commercial greenhouse practices, there are situations where even a few hours of additional electric light makes all the difference.

Featured image courtesy of  Trixi Skywalker.


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Grubbycup was raised on a family-operated organic dairy farm in central California.