Note: Measurement of light is a complicated subject. This article can be approached from at least two levels: one level would be to gain an intuitive sense of the graphs, whereas a second level would be a more in-depth approach where individual parameters are explored. For those interested in the latter, references are given at the end of the article.
In Minnesota the number of hours of light essentially doubles between the winter and summer solstices, 8 and 16 hours, respectively. (Figure 1).
M. Pidwirny (PhysicalGeography.net)
Figure 1: Hourly variations in insolation received for a location at 45° North latitude over a 24 hour period.
The low angle of the sun reduces the number of light photons or energy per unit area meaning that the energy received at mid-day on December 21 is less than one third of that received at mid-day on June 21 (Figure 2: Sun declination angle relative to energy received at 41.7 o N latitude).
Figure 2: Season Comparison of Solar Zenith Angles in Logan, Utah 41.7 degrees N Latitude
In fact the total light energy received in December on average is closer to one quarter of that received in June and July (Figure 3: outdoor daily light integral US).
To survive the winter we bring our plants inside and further reduce light reception through the shading effects of the building. Because plants use light to produce needed metabolic compounds such as sugars and starches, this drastic reduction in sunlight means we have the opportunity to starve our plants.
To have a good experience with houseplants, we need to choose those plants that can tolerate these "lean" conditions. I have two categories of houseplants, plants that stay inside year-round, and plants that winter inside and summer outside. In the first group I have had success with: African violets (Saintpaulia ionantha), Phalaenopsis orchids, cacti, Cyclamen (Cyclamen persicum), Schefflera (Schefflera actinophylla), and Norfolk Island pine (Araucaria heterophylla). In the second group I have had success with Meyer lemon (Citrus × meyeri), tropical hibiscus (Hibiscus rosa- sinensis), banana (Musa 'Dwarf Cavendish'), pygmy date palm (Phoenix roebelenii), and cycad (Cycas revoluta). All of these plants have the ability to tolerate lower light conditions.
Match your home micro climates to the plants' needs
The best way to handle this is to understand the light needs of your plants and the amount of light entering your house through various windows. The amount of light energy is greatest from the south > west > east > north. In the winter the sun rises south of due east and sets south of due west. Windows facing directly east and west receive sun coming at an oblique angle to the window, reducing energy received.
I have placed my African violets (Saintpaulia ionantha), Phalaenopsis orchids, cacti, and Cyclamen (Cyclamen persicum) in a corner with south and west facing windows. These plants stay here year around and avoid direct summer sun due to plant and building shading effects (photo 1).
To improve the location I washed the windows and re-potted the orchids to improve spacing (photo 2).
The other plants [Meyer lemon (Citrus × meyeri), tropical hibiscus (Hibiscus rosa- sinensis), banana (Musa 'Dwarf Cavendish'), and pygmy date palm (Phoenix roebelenii) have been placed in western facing windows.
Photo 3 shows the decrease in light level from the center of the room to the window, a 36 fold decrease.
Photo 4 shows the plants positioned to optimize their light reception.
Recognize the @ 75% decrease in light energy received from summer to winter solstice. Choose plants that tolerate lower light levels and select locations in your house affording plants adequate light. Recognize the dramatic reduction in light energy as you move away from the window.
Korczynski, P., J. Logan, and J. Faust. (2002). Mapping monthly distribution of daily light integrals across the contiguous United States. HortTechnology 12 (1) pp. 12-16.
Lopez, R. and A. Torres. (2010). Measuring daily light integral in a greenhouse. Commercial Greenhouse Production. Purdue Department of Horticulture. Bull HO-238-W http://www.extension.purdue.edu/extmedia/HO/HO-238-W.pdf (accessed 12/2/2012)
Pidwirny, M., and S. Jones (2010). Daily and annual cycles of temperature in Chapter 7: Introduction to the atmosphere in PhysicalGeography.net, Fundamentals eBook. http://www.physicalgeography.net/fundamentals/7l.html
Sunmaster (2012). PAR Watts, Lumens, Photons, Lux and Watts. http://www.sunmastergrowlamps.com/PAR_Watts.htm