| New construction and window replacement in existing homes utilizing LowE (low emissivity) glass should raise questions about growing houseplants behind this glass for indoor gardeners. Good information is hard to find and the blogs are full of hearsay and simply bad science.
First, here’s a little information about LowE glass and then a little about how plants use light and grow.
When you buy windows today, there are many choices about how they are glazed. Pittsburgh Plate Glass alone offers more than 30 different LowE glass products for architectural use. LowE glass is manufactured with one or more layers of metal oxides or microscopic silver coated on the glass. These coatings affect how light is transmitted, how heat is reflected, and the insulating properties (R-value) of the glazing.
The primary reason for installing new windows is energy conservation and the ratings on the window reflect their performance in terms of insulating value, solar gain, and heat loss. Often overlooked is the value for Visible Transmittance. Visible Transmittance is the amount of visible light the glass transmits. Pay attention to this last value for houseplants. The US Department of Energy has a great website to help understand the various ratings: www.energysavers.gov.
Photosynthesis in plants requires sunlight. Most plants absorb the blue and red portions of visible light and reflect green - that’s why they look green to humans. Photosynthesis falls off very quickly in the infrared portions of light and a little less quickly in the ultraviolet region. LowE glass that absorbs these invisible parts of the spectrum has little affect on growth. These are the primary parts of the spectrum blocked by LowE glass.
What makes the greatest difference in plant growth is the amount of visible light absorbed by the glass. To check our own windows, I used a light meter and calculated the amount of light transmitted. The old glass in the greenhouse allowed 90% of the light through while the new LowE windows we purchased last year allowed about 50% of the visible light through. An older LowE window took the prize - it blocked 84% of visible light! So a window with a Visible Transmittance of 65 will only allow that much of visible light to pass through.
So here is the problem when growing plants indoors. Geraniums may have done well in an older window but are starved for light behind a LowE window. Plants that require lots of light probably will not do as well with newer windows. On the other hand, plants that require moderate to low light have a new home. Since conditions vary greatly, it is probably best to experiment to find plants that thrive in this new environment.
Lists of houseplants and light requirements can be found at: www.savvygardener.com/Features/houseplant_care.html or at http://en.wikipedia.org/wiki/Houseplant_care. Many houseplant books have lists of plants and their light requirements as well.
By WALLY PECK,
Beltrami Master Gardener
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