By Wally Peck - Beltrami County Master Gardener
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
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: http://
lists of plants and their light requirements as well.
