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[nafex] Greenhouse Glass

For Minnesota Del:

1.  Normal window  glass, which is composed of silica, will filter out a
substantial portion of the shorter, ultraviolet waves of the spectrum.
Polycarbonate (plexiglass) and lexan do not, nor does "quartz glass",
which is very expensive and rarely used.  Double pane glass would
presumably be twice as efficient at filtering out uv.
2.  "High-E" glass is an attempt to also filter out the red end of the
spectrum, the longer waves which include heat, and reflect it back into
the house or away from the house, depending on where the heat is coming
from.  (Heat is an invisible infrared wave.)
3.  Plants can do without a lot of infrared, but prefer quite a bit of
ultraviolet for healthy, stocky growth.
4.  A few years ago I built a large (27'x14'), south-facing solar
structure attached to my house.  The glass is tempered, double-paned,
tinted, and even has a reflective coating on the roof panels.  The
structure has no provision for internal heat, other than space heaters I
take out there on the coldest winter nights, but I grow plants out there
year-round.  Perennials like geraniums, kalanchoe, and especially
succulents, seem to do marvelously, and I can keep things like million
bells going forever just by taking slips and repotting them.  Amaryllis
are in full and exuberant bloom right now.  In the summertime you can
turn off the fans and sit out there and cook as if you were in a sauna,
without ever getting a sunburn because there is so little ultraviolet.
5.  But I have noticed that when I start seedlings like tomato, pepper,
and annual flowers, they appreciate an additional shot of light from
something like flourescent bulbs for healthy, stocky growth.
Flourescent light contains more of the uv component than incandescent,
and the closer to the plants, the better, since the weak uv waves
produced by the bulbs are filtered out even by the atmosphere.  The best
bulbs would be borrowed from a tanning salon, but they are probably
pretty expensive.  You could bring a couple of flats along when you stop
by to get a tan, but the salon people might think you are crazy and call
the police.
6.  So in the final analysis, it's a trade-off.  Growing under glass
gives seedlings the heat they need to germinate and grow, but you are
going to give up some of the uv provided by natural light, which is
important to their onward development.  I think that is why many
commercial greenhouses that grow seedling plants use polycarbonate or
fiberglass panels rather than glass.  Even though fiberglass appears to
filter out more of the light, it allows more of the uv rays to pass.  So
does cloudy/transparent polyethelyne.
7.  Don't let me discourage you from building a greenhouse.  I think
they are great and I have a lot of fun with mine.  But be prepared to
give seedlings a little extra uv light from some source, or you may have
plants that become a little leggy before it's warm enough to set them

Rgds, Don Yellman, Great Falls, VA

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