protected cold frames

Jane McGary
Wed, 21 Jan 2009 11:07:04 PST
Regarding cold frames, UIi mentioned "polycarbonate (Plexiglas)" but 
I think these are two different things, at least as the terms are 
used in North America. I have Plexiglas on my older frames, and it is 
not unbreakable; I've had it break when the wind blew something large 
into a light, and also once when somebody stumbled and fell against 
it. As I know the term "polycarbonate," it's a somewhat more flexible 
material that comes either as Kathleen mentioned in double or triple 
wall forms, or a corrugated form. I have the latter on my new frames, 
and it doesn't break but can tear if struck hard by something sharp.

The main objections to the flexible polycarbonate are that it tends 
to lose transparency with age, which glass and plexi do not, and that 
it is very hard to get tree pollen off it, whereas removing pollen 
from glass or plexi is not difficult. The pollen, in a wet climate, 
not only diminishes transparency but also hosts mold. For this 
reason, the bulb house I'm planning to have built after I move to the 
city (to a place where there are many large trees in the 
neighborhood) is going to have a glass roof. I'll have to hire a 
window-washing company to clean it at least once a year, but the 
light transmission will remain the same, and there will be no trees 
near enough to drop limbs on it. In the warmer environment of the 
city, the bulb house will not require enclosed sides, just a roof for 
moisture control. Severe cold spells can be dealt with by microfoam 
blankets such as I use here inside the frames, held down with bird 
netting and groundcloth pins.

If I were building frames that would be expected to last only a few 
years, I'd still opt for polycarbonate because it is light and easy 
to work with.

Different kinds of automatic vent controls can be obtained from 
Charley's Greenhouses in Washington; they have a website and print catalog.

Jane McGary
Northwestern Oregon, USA

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