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