Originally Posted by Donwill[_2_]

kerrygirl wrote:
Hi everyone,

I'm new to gardenbanter. I have started a new step in gardening, I grew
seeds for the first time this summer. I have been trying to educate
myself on glasshouses and polytunnels but I'm still not sure, at the
moment I can't afford a glasshouse. A lot of the seed packets say '
protect from frost ' would a polytunnel protect my seeds during winter
or is it used for spring growing. Advice very much appreciated. I live
in Ireland.

kerrygirl.





The problem with polythene is that it's transparent to long wave(LW) IR
radiation so you don't get the "glasshouse effect" which entraps the
LWIRR within the glasshouse. AFAIAA (old age& poor memory :-( ) when
polythene was first applied to horticulture in the early 50s a simple
experiment at the NIAE which was to set up 3 cold frames one with glass,
one with polythene and one without any cover, I think the crop was
lettuce.They found that on some clear nights when the air temperature
above the crop was warmer than the ground temperature there was frost
observed on the polythene covered lettuce but none on the glass covered,
and none on the uncovered, the last observation was due to the relativly
warm air flowing into the cold frame.
So in certain circumstances it can be colder under polythene than
outside. However it may be that modern polytunnels are covered by
polythene which has been treated in some way to reduce the LWIR back
radiation, I don't know the answer, perhaps some one more knowlegable
than myself can answer that one.
Polytunnels are very good at stopping cold drying winds from damaging
a crop as are glasshouses of course.
Hope that helps
Regards
Don

--
Bold is Brown with the People's Purse.