Greenhouse Igloo Effect
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Mid-Winter Update -- 03 January 2009

It took a while getting here, but winter definitely arrived during the middle of December, as below-average temperatures and above-average snowfall hit the area.  This was the test I was waiting for to evaluate my handiwork, and was the reason for building the greenhouse in the first place.

Overall, I am pleased.  The greenhouse has stayed consistently 7-10 degrees warmer inside than the air outside.  This is particularly true on sunny days when it warms-up significantly inside.  It also stays very humid of course, and blocks the dry wind, which is just as important as buoying temperatures.

Once forecasts called for overnight lows to fall below 10 degrees, I added supplemental heat in the form of a small space heater given to me by my former landlords.  This was very helpful, although I haven't seen this month's electricity bill yet.  One of the goals of the greenhouse was to not let temperatures fall below 20 degrees.  This is kind of a break line for a number of the plants that I have.  Secondarily, it is preferable that the air rise above freezing each day, or at least every 48 hours.  At the least, several of these plants cannot be allowed to freeze solid in their pots.  I think we did alright in this regard, although it did not thaw on every day.

Another interesting development was the advent of snow.  We had a heavier snowfall than our usual "inch or two", as there was as much as 8 inches in the yard.  I decided to bury the greenhouse to see if I could achieve an insulative "igloo effect".  Not only would the snow possibly provide an insulation layer, but would again keep more temp-wicking wind off of the plastic surface.  The wind also pulls at the seams over time, and I have already observed some stress.  The snow would cover seams for a while, and weigh the whole structure down...


After the first 5-inch blast, a second round of snow came, adding another layer to the igloo...



You can see by the arrival of the sun on this day -- the Hibernal Solstice and "shortest day of the year", that I placed the greenhouse superbly to capture and maximize the lowest sun of winter...


Ultimately, the greenhouse has been successful so far.  The lowest temperature reached in my backyard so far this year has been exactly 0, but the temperature inside the greenhouse has not yet fallen below 19.8.

One possible issue that I will be making keen observances of, is the possibility that the greenhouse may be "too successful" during the day.  For example, as I write this it is 18 degrees outside on a sunny Saturday morning.  However it is already 64 degrees inside the greenhouse, and what has typically been the warmest part of the day inside the greenhouse is still over an hour away.  The overnight low inside the greenhouse was 21 degrees.  These wild diurnal temperatures swings may not be good for some of those plants.  It could approach 80 degrees inside the building today, yet freeze again tonight -- day after day.  Since I have no easy venting mechanism built-in to the structure, I pretty much have to let this ride.

Unfortunately, I procrastinated in November and did not get my Golden Barrel Cactus out of the ground and into the greenhouse before the cold hit.  It comes from central Mexico and is not exactly known to be adapted to Eastern Washington.  I buried it in leaves and snow and hoped for the best.  Now that it is being uncovered, it is still firm and the color is good.  Maybe it will pull through.  I'm going to cover it with plastic to create a "greenhouse in-situ" and see what happens...

That's all for now.  I hope to start planting broccoli and spinach in early February.


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