Ignoramus2772 wrote:

The idea was to extend the growing season by a month in both spring
and fall...


Looks like it would work but I see a couple things you might want to
address.

It appears to have a double ridge pole. This will increase the strength,
but when the plastic is on it, rain/snow will accumulate on the flat
section on the top. If it freezes, you have a block of ice up there.
That's not a problem in itself, but if the ice is thin enough and a wind
comes up, it could dislodge the ice (which could have sharp edges) and
tear holes in the plastic. Similar things have happened to me when I had
an ice storm that accumulated on my greenhouse. Get some UV treated
repair tape for that possibility.

The rain is also likely to collect at the bottom of the roof line as the
plastic sags. Not really a problem: you can just take a push broom on
the inside and bow the plastic outwards to dump the collected water.

It isn't clear from the picture how you are going to provide ventilation
when the plastic is on. Ventilation is essential, even in cold weather.
A sunny day can bake the plants even if it's cold outside.

The easiest way is to make the sides so that they roll up. Just roll the
plastic around some battens at the bottom. The battens should be tied
together so that the whole length of the greenhouse will turn and roll
the plastic on the batten. At the top, you put some strings that you can
use to tie the roll of plastic in place. The disadvantage of this is
that you have to be home to watch the weather and you have to be home
when the cold night temperatures turn to warm day temperatures (and vice
versa). For a sunny cold day, just open the front and back doors to
provide some circulation.

A more automated way is to provide an exhaust fan and thermostat. I just
put a door on the end that opens inward on a spring. When the fan comes
on, it pulls the door open to provide cooling air. This is more work but
you don't have to watch it. For a small greenhouse like this you could
probably use some cheap plastic box fans (around $20 or less each) with
a thermostat built in. Frame them in and put a flap of plastic over the
outlet so that they aren't open all the time. I'd use two of them on the
leeward end. I had a 30" exhaust fan, so I used it. Bigger is better (up
to the point where it doesn't fit onto the end wall). If you use a large
exhaust fan, put screen over the fan for safety. It can chop off
fingers. If you don't do this simple thing, your homeowners policy may
decline to pay for injuries to some kid who wanders into the greenhouse.

A small greenhouse like this can be heated when necessary by a small
electric space heater. All you really need is to keep it above freezing.
I've used two of them in a medium size greenhouse successfully. Gas or
kerosine heaters can be used, but keep all appropriate safety
precautions in mind.