To be honest, this post had to wait a bit because I feared disaster. Luckily, our first season with a greenhouse has been a near-complete success.
Purchase and Assembly
We purchased a kit from Farmtek. In particular, we got a 9'x12' model shown here. It was not cheap and we added a base that gets attached to the ground by long metal stakes. I highly recommend that, because greenhouses act like sails in a heavy wind. Imagine a $3000 investment blowing across the yard in a storm, to end up a tangled mass of bent aluminum and cracked poly panels. Buy the base.
The kit requires patience, as well as a few common tools. I used some ratchet-driver wrenches and screwdrivers. Since the poly panels are held against the metal frames with spring-loaded clips, only a fool would not wear goggles when working on that part of the project. A carpenter's square is a must, too, for early work. If the frames are not straight when screwed together, the panels won't fit later, and the builder will need to loosen screws, move things, and lose time in the process.
It took me about 30 hours of work, half of that with a helper, to get the thing built. We started in late winter. Before I began I leveled the area and put several inches of rock dust, then gravel, over weed-block fabric. A few weeds have still found the site comfortable for sprouting, but they have been easy to pull out.
The panels were the most difficult part; screwing together framing is easy, if time-consuming. The panels have open cells to trap air and edges that fit into plastic channels resembling the binding for acetate report covers. The edge-channels got easily mixed up in the bags and we had to unnecessarily cut some down that should have been saved for larger panels. They are wickedly difficult to slide on at first, but once we had the hang of it, that got easier.
The company provided several tubes of excellent silicon caulk for sealing out air; the metal clips alone for the panels do not suffice when the winter winds blow. And we often start seeds in February. The finished greenhouse had two loads of snow on the roof from late-season storms.
Overall, most things fit well until we got to the roof-vents. They were cheaply made and ill-fitting. I cut down some poplar strips and painted them white. They braced the vents and sealed them fairly well. Water will still drip in during downpours, and one stream ruined a small tray of Ghost-Pepper seedlings that I grow to heat up our Thai Dragon peppers: no small loss if you hate hot food!
I did not have time to install the automatically opening vents that respond to sunlight; when the plants are all in the ground, I will go to work.
Seeds will not germinate when the soil is too cold or hot, too wet or too dry. I found that my Thai Dragon peppers, our cash crop, were particularly finicky. 2015 seed I saved did well, but not Thai seed left over from last year's bulk purchase, then frozen. 2016 seed purchased as a last-ditch effort to have enough plants did wonderfully. Then some of the Thai-sourced seed from 2015 made a late emergence.
Bigger growers use heating pads under trays, and I think I will too for peppers next year.
A key element for any greenhouse is a max/min thermometer. It reads those ranges and can be reset daily. One cloudy morning I left the vents and door closed. By noon the sun was beaming down and I raced home to find seedlings wilting. I nearly lost 100+ plants since the greenhouse was at 120 degrees. I got lucky and saved them all, by opening the greenhouse wide and misting everything little and watering the rest. Still, it was a close thing.
Damp Soil But No Floods
Mold will kill your seeds fast. The rule I've heard from professional growers to to mist the soil only when it is dry. On cloudy days, the soil may not dry out completely. On sunny ones, I found myself misting the seedlings again in late afternoon, when I got home from work.
The right mix of soil is everything. I used some planting mixes for some trays. For others I put a few inches of garden soil, from a 20 ton pile we had delivered. I smoothed that layer, added the seeds, then topped it with seed-starter mix to the depth needed by a particular seed (1/4 inch for peppers).
When the seedlings get tall, I just use a common metal watering can and water the lot. I have always liked to keep my potted seedlings in trays with a half-inch or so of water in the bottom. I found this develops really good root systems, as compared to only top-watering the seedlings.
Now we are hardening off the seedlings and planting at a breakneck pace. I think I raised over 1000 plants this year. Yikes. I'm giving away the extras.
Would I do it all again? Absolutely. I might have looked for a different vent system. When all of our plants are in the ground, I'll unbox the automatic vents I purchased from Farmtek and install them; they respond to daylight and are solar-powered.
And, I did say "Buy the Base," right? And stake it down, right? Here's what happened to Scottish greenhouse, an image I found online, in a big Scottish gust. I'd need more than a wee dram if I came outside and found that.