We live in Southeast Texas, Zone 9, winter is relatively mild but we do have freezing weathers every now and then. All it takes is one hard freezing night, and the tropical bonsai will say good bye. As I am writing this post, weather forecast warns of “arctic assault” freezing temperatures for the next four nights.
I use portable greenhouse to over-winter my tropical bonsai such as ficus, fukien tea etc. I leave pine, juniper, elm, azalea etc. outdoor.
Over the years, I have used two brands of portable greenhouse. I found the white 10′ x 10′ King Canopy product is much superior than the green one I bought on eBay. The King Canopy is now in its 6th winter, and it is still good; but the green canopy degraded and became flaky after the 3rd season. I think the King Canopy has a better UV stabilizer, the plastic material is thicker, 6 mil thick, and has better tear resistance reinforcing webs embedded in the plastics, running vertically and horizontal. They also sell replacement canopy should one needs it..
Set up is quite easy. I usually set it up myself, of course a second person would be helpful. The metal tubes are heavy duty and slide into fixed angle metal joints for the pitch of the roof. The canopy comes in one piece, all I need to do is put it over the metal frame, tie it down to cross-bars located near the bottom of the frame with bungee cords. I anchor the metal poles with camping stakes, and use cinder blocks and heavy flower pots to hold down the extended flaps on the ground.
Will the wind blow it away? I have not experienced it, and we often have 20-25 mph wind. During bad weather, I make sure to zip up the front entrance and back window flaps. This prevents a sudden gush of wind entering the tent; if the wind has no where to go or cannot escape fast enough, the consequence would be for it to lift the canopy upwards like a balloon. So zip up on windy days. On sunny days, I open both openings to allow air circulation.
When night temperature drops below 35°F, I light up a 1,500 BTU propane gas tank top heater. It keeps the greenhouse at least 10°F warmer than the outside temperature. The objective of over wintering in a greenhouse is to keep them warm enough and out of the cold wind. For people living in colder regions, a higher BTU or double tank top heaters might be needed.
I had used electrical heater before. Several years ago we had a severe winter storm, the power went out in the middle of the night; the heater went off and some plants suffered freeze damages. After that I switched to propane gas heater, heating is not interrupted during power outage.
When storing bonsai in a greenhouse, one should not pack it full. I leave walking space so I can check the bonsai during watering. I now use two 10′ x 10′ King Canopy, and have been happy with them.
Around mid-March, after our last frost date, I take down the greenhouse. In our hot summer, I set up the metal frames and put a 50% shade cloth over it to filter out the sun. Two uses: a greenhouse in the winter and a shade tent in the summer.
One summer , I found a hawk perching on the shade tent!