The over winter greenhouse

I use a gas heater with a thermostat set to 54 degrees for tropicals. This is to save plants from freezing only, the 2 coldest months of the year, and to extend the growing season 6 to 8 weeks of the year. Without artificial heat, a hoop house of this size will only extend the growing season by 4   weeks, or one zone.  So some of the elephant ears in this image would go dormant in January... When its 25 degrees out for 10 hrs. it gets into the high 30s inside just before dawn for 1 or 2 hours, so the most tender of plants, in the coldest 1 or 2 months, will go into dormancy. I have found over the years that some bulbs do not survive these short periods of cold then warm, but rot, and would fare better in the ground outside covered with mulch. However this is a plant collectors green house. Hardiness, moisture requirements, mature size, disease and pest restance, behavior, shade sun tolerance are unknown, as many of the plants have only recently been discovered in far away lands by plant hunters, spent a period in the USDA quarentine station, then sent to tissue culture labs or propagated for plant collectors like me who grow them in different environments to find where they fare best. 25 to 30 percent are finicky and hard to grow, 60 percent are worth having beside something prettier, but about 5 in 100 are wondrous, and make the horticultural industry.

These palms are too close together.

The location of your greenhouse is important. These are often temporary structures in a subtropical climate, built as inexpensively as possible. Many gardeners aren't sure just how deep they want to get into it, so it makes sense to start small. If your serious get the heavy duty poly covering. The light duty will only last a year, then get brittle. The heavy may go 5 to 6 years if you get all the wrinkles out and stretch it tightly. Put your covering on in the hottest sun, in the middle of the day, and pull it as tight as you can rolling the ends at least 3 times before stapling with 9/16 x 14mm staples. Make sure the staples are smashed down tight holding the poly tightly or it will rip during the contraction in cold weather when it gets tight. Any wrinkle that is moved by the wind will not last. There is heavy duty patching tape for these problemed areas.
 Even an expensive professional greenhouses can't take the high winds that they will sooner or later be exposed to. A location sheltered from the prevailing winds, storms, and northern fronts will keep you from worring in gusts of wind . If there is nothing like this around, you could plant them. A large clumping type subtropical bamboo did much to save my house during 12 hours of 140 mph winds in Hurricane Rita in 2005 (bambusa textillis). It comes in a large and smaller form. It is hardy to 15° F when it gets frost bitten. Here in zone 8b it froze to the ground 30 years ago @ 8° F, and recovered in about 3 years as pretty as ever. If you don't have much room you should try something smaller. Southern live oak, bald cypress, and clumping bamboos made up the bulk of what was left when the wind subsided after Rita.

Make sure the location has good drainage. Water from rains and watering plants should drain away sufficiently well. Imported sandy soil is usually necessary to some degree. During the rainy season would be the time to check drainage before you build anything.

Half as much magnification would be better.

Growing is a joy if your setup is done properly, but if it isn't you will be more likely to fail and become discouraged before you start.

USGS The old people know.

Overwinter greenhouses often have too many plants in them. This allows spread of pests and disease quickly. You must realize the forces of pests and diseases, though low on the food chain, are bigger than men by proportions unimaginable. If you can, space plants leaving 12'' or more between. Do not wet the foliage in a green house and do not water excessively, but keep the ground or floor as dry as possible. Plants need less water this time of year. Keep them a little on the dry side.

Though only little bugs, small infestation of white flies, mites, or aphids can become almost as bad as the cold temps outside. Get yourself a pocket microscope ($10 on line). The one depicted here magnifies too much, so try one with less magnification. Go online and study spot, fungus, aphids, white flies, and mites. Spot or fungus will be a problem in greenhouses that stay too damp. Keep your plants 18'' off the ground or floor for plants subject to spot such as palms.  Spot and fungus live on the ground and contact plants when a drop of water hits the ground and splashes the tiniest particle of moisture on them. Even condensed moisture dripping from the poly top starts infestations when it drips to the floor and splashes on the plants. Then when your moving plants around, one that has spot touches the other and bingo.

Your watering wand spreads spot from one plant to the other while watering. Most tropical plants eat spot and fungus for breakfast so don't worry about spot on them so much, only the pests.

Pointed slightly downward.

 A greenhouse crowded with plants is absolutely no good without air movement. Fans help keep humidity consistent throughout the space, and pests find it tuff to suck sap in the wind. I bought a couple cheap but bad ass little fans from Walmart and one runs during summer to tend the few plants that stay inside. It has been running for eleven months 24/7 and it's so quiet all you can hear is the air in the blades.

On warm days open your house up and let the natural breeze through. Sometimes when it's cold out,  the best way to control the pest is to drop the temperature inside the greenhouse and spray everything in it with a soap (fatty acid only, Palmolive green dish detergent) and water mixture. If a plant is infested really bad with pests, put there butt outside in 33 degrees.

You can burn more sensitive plants with soap, so mix properly. This is your best weapon against these pests. You can spend 30 years trying every thing else, but you will come back to it. Spray bottom of leaves first, and then top until run off. Good luck