It’s never too late to build a greenhouse for tropicals and other cold sensitive plants. Stores and mail order sources sell small units ready to use. These are great for protecting just a few plants through the winter.
Building a small greenhouse is a 1 or 2 weekend project. The easiest and most cost effective structure is one constructed with a wood or pipe frame and covered with polyethylene plastic. Treated lumber is safe and long lasting. Pipe can be purchased already bent from greenhouse companies or you can buy new or used pipe and pay to have it bent to your desired shape.
Greenhouse shape and size will be dictated by the type of plants grown. Bedding plants will require benches and hanging baskets will need a sturdy frame to hang from. Tall tropicals will need their special place. Build at least a 10 foot tall structure to aid in heating and cooling.
Cover the greenhouse with a single piece of 6 mill polyethylene plastic. Staple it to wood for a fast and effective air tight covering. Clear fiberglass and rigid polycarbonate coverings cost more but do not have to be replaced every year. A fan is recommended to cool the greenhouse on sunny winter days and will provide air circulation for healthy plant growth. Gas or electric heat should be able to maintain a 45 degree minimum temperature even on the coldest nights. Gas heaters must have thermostatically controlled solenoid valves and electric heaters must have tip over switches and be connected to a GFI circuit.