There are ways to start gardens early, when the weather is still cold.
If you have room, you can buy or build different kinds of enclosures to protects new plants or seeds.
Location is everything. If the winters are very cold, you can’t keep anything over the winter. Perennials, which are plants that will die and come back in the spring can stay outside.
If you live in an area that has weather that isn’t too cold, you can start seeds and young plants in March. Warmer areas allow for growing plants through the winter months.
I have found pictures of four types of enclosures.

The greenhouses and Orangeries are elaborate enclosures. They have, in cold places, heat and ventilation.
A glass greenhouse has panels that open in the roof to let in air. They can also have  panels on the sides. Sun pours through the glass to provide the energy for plants to grow.

Orangeries are buildings with a lot of glass.

Hoop frames are enclosures with metal hoop frames and are covered with special plastic. Hoop frames can also be covered with shade cloth to cut down on the amount of sun that comes into hoop frame. The shade cloth can be put over plastic or in warm weather, used by itself. Too much sun will make a plant enclosure too hot and the sunlight would burn the plants.

Cold frames are usually small. They have glass or plastic tops. The frames can be metal or wood. People use these to start seeds and very young plants in their yard. Cold frames are used when it’s still too cold to plant in the ground. In less cold regions, cold frames can be used all year round.

All the pictures are from Wikipedia Commons.

The first picture is of a simple cold frame  by,Moormand.

Coldframe_1 Moormand WCAnother cold frame  by, KVDP. There is a reflection of a building on the top.

cold frame KVDP WCThe next two pictures are of a hoop frame, covered with plastic. It is D-Town Farm  by, Michael Barera. First is the outside and second is the inside of the hoop frame.

D-Town_Farm_Sept_2011_09 Michael Barera WCD-Town_Farm_Sept_2011_10 Michael Barera WCThis is a picture of greenhouses with side panels open  by, Chong Fat.

greenhouses Chong Fat WCThis is Starling Lane Winery greenhouse in British Columbia, Canada  by, Kyle Flood. There are panels open close to the top.

Starling_Lane_Winery_Greenhouse Brit Colum Canada Kyle FloodThe last two pictures are of the Tower Hill Botanical Garden Orangery in Boylston, Massachusetts. The first is the outside  by, Daderot. The second is part of the inside  by, Moffet.

Orangery_-_Tower_Hill_Botanic_Garden Mass Daderot WCLimonaia_in_Tower_Hill_Botanical_Garden Moffet WC