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On those cold nights in early autumn when we are heading towards the first frosts, or in those same nights in early spring, usually when we're still getting the occasional frost, you can protect your plants from the cold. You can do this in a few ways. Planting them in a greenhouse or polytunnel, covering them with coldframes or fleece, or putting them inside. I'm going to go through all of these methods and will refer back to these in the plant specific posts.
Greenhouses
A transparent glass or plastic enclosed structure. They can protect plants from frost and many pests, with windows to open for airflow and pollinators. There can be beds in there for growing plants in their final position, or you can place pots/trays of seedlings or fully grown plants in them. You'll need to manually water any plants that are in a greenhouse. If you have a heated greenhouse you can grow things year-round.
Polytunnels
A tunnel covered with translucent polyethelene, usually with a steel or plastic structure. They work in the same way as a greenhouse, but can be moved and are usually much cheaper. They heat up to make them warmer than the outside air and can protect from frosts, as well as many pests. Place it in the garden to cover plants in their final growing position or use it to protect potted plants and seedlings. Open vents and windows for airflow and to let pollinators in. You'll need to manually water any plants that are in a polytunnel. This is the method I'll probably be using next season.
Coldframes
They are like a box that is either fully transparent or at least a transparent lid. It's placed over your plants, or plants in pots placed in them. They protect from frosts. Useful for protecting plants from one of those random one-off frosts or getting a headstart on a small number of plants. Open the lid during warmer days so that the plants don't overheat. Plants will need to be manually watered. They offer protection from some pests.
Fleece
A lightweight, porous fabric that is wrapped around or covering plants, protecting them from frosts and many pests. Used to cover plants in pots or plants in the ground. Plants inside a fleece can sweat during the day in mild weather, as well as getting damp during the night, so it's a good idea to open up or remove the fleece during the day to prevent rotting of plants. These plants won't need to be manually watered, unless you go through a dry spell. If using for seedlings, make a frame and cover the area with fleece.
Inside
Plant seeds in trays or pots inside, or grow potted plants inside. Plants inside are protected from almost all pests. South facing windows provide the best natural light, but this isn't always enough, so you can use grow lights to provide plants with enough light to be healthy. Plants will need to be manually watered.
Spring
Greenhouses and Polytunnels
Plant seeds in trays or pots within the structure. Or plant into their final growing position in beds in the greenhouse or the ground in a polytunnel. They should get warm during the day and will protect from frosts at night, allowing you to get a few weeks headstart. At this time of year, provided the structure isn't placed in the shade, plants should get plenty of light.
Coldframes
Plant seeds in trays or pots inside a coldframe, or place the coldframe over plants that are in the ground. This should protect plants from frosts and most pests and get warm during the day, giving plants a few weeks headstart, or protecting from one-off frosts.
Fleece
Not used as often for fully grown plants in spring. Wrap fleece around a frame and place over seedlings in their growing position or place pots inside to protect them from frosts.
Inside
Plant seeds in trays or pots, or grow plants in pots inside. Having them inside will protect them from frosts, and can get warm, depending on the temperature of your home.
Autumn and Winter
Greenhouses and Polytunnels
Plants can be moved into the structures, allowing you to extend the growing season and protect from frosts and pests. Or planted in a bed or ground within the structure if you plan ahead and want an extended growing season.
Coldframes
Place the coldframe over plants that are in the ground. They should protect from frosts and most pests and get warm during the day, extending the growing season and protecting from one-off frosts.
Fleece
Cover or wrap around plants to protect them from frosts and allow them to survive longer, taking off during warm or sunny days.
Inside
Move potted plants inside to protect them from frosts. Grow lights preferred here as it isn't as light at this time of the year.
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