From: DK Books - How to Grow
Some plants and containers need a little help to get them through cold winters but they can be left outside if you provide some protection when temperatures dip below freezing. Tender plants must be brought inside in winter because they die when exposed to freezing temperatures, but those that can survive a few degrees of frost should survive outside in all but the coldest regions.
Plants to Protect
Many slightly tender plants survive low temperatures but not cold, wet soils, so ensure yours drains freely before planting. Other plants are not killed by frost, but their flowers may be damaged. Examples include peach trees, magnolias, and camellias, which suffer when frosted blooms thaw too quickly in warm morning sun. Young leaves and buds of hardy plants can also be sensitive to frost, so don’t feed in late summer because it promotes vulnerable new growth. Also, allow herbaceous plants to die down naturally so that the leaves fall over the plant, forming a protective blanket, and apply a thick mulch over those that may suffer in low temperatures, like Alstroemeria or diascias.
Use chicken wire to secure a straw blanket over frost-sensitive plants that prefer dry soil conditions (image 1).
The flowers of star magnolias are damaged by frost; protect them with garden fleece (image 2).
Top Tip: Overwinter Bananas and Tree Ferns
Popular for tropical gardens, Musa basjoo is one of the hardiest bananas and tolerates winters outside if protected from cold, wet conditions. First, cut down the stems and remove the leaves. Attach chicken wire to bamboo canes set around the plant to form a cage, and pack it with straw. Treat tree ferns in the same way: make a cage around the plant, fold the fronds over the top of the stem, and pack straw around it. Both bananas and tree ferns suffer in wet winters, so top the wire cage with a waterproof covering, such as clear plastic sheeting. In very cold regions you can also add a fleece wrapper. Remove the protection in late spring or when you see new growth.
Cloches for Crops
Some vegetables that overwinter in the soil benefit from a protective cloche or a layer of straw. Likewise, crops that are sown early in spring may grow more quickly if kept snug when frosts strike. A wide variety of cloches is available to buy, or make one yourself from recycled materials.
Choosing a Cover
Winter root crops, such as parsnips, carrots, and leeks, are difficult to lift when the soil is frozen, so cover them with a layer of insulating straw in autumn. Cold frames are ideal for spring-sown frost-hardy seedlings in trays or pots, which will be transplanted outside later in the year, while a cloche is best for crops that are sown in situ in early spring, such as lettuce, arugula, and Oriental greens, or for overwintered vegetables like broad beans.
Cloches can be bought already constructed or as kits, or if you want frost protection for just a few weeks each year, a home-made type made from a few sheets of clear plastic may suffice. Alternatively, make a more permanent tunnel from wire hoops covered with clear plastic; leave one end open for ventilation.
Cloches are an investment and should last for many years; choose from pricey decorative glass types (image 1) to cheaper plastic models.
Cloches can be easy to make with two sheets of clear plastic pinned together with pegs (image 2).
Wrap Up Your Pots
Container plants can suffer in winter on two fronts: roots are more vulnerable in pots because they afford less insulation than the soil in the ground, and the pots themselves may crack or break during icy periods.
Some containers are more vulnerable to frost damage than others. Stone, metal, and plastic pots will sail through winters unscathed, while terracotta often cracks in frosty conditions. Terracotta suffers because it is porous and when moisture from the soil and rain leaches into it and then expands as it turns to ice, the pot cracks. So, unless you pay a premium for containers that have been fired to high temperatures to reduce their porosity, you will need to take steps to make sure yours stay intact. Either remove plants and soil and store pots inside, or, if they are housing a prized plant, wrap them up with hessian or bubble wrap. Cover the soil, too, so that it does not become saturated. Another tip is to line the pot with bubble wrap before you plant it up, thereby forming a barrier between the soil and the terracotta.
Slightly tender potted plants are best wrapped in horticultural fleece in the winter. Also tie together the leaves of strappy plants, such as cordylines, to protect their crowns from snow and ice.
Line terracotta pots with bubble wrap (image 1) to keep them from absorbing moisture and cracking when the water turns to ice in cold weather.
Wrap tender plants and vulnerable pots with hessian or bubble wrap (image 2) to keep them warm.