Containers get too hot and dry in summer and conversely they get colder than a surrounding garden in winter because a greater area is exposed to the elements.
Plants in containers therefore need special attention in cold winters and may well have to be protected, however warm the microclimate of the individual patio, windowsill or roof garden. Roof gardens are particularly affected for being open to the elements they are more likely to be buffeted by wind and storms.
Hardy Winter Vegetables
The container kitchen gardener has less to worry about than the container gardener growing a variety of flowering plants and shrubs. Nearly all vegetables are annuals or grown as annuals and many will have been harvested and removed from the containers before the onset of winter. Any vegetables in garden that remain in containers over winter, such as broccoli, are hardy enough to survive cold and frost.
Garden herbs are a different matter. Perennial herbs, such as rosemary, thyme and marjoram, all need winter protection. If they cannot be brought inside the house and kept in a cool, frost-free environment then they can be wrapped in protective fleece or hessian during the hardest months of the year. Garden fleece is excellent for it allows light and moisture to penetrae, although it is flimsy and liable to tear if subjected to strong winds and sharp corners. Half-hardy or delicate flowering shrubs also may need protection, most roses are fully hardy but a few, such as the Rosa banksiae, are not and should be covered in severe weather.
Looking After Containers
Plants are not the only things that require protection. Terracotta pots may well crumble and crack if they are subjected to extremes of frost and rain. Bind them under the rim with a wire, alternatively wrap them in sacking to guard against winter frosts. An alternative protection for pots and garden plants in very hard weather is plastic bubble wrap. Beware: it does not allow the plants to breathe and can contribute to disease.