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Plants are more sensitive to the frost when grown in pots than in the ground. Do not take any risk and shelter them from frost!
The roots, their real Achilles heel
In all plants, the roots are always more fragile facing frost than the branches. So even in plants which seem resistant enough to withstand winter cold, wounds can appear at roots level during winter. Indeed, the roots do not have any protections like bark or buds have. However, you don't need to protect all the plants. Nevertheless, a few degrees often make a difference.
Pots which stay outdoors
Totally hardy plants like Pine trees or Maple trees do not need any particular care as they withstand frost, including at roots level. It is mostly plants which are not totally hardy in most areas, like a Windmill Palm (Trachycarpus) or an Olive tree grown in pots which you must protect in priority. The best solution is to protect them while still keeping them outdoors by wrapping them in insulating material, like a wintering fleece or plastic bubble-wrap. This latter is however not recommended as it let air through and this encourages the appearance of mould under the protection during winter. However, it can be sufficient to protect the roots, wrapping the pot and letting the stems in open air.
Pots to bring in
All plants which do not tolerate the minimal temperatures usually seen in your area must be brought in a shelter, where they will be hot enough not to freeze. As winter draws near, bring the pots near walls or the entrance so as to be ready to bring them in, as soon as cold is forecasted. On terraces and balconies, set up a little greenhouse which can be dismantled later on to house your potted plants. Beware though, these little greenhouses are useful to protect plants from cold wind, snow and slight frost but do not protect your plants if temperatures fall below -3°C.
Keep them dry
In all cases, do not forget to raise the pots. You must prevent humidity from stagnating at the bottom of pots and between the pots and the soil. Roots would find themselves soaking continually and this would be as harmful as frost! Place wedges underneath the pots like small pieces of wood, pebbles, etc.
Reduce watering
In winter, the plants' metabolism being reduced, their need in water is reduced too. A plant withstands cold better if it is slightly thirsty whereas it is more cold-sensitive than normal if it has been watered too much. Space out watering and let the soil dry on its surface before watering again. As for pots which are left outdoors, pay attention to rainwater : it rains a lot in winter and pots which have water-reserves risk to find themselves soaked : Set them back so that they do not receive too much rainwater. - Photos (4)
Protecting your potted plants in winter
For this Palm tree, the part containing the roots must be protected from frost if it cannot be brought into shelter during winter
Author: B&G Media
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Protecting your potted plants in winter
A Pine tree: Not sensitive to frost growing in pot, even without protection.
Author: B&G Media
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Protecting your potted plants in winter
Wrap the entire pot in an insulating material.
Author: B&G Media
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Protecting your potted plants in winter
Raise the pot at least 2 cm using small blocks.
Author: B&G Media
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