Severely cold temperatures, below which the plant can normally tolerate (is used to experiencing in its natural habitat), frosts and even cold and especially strong winds can all affect, damage and even kill plants.
The main damage is caused when water freezes. As it does so, it expands, rupturing the plant cells and causing them to die.
There are two main types of frost. Air frost occurs when the temperature of the air falls below freezing point (0C/32F). Ground frost and grass frost occur when the temperature of the ground/grass falls below freezing point. Ground frosts are more severe than air frosts.
New and young plants are usually more susceptible to cold and frost damage than fully established ones, since they haven’t built up their natural cold resistance and their roots haven’t become established in the new soil or compost. Young plants may also have been grown in a heated greenhouse and the sudden change in temperature causes significant damage.
While it’s obvious that so-called tender plants – including most half-hardy annuals and bedding plants and some “exotic” vegetables, such as tomatoes, cucumbers, peppers, aubergines and squashes – will be damaged or killed by frosts or temperatures below or just above freezing point (0C/32F), many so-called “hardy” plants can similarly be damaged. This is particularly true of the new, young growth in spring, since it has not yet developed the toughness of older, more established growth.
Plants can also be damaged when the soil or compost freezes. At the very least, this prevents the plants’ roots from absorbing water, so they take on the symptoms of drought [LINK]. At the worst, the roots are actually killed, meaning the whole plant starts to wilt and may eventually die.
If the soil or compost only freezes for short periods – such as overnight, but thaw out during the day or for only a couple of days on end – the damage will probably be minimal or slight. However, if the freezing goes on continually for several days, then severe root death occurs and the plants will suffer much more damage and are often killed. Sometimes, the damage isn’t noticed until mid- to late spring, or even later, when the plant comes into growth. Only then do they need large amounts of water which, because the roots are dead, they don’t receive. They then wilt and die for no seemingly apparent reason, the damage having been done some time ago.
Plants growing in containers are more susceptible to cold weather damage. As the roots are above the ground, they don’t have the benefit of the surrounding soil for insulation, which plants growing in the ground have.
Cold winds, especially strong, cold winds – can also cause similar damage – especially when combined with cold temperatures and frost. The wind acts like a vacuum cleaner – literally sucking water out of the leaves, and more quickly than the roots can replenish it, causing them to be damaged and/or die. This is worse when the soil or compost is frozen.
Plants regarded as hardy, protect themselves in a number of different ways from cold weather.
Deciduous plants drop their leaves.
Some accumulate materials, such as sugars and amino acids, in their cells that act as “anti-freeze” lowering the freezing point of the cell. The shortening day length in autumn induces this to happen.
The cell contents of some plants “super freeze”, meaning the cell contents remain liquid even though they’re below freezing point. This is a far more effective mechanism. Plants have to experience several days of cold weather before the freeze for this to happen, which explains why even hardy plants can be damaged by a sudden autumn frost.