Supercooling Helps Plants Resist Frost

When a solution's temperature falls below freezing point without ice crystals forming, it is said to be supercooled. Some plants are able to do this, and so can withstand freezing temperatures (many fish can do this as well). We saw recently how plants can avoid ice damage by removing fluid from the inside to the outside of their cell walls, where it can freeze between the cells without causing damage.

This article not only illustrates supercooling, it also shows how the latent heat of fusion released by this liquid as it turns to ice raises the temperature to the extent of 80 calories per gram of ice formed (This also illustrates the usefulness of spraying crops with water on the evening before a frost).

It also mentions that plants induce this supercooling by producing such compounds as sugars, amino acids and other solutes; also that plants produce antifreeze proteins and other low molecular weight compounds, which 'provide freeze tolerance by inhibiting ice crystal growth and the nucleation of ice crystals' (crystal structures form around a nucleus - usually a solid one such as dust (though obviously not inside a cell)).

Not As Much Frost Damage As Expected?

Such a prolonged bout of freezing temperatures so early in the winter is uncharacteristic for the UK; I am sure I'm not the only one caught out. However, at first sight, thank goodness there doesn't seem to be as much damage as expected.

I was really surprised when dahlia tubers close to the surface and pulled up with dead stems were not frozen; they had only had a covering of loose, dead leaves. I find leaving the old vegetation gives much protection to the over-wintering tubers - it's a bit messy, and it also shelters pests, but it is good for both the tubers and the soil (dead leaves gradually break down and help structure; many are even pulled down by earthworms).

However, the winter is still young. As usual, I have probably spoken too soon.

Hydrangea Mops Protect Buds From Frost

I usually leave the foliage on old plants to give some sort of structure to the garden during the bleak winter months. Often these look untidy, but they come into their own when the frost arrives, and can on occasion look quite spectacular. However, there is more to it than this. These frozen hydrangea mops are protecting the buds on the lower parts of the stems from frost. They say this is because cover cuts down heat loss to the atmosphere by radiation, though I find this hard to appreciate fully.

I have found that frost seems to be far more damaging if it is allowed to settle on the plant - even glass gives considerable protection in this regard, though will do little to cut down heat loss through radiation (radiated heat passes straight through it, as it does through a tungsten light bulb).

As far as the hydrangeas are concerned, best not to remove the mops until late spring when the stems can be cut back to strong buds.

Watch Sun on Frozen Evergreens

We have recently seen how plants can survive hard frosts by reducing the water content in their cells. And if things get really bad, the roots can freeze as well. The plants react as they would in drought: they shut down and begin to wilt, as seen here with this frozen fatsia, which has dropped its leaves down by its side awaiting the thaw.

This is a particularly dangerous time for many evergreens, especially young ones with tender tissue and shallow roots. A drying wind could leave them unable to replace any water which might be subsequently lost, thus causing scorching. Strong morning sun is also a problem on such plants as camellias which might have similar difficulties in frosty weather.

Patchy Triphylla

Due to recent hard frosts, I have brought the triphylla fuchsia indoors (they don't like being too cold). Now that it is growing a little, you can probably see the problem: growth has become patchy, limited mostly to the perimeter of the plant. The withering leaves reported earlier were confined to the centre, and this tendency seems to have been stopped by watering, indicating that the core of the plant was too dry.

TWO BIRDS WITH ONE STONE
The problem has been how to water without bringing on botrytis through the subsequent cold wet conditions. So I have had to keep it dry to fight the botrytis. Growing indoors should 'kill two birds with one stone': It should allow watering, whilst controlling botrytis (due to being warmer). The only problem might be that it goes leggy. I have given it a full sun position to try to avoid this.

The Worst of All Frosts

In the UK, a bitter east wind (forecast for later in the week) provides the worst of all frosts - a penetrating one. It might take more than horticultural fleece to protect sensitive plants.

Fleece insulates plants by preventing radiant heat loss and to some extent restricting air movement around them. But I don't think it will be strong enough to protect against strong air movements; these will brush aside the still air (because it's porous) and allow freezing temperatures to reach the plant surface.

Much protection can therefore be obtained by blocking up cracks in doors and windows through which this wind can pass. I am also going to cover plants with newspaper overnight to help reduce air movement around them. Fortunately, daytime temperatures should remain above freezing.

How Frost Damages Plant Cells

An interesting report from the Food and Agriculture Organization of the United Nations states that it is the formation of ice within a plant which causes frost damage, not the sudden change in temperature. A turgid (full to capacity) cell's liquid contents expand as they freeze and can rupture the cell wall. This damage within individual cells is called 'intracellular'. It often occurs when temperatures drop below freezing point very quickly.

If temperatures fall more slowly, the liquid in the cell has more time to be forced outside through the cell wall. If it freezes here, the damage is called 'extracellular'. It has the effect of forcing the plant cells apart and dehydrating the cells.

DROP IN TEMPERATURE IMPORTANT, NOT LENGTH OF FREEZE
Within a reasonable amount of time (less than 24 hours), it is not so much the time endured at freezing temperatures which affects the degree of damage, but the amount of the drop - the lowest temperatures causing the most damage.

DROUGHT-RESISTANCE IMPORTANT
If ice forms outside the cell (extracellular), even though the cell wall remains undamaged the plant can be slowly killed. This is because the saturation vapour pressure is lower over ice than over liquid water, causing water to move out of the cell towards the ice, and thus death by dehydration.

FROST HARDENING
However, if exposed gradually, plants can harden themselves against frost damage in the following ways:

• removal of water from cell contents, making them more concentrated. This has two effects:
• it lowers the freezing point, and acts as a kind of antifreeze
• it makes the cell flaccid, thus pulling the contents away from the wall and thus making room for frost expansion (see diagram)

• decreasing ice-nucleation active (INA) bacteria concentrations during cold periods, thus allowing super-cooling to occur (cooling of contents below freezing point without the formation of ice crystals)

This hardening occurs naturally as a season become progressively colder. However, a warm spell can cause a reversal to occur much quicker than the hardening, especially if growth is resumed.

Fuchsias Caught by Frost?

Sunday night's frost was hard for October - looks as if I might have been caught out with the outdoor fuchsias.

I had been awaiting a slight frost to kill the foliage, along with any insect pests living on them, before storing. Too much frost can kill the stems, as well. Still, 'no use crying . . .', so to speak. 'What's done is done', and all that. I put them into the garage before it started raining.

Once the dead leaves have decided to drop, I will completely remove them all, spray the plants with a pesticide, then the plants and compost surface with a fungicide, and await the worst. It should be obvious by mid February whether they have survived or not.

At least the compost is not too wet. Even so, I shall have to watch out for signs of botrytis appearing on any dead material lying on the compost.

NO LIGHT NEEDED IN STORAGE
Fuchsias can be stored in any frost-free place such as a garage or attic. They can even be buried in the ground. Light is not necessary until new growth commences.