An A To Z Of Frost And How It Affects Our Plants

Frost facts and figures . . . together with tips on preventing frost damage 

Frost is often seen to be the gardener’s enemy, causing damage to our plants and potentially destroying weeks of work. Our guide should help you to understand what frost is, what causes it, what types of frost exist, how it affects our plants and what we can do to protect them.   

Air Frost
Air frost occurs when the temperature of the air falls below 0 degrees Celsius / 32 degrees Fahrenheit and is usually measured at a height if 1.2 metres / 3ft 4in above the ground surface. 

Waterproof Max Min Thermometer A robust digital thermometer which is suitable for use outdoors, it has a triple display simultaneously showing maximum, minimum and current temperature. Switchable between degrees Celsius and Fahrenheit it has a temperature range of -20 degrees C to plus 50 degrees C.  Click here to see full details.

Accurate to within plus / minus one degree Celsius this waterproof thermometer is ideal for checking the temperature in your garden.  

Black Frost
This is a very damaging frost as it is severe enough to blacken vegetation without there being a visible frost.

When we think about frost we expect to see a white covering but with black frost although the temperature drops below 0 degrees Celsius frost does not form because the air is so dry – without the moisture the white frost isn’t formed.

If you’re thinking that the frost not forming is a good thing, then think again. Black frost can cause more damage to plants than white frost as it directly attacks the internal structures of the plants, with ice crystals forming within them. When the ice forms a pointed shape inside the plants it tears the internal tissues of the plant making the membranes dry out and the plants die.

The reason it’s called ‘black’ frost is that the damaged parts of the plant turn black. However, on the positive side, this damage only occurs on evergreen plants as it only damages plants where their vegetative state is active.

Top Tip
Although it's easy enough to move shrubs in pots into more sheltered areas to protect them from frost you may well have evergreens in your garden border than you can't move. So to help protect them you can apply a thick layer of mulch around them to prevent the ground from freezing. And you can wrap the plants in a couple of layers of garden fleece to insulate them from the frost. 

Cloches For Instant Frost Protection 

Cloches are helpful in protecting your plants from frost damage due to the following effects:

• Sunlight warms up the air inside the cloche – so that’s why you would use polythene or glass cloches or fleece cloches which let the light through.
• The cloche traps the warm air inside, preventing it from escaping – so the air continues to heat up and the soil covered by the cloche is heated as a result.

In this way a cloche acts as a ‘mini greenhouse’ increasing and trapping any heat which the sunlight provides – but just like a greenhouse the plants will now rely on you to provide the water and fresh air they need to remain healthy. 

Did You Know ?
Using a cloche can increase soil temperatures by up to 10 degrees Celsius. Depending on the cloche your choose this could allow you to protect a single shrub from frost damage or a complete row of plants. 

Tunnel cloches can be used to protect autumn sown crops from frost damage and other weather damage by providing a protective layer around them. They will prevent rows of young plants from being damaged caused from heavy rain or hail.

Dew Point And Frost

Frost will form when an outside surface – this could be a car, a greenhouse or your plants – cools past the dew point. The dew point is the temperature below which water vapour in the air turns into a liquid – if the temperature is cold enough the liquid freezes to form ice or frost.

Effects Of Frost On Plants 
Plant cells can be damaged or even destroyed by frost but do you know why?

Basically, when the temperatures fall sufficiently for frost, the water inside in the plant cells can freeze. When it freezes the water expands and with nowhere to go it breaks the cells walls. As these cells are responsible for carrying the plant’s nutrient juices (sap), this process stops and the plant dies. 

Fleece As Frost Protection
Often called fleece, garden fleece or horticultural fleece, this lightweight, spunbonded material is a versatile product which can be used to protect your plants from frost, wind, hail and pests. The advantages of using fleece as a plant protection (compared to using clear polythene for example) is that it is porous - allowing air and water through - helping to create a healthy environment in which your plants can grow.

Spun-bonded garden fleece will be UV stabilised to withstand sunlight, tear-proof, rot-proof and washable, making it a durable material which can be used all year round.

Standard fleece with a weight of about 18gsm will protect plants to just below freezing, whilst more heavyweight fleeces of 30gsm will protect plants when temperatures fall to approx. minus 3 degrees Celsius. You could use more layers of the fleece to increase the protection it provides, so if you used two layers of the 18gsm fleece you would then have a weight of 36gsm and the corresponding frost protection.  

Cosygrow Garden Fleece Our Cosygrow fleece is 2 metres wide and can be purchased as many metres long as required. This makes it perfect for covering long rows of plants at your allotment. It is 18gsm so lightweight enough to be put directly over plants for a couple of weeks without causing damage.  Click here to see full details.

When leaving fleece over plants for more than two weeks it is normally recommended that you create a frame to hold it in place over your plants, such as covering our Cloche Hoops with fleece. 

Ground Frost
A ground frost occurs when the temperature of the ground falls below 0 degrees Celsius / 32 degrees Fahrenheit and is measured at a height of 5cm / 2inch above the ground surface. Ground frost can occur even if the air temperature (measured at 1.2m / 3ft 4in above the ground) is slightly higher than 0 degrees Celsius if the sky is clear, because any heat that the ground has absorbed during the day radiates to the sky. 

Hoar Frost
The ice crystals of this frost look like aged white hair or beard and it is for this reason that it is referred to as ‘hoar frost’ (the dictionary definition of ‘hoar’ is greyish white, grey or grey-haired with age).

This type of feathery hoar frost is formed because of specific weather conditions – different to the conditions which cause ground frost. Ground frost is formed when the air is cold and still, with water in the air condensing on solid surfaces and when the surface temperatures fall to 0 degrees Celsius or below it then forms ice crystals.  

However, hoar frost is formed when it hits a surface which is already below 0 degrees Celsius and ice crystals form immediately. As more water vapour is frozen the ice crystals continue to grow – you often see pictures of hoar frost shown on tree branches as on cold, still nights, tree branches are unlikely to rise above 0 degrees for several hours.

Insulate Your Greenhouse For Frost Protection
The benefits of having a greenhouse in the colder months is that it will protect your plants from the weather conditions – wind, rain and snow – and will keep them a few degrees warmer than they would be if left outdoors – you’d expect your greenhouse temperature to be 2-3 degrees Celsius above the outside temperature.

You can increase this difference further by insulating your greenhouse, lifting it 5-6 degrees Celsius above the outside temperature, which if often sufficient to keep your plants frost free.

There are lots of options when it comes to greenhouse insulation, so it’s a case of looking at the pros and cons of each type before deciding. Here are a few examples:

Use Cardboard – on the plus side this is a cheap option as you can recycle old boxes – but on the negative side it will stop light getting into your greenhouse. If you’re going to use cardboard just use it for the first couple of feet up from the ground, so light can still enter through the glass above.

Use Fleece - this soft, flexible material is great for throwing over benches or shelves packed full of plants as it is lightweight enough not to damage them. However, if you use it to line a greenhouse it won’t be as effective as other materials just because it is thinner and will obscure some of the light.

Use Insulation Foil – usually used in loft spaces, timber floors and wall linings to reduce heat loss, this type of insulation is effective but would block all the light. If you wanted to use this in a greenhouse only use it on the north facing side of your greenhouse, so that light can enter through the rest of the greenhouse, with the reflective surface helping to reflect both heat and heat and light back into the greenhouse.

Use Bubble Wrap – the most popular choice, ideally choose purpose made horticultural bubble wrap, such as our heatsheets – which is UV stabilised so that it lasts longer than bubble wrap used in packaging. You can line all your greenhouse with this type of insulation as it will still allow some light in whilst effectively double glazing your greenhouse.  

Fixing bubble wrap in your greenhouse is easy to do in aluminium or wooden greenhouses.

To fix this type of bubble insulation into a wooden greenhouse you can use drawing pins or a staple gun.

Jack Frost 

"Oh, little Jack Frost get lost, get lost
Little Jack Frost get lost
You know you don’t do a thing
But put a bite on my toes
Freeze up the ground
And take the bloom from the rose ..."

Although we now know all the scientific reasons for why we get frost, in the past this wasn't the case. Just like now, the weather was still a main topic as it affected peoples lives and without the scientific know how people made up their own explanations and this is where the myth of Jack Frost originates. 

Jokul Frosti The original ‘Jack Frost’ Jokul was an ancient, wicked Norse giant who served as the personification of ice and snow. Giants were seen as violent, fearsome characters and as the winter weather would have been more severe in the Norse regions then choosing a giant to represent this dangerous weather is understandable.

However, in areas of Europe where the weather isn’t as extreme the giant morphed into Jack Frost, more of a sprite who would scamper around painting elaborate pictures with ice . . .

Killing Frost 
This term is used if a frost period has been severe enough to end the growing season or delay its beginning. For a killing frost to occur the temperature will need to have fallen to minus 2 degrees Celsius. It is at this temperature that plants can freeze – in fact you will find a killing frost also referred to as a killing freeze – with all but the most cold-tolerant plants dying.  

Landforms
Did you know that ice can be strong enough to break rock? This physical strength of ice, caused by cold temperatures, is seen by the periglacial 'landforms' which are sometimes seen on the landscape.

Luckily for us these landforms only occur in the cold or periglacial (near-glacial), areas adjacent to or beyond the limit of glaciers. In these areas intense freeze-thaw activity takes places which can produce periglacial landforms.

These landforms are created due to the unique behaviour of water when it changes from a liquid to a solid when it freezes. When water freezes its volume increases by approx. 9% and this increase is even greater when there is ice growth, when air becomes trapped. So, the water in its liquid form, which could fit into a small space, now needs a greater space to fit – if the water is caught in a crack or pore space in rock this increase in volume can exert sufficient pressure to break rock.

Mordvins 

Did you know that some cultures worship frost . . . one being the Mordvins. Being one of the larger indigenous people of Russia, there are about one million Mordvins who live in or around the middle Volga provinces of Russia.

When you read about the Mordvins you will see mention of Ivan the Terrible using them to build bridges and clear forests as the main occupation of these hard workers was agriculture.

Although they are now mainly Christians some authorities still say that the less Russified Mordvins continue to worship trees, water, the sun, the moon, thunder, and frost. 

Needle Ice
Needle ice, also referred to as ‘frost pillars’ or ‘frost columns’ is an interesting type of frost as it is formed by groundwater.

It forms when the soil temperature is above 0 degrees Celsius and the surface temperature of the air is below 0 degrees Celsius. Liquid water underground rises to the surface of the soil by capillary action and when it comes into contact with the cold air it freezes and contributes to a growing, needle-like column of ice. 

This is clearly illustrated on the photograph as you can see on the top of the ice needles small pieces of the red clay soil which has been lifted as the needles have grown. Ice needles will typically grow to be a few centimetres tall.

Needle ice can kill seedlings when it forms, as when the ground hardens in the cold, the roots and stem of the seedlings are gripped by the soil. When the needles form, they then get pushed up and out of the ground. Even when the needles melt, due to the disturbance the seedlings will usually die. 

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Plants which can survive frost 

When it comes to plants which can survive frost and flourish in the colder months of the year, adding colour and structure into your garden, then you need to look at those plants which are termed as ‘hardy’. Plants in this category can resist adverse weather conditions including frost and cold winds so should survive most UK winters.

Plants falling into the hardy category can then be defined even further by dividing them into hardy annuals, hardy perennials, and hardy biennials:

Hardy annuals – tolerating light frost these fast-growing annuals can be sown, will grow and flower within a 12 month period and will the need to be replaced. This makes them ideal for pots and containers, to fill a gap in a border etc.

Hardy perennials – these plants are great for budget conscious gardeners as they not only will tolerate frost they will also come back year after year, generally you’ll get at least 2-3 years of growth or even more. A good example of a hardy perennial if the hellebore. 

These hardy plants usually have evergreen, leathery leaves either solid green or some varieties have marbled or silvery leaves. Available in a wide range, their attractive foliage and flowers are great additional colour in your garden during frosty or snowy weather.

Hardy biennials – flowering in their second year, these biennials will survive an average winter growing a strong root and leaf system during the first 12 months. In it’s second year these plants will flower, set seed and then die off. Examples of these biennials include primulas, foxgloves, canterbury bells etc.

Quotes and sayings about frost 
It does seem that we have always been preoccupied with the weather! As a result there are lots of quotes you’ll hear about the weather, some about frost including:

‘Clear Moon, Frost Soon’ this quote is true as when the night sky is clear allowing you to see the moon, the ground cools more rapidly as there is no ground cover to keep the heat in. So, if it’s clear enough to see the moon at night and the temperature falls enough then you can expect there to be frost.

Other quotes include . . .

• ‘For every fog in March there’s a frost in May.’
• ‘For every frost in October, a snow in the Winter.’
• ‘Full Moon in October without frost, no frost till full Moon in November.’
• ‘Hoar frost on May 1st indicates a good harvest.’
• ‘If this day (August 24th) be misty, the morning beginning with a hoar-frost, the cold weather will soon come, and a hard winter.’

Rime

Usually occurring in very cold, wet climates rime is a frost that forms quickly on exposed surfaces.

It’s formed from supercooled fog or cloud, hence the term ‘freezing fog’ and appears on the sides of objects, such as tree branches, which are facing the wind. For rime to form temperatures will normally have fallen to between -2 to -8 degrees Celsius and there will be high winds.

If you’re working outside in your garden in freezing weather – even it it’s just to tidy or to harvest winter crops – then remember to keep your hands covered from the cold. 

Symptoms of frost damage

Plants can be damaged by frost in early spring when tender, new growth could be damaged. Or in autumn / winter when evergreen shrubs can be affected by hard frost.

The tell-tale signs of frost damage is that the leaves of the plants, which should be a nice, healthy green colour, will change to have a scorched appearance, turning brown or black over time indicating that part of the plant has died. With many plants frost damage can be seen straight away, but with the hardier, particularly woody shrubs or trees, it may take several months for you damage to become visible.

Tissue Damage 

Plant tissues will be damaged by frost if ice forms within the cells of the plant. The most frost sensitive parts of a plant is where there is new growth, however the general health of the plant can also affect it’s ability to withstand cold temperatures.

With new growth being vulnerable to frost damage, frosts in early spring can cause immediate damage to young plants, including vegetable and salad crops. Depending on how low the temperature is will determine whether there is minor damage, from which your plants could recover, through to critical damage where your crops could be devastated and you’ll need to start again. 

Using Smoke To Protect Vines From Freezing

OK, not many of us are lucky enough to have a vineyard, but it’s still interesting to see what methods grape growers use to help prevent frost damage – as a spring frost can cause devastation, causing young shoots to shrivel and brown, potentially destroying the potential crop for the year.

When the grapevines are dormant in the winter months they can tolerate the colder temperatures, so it’s really only in the spring when new leaves start to grow, and embryonic flower clusters emerge that frost can get a chance to damage these younger, more vulnerable parts of the plant.

From picking the correct site for the vineyard, to using the correct trellising method will all help to protect vines from frost damage. However, to help prevent radiation frost which can occur on clear, still, spring nights when temperatures can drop, many growers will light large paraffin candles or torches placed every few metres throughout the vineyard. These are sufficient to increase the temperatures by 2 to 2 degrees Celsius, often sufficient to prevent frost from forming. Other methods include creating straw fires amongst the vines – the aim of these fires is that the smoke created acts like cloud cover, preventing the warmth in the ground from rising and the freezing air from sinking down.   

Verglas

A thin coating of ice that forms on a solid surface, verglas usually appears overnight when temperatures drop and the moisture from either dew, mist, or rain freezes. Or it can form when snow or ice melts and refreezes.

You tend to hear about this type of ice when accidents happen in severe weather and people are affected. A good example is the winter of 2009-10, when on the 12th January a warm front moved across the country causing freezing rain and heavy glaze. This was particularly seen in South and West Yorkshire and resulted in many accidents due to the glaze affecting road surfaces and A & E departments in Sheffield, Rotherham and Barnsley were inundated with people suffering from broken bones, fractures, and sprains.  

What Causes Frost To Occur? 
Frost occurs with the combination of these three vital components:

Chilly Air – the air needs to be cold as no matter how little cloud or how light the winds are you won’t get a frost if the air is inherently warm.

Clear Skies – this allows the heat from the ground to escape into space. This happens most often during the autumn and winter months when the nights are longer than the days, resulting in more hours of the day when heat can escape from the ground and the temperature fall.

Light Winds – these are crucial to prevent mixing the air close to the ground with the air just above. As the air cools from the ground upwards, any mixing from above will bring the warmer air towards the ground, preventing the temperature from falling further.

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Our A to Z posts are created from our personal knowledge, information gathered by speaking to other gardeners or manufacturers in the gardening industry, by reading gardening magazines and devouring information from books and the internet. We aim to be as accurate as we can, so if you find a mistake, please remember, we’re only human.