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Eucalyptus foliage encrusted with hoar frost

Hardiness and Eucalyptus Trees in the UK

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Frequently Asked Question: Are Eucalyptus hardy for the UK?

Summary of this article:

What is Hardiness in plants – how does it all work? We shed some light on the subject

Of the 800+ species of Eucalyptus, several are sufficiently hardy to cope with an average British winter once established.

Some species of Eucalyptus are hardier than others.

Selecting the right species is key to the Eucalyptus surviving in your microclimate, but how you treat it thereafter, along with the tree attaining age and gaining collateral, will determine its overall, long-term success. The older a Eucalyptus, the hardier it becomes: we explain why.

Understanding Eucalyptus biology and what you can do to help your tree prepare itself for winter, along with alleviating some of the environmental stressors, can significantly help your tree survive extreme weather. We explain the triggers Eucalyptus require to move from ‘summer mode’ to ‘winter mode’ and how you can help.

There are many abiotic stresses to be endured by Eucalypts and plants in general. These are listed at the end of this article.

There is and always has been a risk to growing trees and shrubs of Mediterranean and Antipodean origin in the UK; this is nothing new. Erratic fluctuations in winter temperatures are here to stay; some winters will be warmer or colder than others and there will be the occasional loss. Seasoned gardeners have come to accept this. This is Gardening 101


FAQ: Are Eucalyptus trees hardy?

Short Answer: yes, some of them are – which is what everybody wants to hear, but the truthful answer is yes, but only if you tick all the boxes.

But first we need to answer a different question. What is hardiness?  What does it mean?

Frequently asked Questions: Is Eucalyptus hardy? Is Eucalyptus winter hardy? Is Eucalyptus a hardy plant? How hardy is eucalyptus? How hardy are eucalyptus trees? Is Eucalyptus frost hardy? Are Eucalyptus hardy in UK?

A Definition of Plant Hardiness

According to the RHS:

Hardiness Rating H4 is hardy within temperature range -5°C down to -10°C

Category: Hardy for an average UK winter

Definition: Hardy through most of the UK apart from inland valleys, at altitude and central/northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Some normally hardy plants may not survive long wet winters in heavy or poorly drained soil. Plants in pots are more vulnerable to harsh winters, particularly evergreens and many bulbs. (Many herbaceous and woody plants, winter brassicas, leeks).

Ref: https://www.rhs.org.uk/plants/trials-awards/award-of-garden-merit/rhs-hardiness-rating     2012_RHS-Hardiness-Rating.pdf

The RHS chart goes on to further discuss Hardiness Ratings H5 (-10°C down to -15°C), H6 (-15°C down to -20°C)   and H7 (lower than -20°C)

Broadleaved Kindling Bark
Eucalyptus dalrympleana encrusted with hoar frost

To be clear, hardiness does not mean that A. plant (enter name of your choice here), if given the title ‘Hardy’, will take any weather that the British Winter wishes to throw at it, without reference to temperatures and moisture levels, microclimate, stress levels, provenance and the age of the plant. 

The Eucalyptus dalrympleana on the left is coated in hoar frost during the winter of 2011. It has since grown into a very handsome specimen.

Hardiness in Eucalyptus

To say Eucalyptus are sufficiently hardy or not hardy for the UK is unqualified; it is meaningless when stated out of context

Nor is it simply a matter of choosing the right species, say Eucalyptus glaucescens, which can be hardy down to -18°C.  Simply plumping for E. glaucescens and hoping for the best or thinking you are sorted is not going to work either.

Plant Hardiness is not binary. It is not an on / off switch. It is not guaranteed        

Hardiness in plants is more of a sliding scale, like the volume on your television with a mixer of treble and bass.

A Eucalypt’s response to frost or freezing conditions i.e. whether it becomes frost-damaged or not, depends on multiple factors

  1. the Eucalyptus species type and provenance i.e. geographically from where the seed was harvested
  2. the environmental triggers it receives – the time of year and the weather in the preceding weeks
  3. the tree’s circumstances – including microclimate, nutritional status, health and vitality of the root system, how old it is and stress levels.

Plants that we know to be generally hardy in winter are not always hardy in summer, when they are actively growing.   If you had some roses in mid-July, full of open blooms with masses of fresh bronzy young leaves and there was an overnight temperature dip to -15°C  you would be surprised, but not be surprised if they all went black. However, roses are usually as tough as nails in a standard British winter, because they have been able to prepare themselves properly. 

Deep Pink Rose
Deep Pink Rose

Plants and Triggers

A short detour for a moment to explain what I mean by a trigger. Why do plants do what they do? More specifically for example, how do Oak trees know when to leaf out after winter? How does a rose know when to flower? How to apple trees know when to start producing fruit?

The answer to these seemingly simplistic questions is all down to environmental triggers and complicated biochemistry; the latter of which we won’t visit in this article! The precise nature of a trigger is specific to a plant genus and species, and depends on what the plant is required to do. For instance, an Oak tree (Quercus robur) leafs out after accumulating ‘so many’ growing degree days. i.e. how many degrees temperature it accumulates over a minimum temperature for however many days. So the trigger for when an Oak tree leafs out is initiated by temperature, whereas an Ash tree (Fraxinus excelsior) is more heavily influenced by day length hours. The Ash will leaf out when the day length in Spring gets to ‘so many hours’.

From pruning a dormant rose in the Spring, to when it begins to flower is twelve weeks of growing from when growth begins. I could not easily find the information on the precise trigger to initiate a rose to begin growing after winter; it may be temperature or daylength or a combination of both. However I do remember a lecture by a professional rose grower, a Mr Sanders, telling us that a rose grows precisely for 12 weeks after Spring pruning and then it flowers, no matter how hard or not it was pruned.

It’s long been known that flowering and subsequent fruit production in plants is a response to stress. If you want to bring a young apple orchard into cropping after a few years of establishment, you grass it down. The grass is an environmental stressor and trigger, which has first call on rainfall and nitrogenous fertilisers. With reduced access to these, the apple tree ‘thinks’ that food and water have become scarce local commodities and therefore it needs to reproduce, to ensure survival of its type. Similarly, you would not grass down a newly planted orchard as this could quite easily lead to tree establishment failure. Newly planted apple trees needs to establish good root systems in the absence of aggressive grass and weeds.

Bob in the middle of summer
Bob in Summer

 

Indulge me, if you will, with an analogy.

This is Bob. He likes to garden.

Here he is dressed for the British Summer Weather – shorts, sandals, cool drink in hand.

Hold that image – we’ll return to Bob in a minute.

In some plants, making their preparations for winter dormancy it is quite clear cut.  

For Oak trees (Quercus robur) the routine is well defined

  1. Receive the correct stimulus or environmental trigger
  2. form an abscission layer
  3. drop leaves
  4. have nice thick coat of bark
  5. go to sleep for several months – dormant – winter sorted. 

They can then take anything a normal British winter throws at them.

But as discussed above, oaks need the correct trigger – something to tell them when to flick the switch to start the process of going dormant, of becoming winter hardened. 

Quercus robur leaves
English Oak – Quercus robur in summer
Bob the Gardener
Bob in his Winter coat

Here is Bob again, now dressed for the Great British Winter

The trigger for Bob to shift from summer mode to winter mode is when his wife tells him its cold outside and the process is for him to put on his coat and boots.

For other plants, such as Scots Pine (Pinus sylvestris), their trigger is shortening day length and the process to become hardy is a period of cooling and chilling.  

Just 6 frosts of only -4°C, over 1 month in the autumn and that pine will be tolerant of an unbelievably cold -32°C.

Pinus sylvestris encrusted with hoar frost
Pinus sylvestris encrusted with hoar frost
Pine trees badly scorched by radiation frost
Pine trees badly scorched by radiation frost

Hardy until a period of warm weather dehardened their foliage and branches. These pine trees became damaged after a sharp radiation frost.

The reverse of this process is called dehardening and it can be very rapid; it can occur when the daytime temperature rises above +10°C. It took only a week at these warmer temperatures for the pine trees to become susceptible to frost damage.

Once the trees have become ‘dehardened’, it will take only one brief cold snap at sub-zero temperatures for the pine trees to be severely scorched and even killed, as in this photograph on the left.

If only hardiness were this simple with Eucalypts, but it’s a bit more complicated.  

Frequently asked questions about Eucalyptus and frost protection? Does Eucalyptus need frost protection? Can Eucalyptus survive outside in winter? Does Eucalyptus need protection from frost? Is Eucalyptus killed by frost? Can Eucalyptus survive outside in winter? Can Eucalyptus survive UK winter?

There are well over 800 species of Eucalyptus and with the exception of 2 or 3 species, all of them originate from specifically Australia, Tasmania and New Zealand.  Those that grow in the north and west of Australia are generally considered only suitable for warm climates or glass house culture in the UK. They are not of hardy provenance. 

At Hardy Eucalyptus, we focus on species that hail from south-eastern Australia, New Zealand and Tasmania; sourcing our seed, where possible from frosty valley bottoms, mountain ranges and high-altitude locations.  Provenance is king.

Eucalyptus dalrympleana encrusted with hoar frost
Eucalyptus dalrympleana encrusted with hoar frost

No Eucalyptus go dormant; not as we understand it in Britain. They don’t stop growing; nor metaphorically go to sleep by shutting down their root system and forming a terminal bud up top, like an oak tree.

At Grafton Nursery, we have found that, when the temp is above +5°C, Eucalypts grow. The warmer the temperature, the faster they grow. You should see what growth can be achieved within only 4 summer months by a Eucalyptus globulus bicostata at +20°C with unlimited food and water! Over 4 metres!

At best, when the temperatures fall consistently below +5°C, Eucalyptus go into ‘suspended animation’.  They are sitting there quietly, with their engine idling, just waiting for the traffic lights to turn green again.  The minute temperatures consistently rise to +5°C, they accelerate hard, growing in the fast lane of life!

However, even if you have selected the correct species and provenance for your microclimate, if the Eucalypt has not received the correct triggers and therefore been able to go through the hardening process and ripen its wood for winter properly, it will not cope with the cold.

Back to my little analogy.

Bruce is Bob’s friend. He’s newly arrived in the UK, on holiday from Oz. He’s been enjoying the unnaturally warm weather of November 2022 and thinks this is going to continue for the next few months. Unfortunately for Bruce, he’s not received the same trigger as Bob and is completely unprepared for what’s going to happen in December. He’s in for a nasty shock!

Choosing Eucalyptus that are hardy for your garden or plantation microclimate in a standard British Winter is important, but it is only part of the story – way less than 25% the story.  Well over 75% of the story is about how these trees are treated by the gardener or grower. 

Our research, at Grafton Nursery, shows that how Eucalypts are deployed and treated thereafter governs how successful and hardy they will be in their garden or plantation setting. Some of that will be in the lap of the gods, like weather, so more of that later. How well the Eucalyptus handles that weather can also be down to you, the Grower. 

Bruce from Australia is visiting Bog in England
Bruce is from Oz and is visiting Bob in England. Unfortunately it is winter and Bruce hasn’t realised because it was unusually warm in November. He’s going to get frostbite. 🥶

 

(25% plant genetics + 75% how the Grower treats their Eucalyptus) x C  = Eucalyptus Hardiness

Where C = weather conditions

So let’s get down to the nitty gritty:-

How to plan specifically for hardiness in Eucalyptus

We are assuming that you are growing your Eucalypts in the ground*.  You will need to take the following into consideration:

  1. Site assessment and choosing the correct species for your microclimate, ensuring that it is not growing in a frost pocket.
  2. Set up is so important. Plant the tree according to our instructions, which have been designed to give your Eucalypt the best possible establishment advantage.
  3. Deploy aftercare tactics to help your tree adapt ready for winter in the UK – see later.
  4. Accept that the British climate can be subject to erratic weather conditions; that on occasion we get thrown a curve ball and temperatures will fluctuate wildly. There is nothing any of us can do about this.

 

If growing in a container, your Eucalyptus will require winter protection in some form. Its root system will be above ground and therefore not afforded the protection offered by warmer soil in the ground.  Over-winter your tree into an un-heated greenhouse. Ensure there is good ventilation. Left outdoors, you will have to wrap the pot.  Further information can be found on our website under ‘Protecting your Eucalypt over Winter’.

*Raised planters are different in that the soil level is higher than the surrounding topography and will be subjected to sub-zero temperatures. Careful consideration needs to be given to growing in raised planters and there must be an acceptance of the degree of risk this scenario poses for young or unestablished trees.

1. Site assessment and choosing the correct Eucalyptus species

You know your garden better than anyone. You are familiar with where it floods in winter, what spots get the most sun, where the frost hangs in winter. You can assess soil type, pH, rain fall and wind tunnels. Is your garden coastal, high on a hill or in a frosty valley bottom?  All of this is valuable data when choosing your Euc.  We have written extensively on the subject and you can find these notes here: where to plant

Every Eucalyptus species listed on our website has extensive notes showing its biometric data and microclimate preferences, to help with the selection process. There are 4 tabs to each species, each one discussing different aspects of that species potential uses, likes and dislikes. All of our hardy species our grown from seed sourced from as hardier provenance as we can find.  Not necessarily so for the ubiquitous Eucalypt found lurking in the dark recesses of odd retail outlets. If the nursery producing these specimens has simply purchased generic Eucalyptus seed on unknown provenance, the hardiness of these trees will be questionable.

2. Set up is so important

There are gardeners and horticultural professionals, who have unwittingly missed or chosen to ignore the Eucalyptus Rules of Engagement.  You cannot do this and hope to get away with it all the time.  A beloved but malnutritic rose may pathetically limp along chucking out the odd occasional bloom. A pot-bound supermarket tomato plant will valiantly yield a tiny fruit or two, in spite of you never potting it on. Not so a Eucalyptus. It is like a race horse requiring careful handling and stabling. Treated well it will grow like the clappers 😊.  Break the rules and the Eucalyptus will fail to establish ☹.

If a young Eucalyptus fails to establish quickly in its first growing season – it has failed forever.   

There is no retro-fix.

You can find our full guidance notes on How to plant a Eucalyptus here

Our planting guidance notes are lengthy, but practical and designed to give your tree the best possible chance to succeed for the next 20, 30 or 50+ years!  So it’s worth making an effort. 

We know these guidance notes work.  100’s of people have followed them successfully with impressive results.

Don’t stress me out!  A Eucalyptus that is stressed by abiotic factors, i.e. non-disease related environmental stressors and biotic stressors such as fungal attack, is more susceptible to frost damage.

Three key takeaways from the planting instructions pertaining to hardiness.

  1. Deep Hole!  Digging a 500-600 mm deep hole and filling it with top soil and sharp sand is an absolute must to get deep rooting into warmer soil horizons. In lowland Britain, the soil temperature at around 600mm down is at a constant +6°C, even in Winter. Roots will be warmer and there will be water available, at this depth.
  2. Don’t grass me up!   No grass or weeds for at least 2 years for a 600-900mm diameter circle around the Eucalyptus – it’s non negotiable!
  3. Root Fungi!  Eucalypts come from an impoverished land and are genetically, heavily reliant on the ability of their root fungi to forage food for them. Don’t starve your new Euc – give it some Rootgrow mycorrhizal fungi at the time of planting. To help the fungus bond quickly and efficiently with the host, drench the roots on day 4 after planting with Alg-a-mic cold-pressed seaweed. You can read more about the biology of this magical product here: LINK

3. Aftercare tactics to help your tree prepare for winter

  1. Zero root-rock whilst establishing. See under stress factors for more details.
  2. Mulch with bark or wood chips to provide a root duvet, to conserve moisture, reduce weed population and to keep the mycorrhizal fungi and earthworms happy
  3. Water– lots of it whilst establishing – see our watering chart under our guidance notes ‘How to Plant’
  4. Feed a worked (pruned) Eucalyptus during first week of April, June, August and mid-September with a high potassium, low phosphorus, low nitrogen fertiliser. Add some Magnesium to assist with photosynthesis, chelated iron to improve metabolism and the occasional teaspoon of slow-release sulphur chips, which provides the building block of tree protein for fibre.
  5. Prune as close to March 18th – National Eucalyptus Day as possible and thereafter lightly prune the annual growth at the end of May, mid-June and July. In a wet summer, a little tip pruning in August is ok, but thereafter no pruning until next March.

The above may seem like a lot of faff, but every plant species has its own special rituals – just look at what goes into producing a tomato!

4. The British climate and accepting risk

How do you plan for hardiness?  In the first instance, you need to know about the biology of a Eucalyptus and the changes it makes to its structure throughout the year. Then it is a question of doing everything in your power to provide the best growing conditions possible as laid out in this article, to help your plant achieve its goals. Finally, it must be taken into consideration that there is and always has been a risk to growing trees and shrubs of Mediterranean and Antipodean origin in the UK; this is nothing new.

Erratic fluctuations in winter temperatures are the order of the day. Remember the winter of 1947 where dynamite was used to extract parsnips from the ground or even worse the winter of 1963, the coldest since 1740 (I don’t recall that one!), where temperatures plummeted to below -20°C? In more recent memory the winter of 2009/2010 was a cruel one.

However, it is not the cold alone that can inflict damage. It is periods of warm weather immediately before the icy blasts that in-fact does the damage. Short bursts of warm weather for only a week can create havoc for plants trying to prepare themselves for or remain in winter-mode. If they suffer damage as a result of the arctic conditions subsequently, it does not mean that the plant is not ‘hardy’ as per the RHS definition above. Is simply means that the plant was not given an opportunity to harden its tissues in time to face the onslaught of sub-zero temperatures.  The ‘trigger’ arrives too late for the plants to act and initiate the hardening ‘process’. 

Battle-hardened gardeners have learned to accept the unpredictable outcomes of erratic weather. It’s acceptance or they have to be prepared to be disappointed every so often. This is Gardening 101.

How are Eucalyptus Biology and Hardiness connected?

We’ll start at the end of the year; October 31st, Samhain or Summer’s End as in the Celtic calendar. Your Eucalyptus has had a good summer; been well fed, had plenty of water and isn’t at all stressed. From July to the end of September, it has been growing very rapidly, shooting skywards, throwing out spaghetti-like-stems without a care in the world for storing food or laying down fibre. It’s been having a blast😊

But now we are at the end of October. Day length is significantly shorter and very specifically the night time temperatures should be regularly hitting +5°C or less. This is the trigger for your Eucalypt to begin the process of preparing for winter hardiness. 

This is the process your Eucalyptus needs to undergo to become winter hardy:

  • Eucalyptus growth seriously slows down from a gallop to a trot. New growth almost ceases; the production of vulnerable sappy spaghetti is no longer order of the day.
  • The Euc uses the potassium and the sulphur you provided in August to turn minerals, water, sunlight energy and resulting carbohydrates into lignin – woody stuff.
  • It thickens its trunk, laying down fibre – this woody stuff – in the heart wood and sap wood; scaffolding for the tree. The root-system is going through a similar process.
  • You will notice that the branches have, as if overnight, become more vertical. They have suddenly lost their springiness and stiffened up. Both branches and trunk are less flexible.
  • Meanwhile, there is a frost-proofing process going on at a cellular level, deep inside the tree.  Cell walls and membranes are thickening.  The sap is also thickening from a fast-moving watery squash to a sluggish, more sugar-rich treacly soup; anti-freeze for trees. If ice crystals begin to form within the sap, cell walls will rupture; a disastrous process if it occurs, which can lead to instant death. However, it is believed that the tree has a cunning plan to prevent this from happening by producing a protein in the sap, to prevent the ice crystals from forming.

And so your Euc is ready to take on sub-zero temperatures down to -4°C for sure.  As the tree ages and acquires a thicker overcoat (bark) and lays down more fibre (a fatter trunk), its ability to tolerate lower temperatures will increase until it reaches its genetic limit. For some species, such as E. kitsoniana and E. saxatilis this may only be around -8°C.  E. dalrympleana is good to around -12 to -14°C once mature. E glaucescens provenance ‘Tinderry’ will ultimately tolerate down to -14 to -16°C in maturity whilst E glaucescens provenance ‘Guthega’ is good to -16 to -18°C once mature.  Snow Gums are even hardier.

What is the Number 1 hardiest species of Eucalyptus?

Audreys Garden Debi - Eucalyptus Debeuzevillei
The Jounama Snow Gum – the impossibly named Eucalyptus pauciflora subsp. debeuzevillei

The hardiest of all Eucalypts being the magnificent and beautiful Jounama Snow Gum, impossibly named as E. pauciflora subsp. debeuzevillei provenance ‘Mt Selwyn’.  We just call them ‘Debbies’

 

 

 

 

Provided the temperatures throughout November, December, January and February remain constantly on the cooler side, with night time temperatures hovering around +5 to -4°C both Eucalypts (and Hilary) will be happy. They will remain ‘inactive’, quietly respiring but not breaking into a trot.

What happens if Eucalyptus are not subjected to low temperatures during October and November?

This is exactly what happened in the winter of 2022/2023. Certainly, for Worcestershire and for many parts of lowland UK, notably Kent, temperatures during October and November were stupidly warm with a couple of weeks registering +10°C at night time, and rising to well over +12°C during the day. These unusually warm conditions are what initiated the damage to Eucalyptus across the UK, not the sub-zero temperatures later in the winter.  The trees didn’t receive the ‘trigger’ to begin the process of hardening for winter.

No trigger = No ripening of wood for Winter = damaged trees during sub-zero temperatures

Many Mediterranean and Antipodean plants that were not in protective custody, but planted out in gardens were still having a ball.  Our friend Bruce (above), along with Olive and Bay were, metaphorically speaking, still sunbathing in Hawaiian shirts and thongs, completely oblivious to the arctic blasts that were about to hit them at the beginning of December.

When the cold weather finally came in with a vengeance, it was so rapid that many plants did not have time to switch their biological constitution.  Worcester had -13°C. Parts of Kent received -17°C.  If plants survived this first short sharp shock of icy weather, they were given what seemed to be a reprieve around Christmas time with balmy conditions again. But this was a double whammy. 

Any plants which had managed to harden up during early December became confused during the late December temperature rise. They thought it was Spring and went right on to de-harden their tissues; only to be hit by sub-zero temperatures again for January and parts of February – alternating warm and cold over a very long time.  Icy conditions hung around for weeks at the ends of sloping gardens in England and frost lurked across large flat plateaux in Scotland. As the days dragged on, the cold needled its way into the stems and trunks of many trees and plants, icy draughts sweeping around the ankles of young and newly planted stock, and seeping into the ground.  Positively Dickensian ☹

To be clear:  For Eucalyptus trees to be sufficiently well prepared to take on the icy conditions of a cold British Winter, their tissues need to be hardened or ripened. They need to experience the correct chilling trigger and undergo a series of nights at no more than +5°C, most likely for 2 weeks or more.  Typically this process happens in October and November. If they do not experience this cold trigger to initiate the process, they will not be able to cope with a cold spell during the winter and will most likely suffer scorch and dieback, as happened in the Winter of 2022/2023.

Growing at High Altitudes can be beneficial

Eucalyptus plantation growers, living at higher altitudes as in Yorkshire, experienced typical cooler weather during November 2022 and also enjoyed good frost drainage. Their Eucalypts received the correct triggers, ripened properly for winter and consequently suffered no damage whatsoever in spite of the icy conditions that followed.

The weird thing we noticed across the UK, during the first half of 2023, is that the plants’ foliage took a long time to turn brown. The leaf tissue must have been damaged during the winter, but behaved like cut foliage. The leaves remained green for several months into the Spring, only to finally turn brown and crispy as the temperatures rose substantially in April and May.

At the nursery, we have many trees planted in the ground. Of the hardier species E. glaucescens ’Guthega’, E. parvula, E. neglecta, E. nicholii displayed no damage at all. It wasn’t until late Spring that an odd solitary spindly branch at the top of the trees turned brown. However, our very exposed E. nicholii did ultimately turn brown at the top and leafed out profusely all the way up the trunk.  By early Summer we could see that those species such as E. urnigera and E. stellulata, that were sitting in serious frost pockets, had suffered quite markedly and turned totally brown, only then to exuberantly sprout new shoots off their trunks and lignotubers.

Eucalyptus urnigera growing in a frost pocket at Grafton Nursery, re-sprouting off the trunk June 2023

Mature Eucalyptus Trees are doing well in the UK

On a positive note, there are plenty of large and very mature Eucalypts across the UK. Champion trees planted as far back as 1950’s that are still thriving today. There are magnificent E. dalrympleana  and E. johnstonii at the Hillier arboretum with girth of around 2.5m and 120+ft tall, planted circa 1955. Several champion trees can be found at Kilmun in Scotland, Markshall Estate and Marwood Hill Gardens, Barnstaple, to name but a few.

Whilst weird winters usually only happen about one year in 11, the last one being the winter of 2010, erratic weather may well become the norm in the wake of climate change. In the November of 2009, there was thankfully cold weather, hardening for winter did take place and many Eucalyptus survived largely unscathed.

What happens to your Eucalyptus at the start of Spring?

Once we arrive at March the process of de-hardening will begin. Temperatures are rising and daylength increases.

Eucalyptus are stepping on the gas and ready to grow.  They convert their frost-resistant, sugary thick soup into watery sap and roots fire up to push out early growth in April.  

It is at this moment that they are most vulnerable. They have dropped their winter defences and any sharp radiation frosts occurring between March and end of May could cause significant damage to the no-longer-hardened Eucalypt.

Having said that, we’ve had some vicious spring radiation frosts at Grafton, which killed cherry trees but not the Eucalyptus.

All being well, the Eucalyptus will start into growth. Any hard pruning, pollarding or coppicing needs to occur before growth substantially begins, otherwise the tree risks becoming exhausted if new growth is pruned off late in April.

Should the Spring weather turn cold again with night time temperatures dropping to around +5°C, the Eucalyptus can still harden and slide back into suspended animation. When the weather warms, the Eucalyptus de-hardens again.

De-hardening can happen at any stage during the winter if the UK experiences strange weather patterns with warmer than usual temperatures.  During late December 2022, the temperatures rose well above the national seasonal average to a balmy 11-12°C in some parts. Great for the central heating bills; not so good for Eucalypts.  They de-hardened within a matter of days. If this warm spell is followed quickly by a sharp drop to sub-zero temperatures and arctic winds, the Eucalypts incur cell damage, because they will not have had sufficient time to harden up their tissues again to fend off the cold conditions. This is what happened in January and March of 2023 and explains why so many young, newly planted or stressed Eucalyptus suffered damage that winter.

What impact does stress have on Eucalyptus?

The genetic limit of low temperature tolerance in Eucalypts can only be reached as the tree moves from the juvenile phase into maturity AND this needs to be combined with a tree that is free from stress.  We all occasionally suffer stressful conditions in our lives and as you know, your health can suffer, you are distracted from the tasks at hand and you take your eye off the ball; not to mention a complete sense of humour failure!  So it is with Eucalyptus. 

If your trees are endeavouring to harden tissues for winter and are dealing with one or more of the stress factors listed below, chances are they will not perform well through a cold winter. You will be able to ameliorate some of these stressors through taking appropriate action, but some of those listed below simply require the passage of time.

22 Major stressors that affect your Eucalypts ability to survive extreme weather conditions.

By the way – these conditions not only affect Eucalypts; these conditions can apply to all plants, but especially those hailing from the Mediterranean or Antipodean parts of the world.  In particular Hebe, Laurus nobilis (Bay), Olive trees, Abelia, Ceanothus, Cistus, Lavender, Rosemary, to name but a few. What a scary thought!  The list below is extensive but not necessarily all encompassing.

  1. Grass around the trunk for 600-900mm diameter. I don’t care if you are watering, the grass and weeds have first call on the water and very little will reach the tree roots of young or newly planted Eucalyptus. REMOVE THE GRASS!    Get the feeling that I am passionate about this particular point?
  2. Growing through plastic or inorganic mulches. There’s a lot wrong with plastic mulches – ask any earthworm! Eucalyptus absolutely hate it and will fail to thrive if grown through plastic or non-organic mulch. Don’t use it!
  3. Bark! Bark Bark! Not having a 150mm depth bark or wood chip mulch for the first 5 years after planting – mulch your trees deeply. They will love it!
  4. Being shallowly planted – not having access to the lower soil horizon at 500-600mm depth. For Eucalyptus to stand a chance of being fully hardy in a bad winter, their roots need to be down in the lower soil horizon of 500-600 mm depth. That’s 2 feet in old money! See our planting instructions.
  5. Having a compromised root system. This point is Eucalyptus 101.  Never plant a Eucalyptus on your land that has been grown in a smooth walled pot. It has been cursed with the wall of death. The Euc is a sad, depressed little thing that is living on borrowed time, and if you are one of those people bouncing up and down screaming that you’ve bought one and its fine – all I can say is…’just wait’! It may take 20 years, but just wait and see.  Having a deformed root-ball is not fun!  It scars you for life. The only safe and happy Eucalyptus is one that has been grown using an air-root pruning technique and for the larger specimens, I mean very specifically an Air-Pot container with lots of spikes and holes. For ‘All you needed to know about Air-Pots but were afraid to ask’ click here  LINK
  6. Root rock. No – this is not a dance. It’s a serious life-threatening condition that irreparably damages your root-plate! However, it is avoidable. Eucalyptus are leafy all year round. This means that in the autumn, when all their deciduous chums have dropped their leaves, your newly-planted Eucalyptus is going to be battered by the gales. Now, whilst a little trunk wobbling is a good thing to encourage the laying down of woody fibre, violent thrashing where the trunk is bending over at 90 degrees to the normal, is decidedly to be discouraged. It can fracture the point at which the trunk joins the roots. This can be avoided by cross-staking your baby tree with a stout stake and securing it to the tree with biodegradable soft hessian tree-tie or in an emergency – ladies tights. Resist driving a stake through the root-ball. I guarantee that way lies death!  Larger trees require our tree stake kit of 2 stakes, cross rail, hessian belt and screws. You can read about it here. Avoid rubberised tree belts – they saw Eucs in half. We’ve done the research so you don’t have to ☹
  7. Lack of water in a cold, freezing winter, when only a few years old. The ground water will be frozen, but this evergreen is still losing water through its leaves. As soon as the weather begins to defrost, water the babies please! Apply 3 gallons (15 litres) of tap water at midday.
  8. Poor nutrition through not being planted with Rootgrow Mycorrhizal Fungi. Eucs are dependent on their root fungi not only for providing the correct nutrition, but also to coat the roots in a protective, moisture attracting gel. The fungi also help mitigate the ingress of soil-borne pests and diseases.
  9. Poor nutrition and being worked.  By being worked, I mean the pruning of your Eucalyptus to control its size and shape or cropping the plant for foliage, fodder or firewood. Through pruning, you will be removing plant collateral – stored carbohydrates in the form of woody material. These carbohydrates are made by the plant by using sunlight, carbon dioxide, water and utilising vital minerals provided by the foraging root fungi. When you remove woody material through pruning to shape or taking a harvest, you weaken the tree and it has to then ‘set to’ and produce more woody material.  If there are no minerals to be found in the soil, the tree becomes exhausted and stressed. Stressed trees die when put under extreme conditions, such as sub-zero temperatures for a pro-longed period of time, like a week or more.
  10. Lack of the correct minerals. Eucalyptus consume quite a high quantity of potassium and sulphur in order to function efficiently.  They also enjoy a fair bit of magnesium and iron. Read about our Eucalyptus Smart Fertiliser here 
  11. Too much of a ‘good’ thing. Eucalypts also love nitrogen, but it is the crack cocaine of the Euc world. It will make them sick and fall over – literally. High nitrate feeds will lead to sappy growth and a compromised ability to ripen wood for winter. Avoid giving them nitrogenous fertilisers such as Growmore.
  12. Toxic food. Australia is a country devoid of any amount of phosphorus. Eucs have evolved to live without it, so avoid bonemeal and high phosphate fertilisers such as Miracle Gro.
  13. Dodgy home-brew. Brewing your own fertilisers may be fun, but risky. It’s not recommended as a good practice for Eucalyptus, as these feeds are of unknown biochemical constitution and tend to be high in nitrogen (see above).  I am also unsure about the pathogen levels of these products. Best avoided.
  14. Living in waterlogged conditions in winter may compromise hardiness, but that may be something you have to accept if you are using the tree to pump water and dry out the surrounding ground. Swamp gums tolerate these conditions better than other gums. View those Eucalypts which tolerate boggy ground by using the sort function in our shop here.
  15. Living in a windy location; coastal conditions, high altitude or wind tunnel. Eucalypts are evergreen, do not form a terminal bud or become dormant in winter. They are losing water through their foliage 365 days a year. Under windy conditions, especially in areas of low rainfall or being grown in a container, they run the risk of not being able to access sufficient water quickly enough to prevent leaf scorch. Spraying the foliage regularly between March and September, with Alg-a-mic seaweed can help mitigate this stressor. You can read about the important benefits of seaweed here. Ensure they receive sufficient water all year round. Deep root establishment will be essential, as will thick mulches.  Administer high potassium (K) fertiliser to assist with sturdy, resilient growth, at the right time of year.
  16. Living in a frost pocket or location in the garden with poor frost drainage; equally a flat plateau where the frost hangs for several weeks can threaten a stressed plantation. This is a tricky situation to manage. Ensure your trees are kept happy and free from all other stressors as much as possible.
  17. Being too young and not having a well developed and established root system to fully function under difficult circumstances. Your tree needs time to develop, but this point illustrates why fast establishment is essential. Follow our planting notes to achieve this.
  18. Being too young to produce a protective bark ‘overcoat’. Young Eucs have only periderm, which is a bit like an orange peel jacket – lightweight and easy to remove. Thick and tough, weather-resistant bark is produced from about years 5-7 onwards. Time and sturdy growth are your best friends.
  19. Being pruned at the wrong time of year; namely between September and end of February. Difficult if you are growing for cut foliage. Annual losses are something a flower farmer has to accept. Tree surgeons pollarding mature Eucalypts in the winter can lead to your Euc ultimately dying through the ingress of disease. Eucalyptus can only heal their pruning wounds, by literally gumming up the holes, when the sap is actively rising in Spring and Summer. Serious pruning, such as re-shaping, pollarding or coppicing should take place around National Eucalyptus Day UK – March 18th. Prune lightly by tip pruning of only the annual growth May, June, July.
  20. Being the wrong species for the geographical location in which it is planted. Eucalyptus are very site specific in their requirements. When choosing your species or variety, do match the Eucalyptus to your specific microclimate. Every species on our site has information pertaining to its requirements, be it soil type, pH, moisture levels, temperature tolerance etc.
  21. Acquiring a disease and being weakened by it. An athlete with a cold is unlikely to win a marathon! There are always varmints threatening to invade our borders but for now, fortunately, Eucalyptus do not catch much in the way of serious pests or diseases in the UK. One of the most serious diseases is Silver Leaf Fungus (Chondrostereum purpureum) and it is difficult to diagnose. This fungus is usually acquired either through leaving a large pruning wound without wound paint and also pruning at the wrong time of year (September through to February). Silver leaf fungus can also be contracted through pruning with dirty or contaminated equipment. Always use cleaned pruning equipment. Wash in hot soapy water and then sterilise with alcohol (vodka will do nicely!)
  22. Being overly mature +/- having a disease like Silver Leaf Fungus. Eucalyptus can and do die from old age. Generally speaking they can live for several hundred years, species depending. However, in the UK, Eucs in a domestic setting can die after a few decades simply from stress. Sad but true! ☹

In conclusion about Eucalyptus hardiness in the UK

Congratulations if you have got this far! Thank you for staying with me on this long and complicated journey. I hope it has been useful.

Well chosen Eucalyptus trees are ideal for the UK climate and can be deployed to fulfil a number of roles. Selecting the right species for the microclimate and the job at hand is key to success. In a domestic setting they are fabulous garden screening trees, small ornamental trees, hedge-screens and shrubby bushes.  Eucalypts make great container plants for a terrace. They provide cut foliage, fodder crops and firewood logs both commercially and domestically. In a wider setting they can be deployed as parkland and street trees.

Furthermore, contrary to certain views on the lack of biodiversity, we have found that bees and pollinating insects revel in the copious nectar and protein-providing pollen; small birds find their evergreen shelter life-saving on a cold winter’s night and pheasants certainly enjoy the crop cover they provide. Mitigation for air-pollution and use in sustainable drainage systems are topics for another article, but we know customers who have successfully used these trees to pump and drain water from their land under difficult conditions, both in gardens and farmland.

They have the potential to be extremely long lived for several centuries, as proved by the population of Champion trees living in Great Britain today.

The key take-aways from this article are to treat them like race horses

  • Select the right species for your project or garden, to do the job you want it to do.
  • Treat them well by not breaking the rules – their rules, not ours.
  • Keep them as free from stress as possible

And they will have the potential to gallop into growth and live to a ripe old age.

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