Guide to Overwintering Outdoor Bonsai



Introduction to Overwintering Bonsai in the UK
This article is designed to give you the basics of overwintering cold hardy bonsai trees in the northern hemisphere. My own experience is of the U.K climate covering zones 7-9 and temperate. However, the basic principles of overwintering remain the same whether you live in a cooler or warmer climate. Therefore it should be possible to glean the basics from what follows. The term ‘outdoor’ bonsai refers to those species cultivated as bonsai than can be considered frost hardy. The term ‘hardy’ is an all encompasing term that refers to a plants ability to withstand extreme conditions including heat, cold and drought. On the face of it any plant that is known to be frost hardy in normal cultivation should overwinter without problems when kept as bonsai. However as with most things in life the situation is not so simple as it appears at first glance. It’s quite possible that your bonsai can overwinter with almost no care whatsoever. However as the saying goes ‘knowledge is power’ and a simple understanding of some basic principles of horticulture can save you the heartache of losing a plant over the winter period. A little knowledge and basic care has the potential to save the loss of many years hard work. Bonsai trees are expensive both in terms of time and money and losing a good one into which you may have invested many years is like loosing a beloved pet. Remember the first rule of business ‘Protect your investment’.

Guide to overwintering bonsai
A UK winter can be a hard time for bonsai trees.

The Crux of The Problem
If you were to look at the end grain of wood under a microscope you would see what appears to be a honeycomb structure much like a bunch of drinking straws tied tightly together. Trees move water through their sapwood (the straws) in order to live, this is achieved by a process known as cohesion. As water evaporates from the trees upper parts a small force is exerted inside the tree that draws water in at the roots. An unbroken column of water stretches from the top of the tree to the tip of the roots. If this column is broken the process is interrupted and growth is impaired or even stopped altogether, in which case the tree or affected parts die back. As we all know from junior school when water freezes it expands. Water will expand by approximately 9% upon freezing. Going back to the drinking straws analogy, if water is trapped within this structure under a very slight pressure with no avenue of escape, upon freezing it creates a very powerful force. If the containing vessel is unable to withstand the force it will rupture. Once the temperature rises and the water contracts an embolism is created in the form of an air pocket. This interrupts the trees water column and can be catastrophic.

So how do trees survive winter freezing? The first answer is simple, by having very strong vessels and cells carrying water. To see this ‘not working’ take a cucumber or lettuce, a nice crispy bit of salad, and pop it in the freezer overnight. Next morning remove your nice hard and healthy fresh food and put in a bowl to defrost. About six hours later see what you get? The second answer is MUCH more interesting. Trees like oaks and ash put up with the loss of all their vessels each winter, relying only on the present seasons wood to transport water. The wood they produce in spring contains many large vessels which are used to supply new leaves. Later in the year they produce wood primarily composed of mechanical fibres and very few narrow water vessels. The wood is described as ‘ring porous’ because it has rings of more porous tissue sandwiched between layers of mechanically dense wood. This survival strategy works but does have some drawbacks. Firstly the trees cannot break new bud until after new wood has formed. That explains why oak in particular is one of the last species to leaf up in spring. Secondly the plants are vulnerable to late frosts. Oak and ash are at their limit in northern Europe. Other trees such as maple, birch, poplar and beech have what is called ‘diffuse porous’ wood. This form of wood consists of large numbers of narrow vessels that are spread more or less evenly throughout. These narrow vessels are less prone to embolisms than wide vessel tissues. The trees prevent embolisms spreading by separating adjacent vessel cells with rows of bars called ‘scalariform plates’ which trap air bubbles. In spite of this the trees still suffer a relatively high rate of embolisms but just like a cheeky teenager the trees have an answer… Ever wondered why some trees bleed profusely when pruned in spring? In spring some trees have a clever way to reverse emboli, they flood their vessels each spring using roots to pump sugar-rich water up into their trunk and branches. This strategy helps maples and birches survive extremely low temperatures as far north as the arctic tundra. The strategy has been exploited commercially: both maples and birches can be tapped in spring to collect sugar rich sap which is boiled down to make maple syrup and birch sap wine.

Put simply this all explains why low temperatures are significant. Freezing water expands and can rupture cells, causing air pockets to form that disrupt the springtime sapflow. The consequence of this can be tissue loss ranging from tertiary twigging to entire branches or in extreme situations your entire bonsai tree. Some trees, as above, have strategies to deal with the situation and others don’t, these are the ones we need to be a little more cautious with.

 Winter for bonsai can be tough

Native tree species won't be bothered by a hard winter.


Your Climate
Before we get into the intricacies of individual trees our first concern must be to understand your local climate. Conditions vary enormously from one garden to another, even within the same street. Prevailing weather conditions are readily affected by such elements as shelter from trees and houses, exposure to wind or shelter from the same. Also your proximity to such factors as a large conurbation or the sea as well as the elevation and orientation of your garden are extremely significant. There are no hard and fast rules when it comes to overwintering bonsai and so I cannot just give you a list of temperatures to which your bonsai will go. The only way to be sure your trees will be safe is to start learning about your specific situation right away. Every year the conditions our little trees have to endure is different, the only constant is change. The only approach we can adopt is careful observation and experimentation. Understanding your specific localised climate is vital. Temperature is just one element of the equation. As well as the factors listed above, rainfall is important, temperature fluctuation within the cycle of a day, exposure to wind and so on are all significant factors. The following pointers may be useful and you should have the answers in order to make a qualified decision on how to overwinter your bonsai.

Are your bonsai native to your region?
If your bonsai are not native to your region what type of winter conditions would they be exposed to in their home region?
What is the lowest temperature you are likely to experience over the winter?
What is the highest temperature you are likely to experience over winter?
How much does the temperature fluctuate between night and daytime?
Does rain evaporate quickly in your garden?
Will pots sodden from daytime rain freeze solid at night?
Is your garden ever exposed to freezing winds?
Are your bonsai exposed to lots of winter sunshine?
How quickly does your soil mix shed rainwater and return to a moist but not wet condition?
Are your trees evergreen or deciduous?
What is the normal air circulation/wind exposure like in your garden?
Were your bonsai vibrant and healthy going into winter?
Have your bonsai trees been in their pots for a very long time without re-potting?
What is the drainage characteristic of the plants growing media?
What is the state of health/vigour of the plant.

There are a million variables but this should give you some idea of what to consider before deciding whether you need to make provision for overwintering your bonsai trees.

Bonsai like bougainvillea will need winter protection in the UK
Bonsai like bougainvillea will need winter protection in the UK

Other Important Factors
There are many factors that determine the conditions a plant can withstand. Just knowing the temperature a plant can survive is not enough. Hardiness of trees cultivated in a bonsai situation can be markedly different from those grown using conventional nursery techniques or in the ground.

Health It’s fairly obvious that a tree in very good health will be more resilient to what the winter can throw at it. Conversely a tree that is weak or poorly established has very low resistance to adverse factors. The surest way to get bonsai through the winter is to ensure they are outstandingly healthy. This is achieved by not re-potting more than is absolutely necessary. Using a well conceived fertilising regimen is important (for established trees). Giving the tree optimum growing conditions over the summer and NOT overworking pruning, wiring etc’ as well as keeping the tree pest and disease free will go an extremely long way to ensuring a good winter outcome.

Origination If your bonsai is a healthy native from your area it is unlikely special winter care will be required. However if the tree is weak for any reason special winter care is a must. Obviously a plant from a very different climate to your own will be likely to benefit from special winter care.

Bonsai Pots and Soil
In my article on Choosing Soil for Bonsai Trees I covered the subject of drainage and capillary action. This is a key point in our ability to overwinter bonsai trees.

Soil selection is only partly responsible for this factor but the key is…

Following on from what we learned earlier about embolisms caused by the expansion of freezing water it follows that the lower the water content within the tree the less likely it is to suffer. As an example an olive that is simply left outside in the rain and freezing will not likely survive a temperature below -1 or -2 Celsius, however if that same tree is kept very dry over winter and protected from wind it will survive -10 to -15 Celsius. Wet roots absorb a lot of water and remain turgid, just add a period of hard frost and your bonsai’s roots will resemble the lettuce from the freezer and turn to mush. Very (very) slightly dehydrated tissues will be much more resilient. So, if you cannot control a trees environment over winter using a greenhouse or other type of cover you will need to select a soil that will allow easy passage of water over the season: the trouble with this is that it could dry out too fast in summer. Getting soil absolutely perfect is all but impossible. Following on from above, also consider the depth of the pot. As I detailed in my article on Choosing Soil for Bonsai Trees a shallow pot has MUCH less drainage than a deeper one due, in part to the effects of capillary action. Another factor concerning bonsai pots is that of root temperature. Trees growing in open ground experience a warming effect from the earth beneath. The root system of a bonsai kept in a cold climate will experience much lower temperatures than it’s earthbound counterpart. Bonsai roots also experience much greater temperature fluctuation that can be significant.

Tilt shallow bonsai pots to aid drainage
Tilt shallow bonsai pots in winter to aid drainage.

The Species of Tree
It kind of goes without saying that different species need different conditions in order to thrive but it’s astounding how often I am asked about keeping outdoor trees indoors and vise-a-versa. Find out where your particular species of plant naturally occurs. For instance olives naturally grow around the Mediterranean region, Spain, Italy, Greece etc’. The climate there is warm and very often dry, particularly in summer. So with a little imagination we can figure the tree will not stand a Scandinavian or British winter of low light and heavy precipitation and cold. However with a little care olives will thrive in Britain and can withstand – 10° of frost. Tree species that are native to your area should do well in your area…. fairy obviously but do bear in mind that a bonsai is a little more susceptible than a comparable plant growing in the ground. Trees termed ‘Indoor’ are typically from tropical regions and are tender (they will not stand frost). These need to be kept above freezing in winter, however a centrally heated house may be too warm. Thanks to the likes of Google it’s very easy to find out about your plant species. Do not rely on ‘Bonsai’ sites or forums to find out about your plants. Go to sites where the information is based on science and horticultural practice rather than hearsay. There are a lot of very good specialist commercial horticultural web sites, plant directories and encyclopaedic media that provide sound proven species profiles. Do make sure you take time to identify your tree properly, common names are about as much use as a chocolate tea pot! As an example we recently bought a tree from a very large importer under the name jasminum nudiflorum, an incredibly tough cold hardy plant. Upon examination it was found to be Brunfelsia that is a semi tropical species entirely incapable of withstanding frost. The moral is to make sure you know what you have got. Then find good sound advice about the species and variety. From there you should be able to make an informed decision on what you need to provide for the plant.

General Pointers for Overwintering Bonsai
From experience overwintering bonsai in the U.K I can state that water is going to be your main enemy. As already stated, excess water will cause problems upon freezing. Keeping the root ball a little on the dry side over winter will benefit your tree no end. More importantly it will benefit the mycelium upon which 99% of trees depend. Most bonsai folk are aware of mycelium in relation to pines because it is so visible. However most every other tree has a similar relationship with fungi, azaleas being an exception. If a trees mycelium suffers badly over winter the tree will suffer in spring. Obviously the microscopic hyphea (branches) of the mycelium are very easily damaged by excess freezing, wet and hydraulic pressure within a bonsai pot. Keep healthy mycelium and you will have a healthy bonsai tree! So it’s plain to see the benefits of keeping a bonsai trees soil a little on the dry side over winter but how dry is dry enough, more importantly how dry is TOO dry? Anyone with experience in keeping bonsai will be able to determine this point but if you are unsure there is a very simple gauge you can absolutely rely upon and best of all it’s free. What you need is a good size weed growing in your trees pot. Just leave one over from the summer crop we all fight against relentlessly. A weed with good size leaves like grass or dandelion is ideal. When the weed wilts it’s time to water, simple! With trees like olives and other Mediterranean drought tolerant species water a week later. REMEMBER IN SPRING AS THE WEATHER WARMS UP RESUME NORMAL WATERING CARE!


Only ever fertilise trees that are in active growth. If you have been following a good fertiliser regimen using a product like Green Dream Fertiliser a little residual nutrient will be available for the plant, held within the soil, for warm periods over winter when roots may be active. Don’t use chemical fertilisers or foliar feed over winter it’s just not necessary.


Fungal Problems
Fungal problems that manifest in summer are often the result of fungal activity in winter. A common problem like peach leaf curl cannot be treated once symptoms appear in summer but need to be pre-empted in winter. If you are keeping bonsai trees in a cold greenhouse, polytunnel or other shelter in winter it’s a very good idea to give the plants a thorough soaking in a good fungicide, like Scott’s Rose Clear Ultra before putting them in for winter. Use Bordeaux mixture for olives and thoroughly soak the bark. This leaves a crazy blue colour but it will disappear in time. Also don’t forget to clean your storage area prior to filling with bonsai and disinfect using Jeyes fluid or similar, simple precautions but tried and tested nursery practice.

Overhead Cover
There are a lot of trees in cultivation as bonsai that could be described as marginally frost hardy. Species like myrtus, morus, Chinese elm, celtus, ceratonia, various quercus and trident maple can withstand temperatures below zero. However without some form of protection these trees will certainly suffer some degree of winter damage. The simple answer with these is an overhead cover open on all sides. Indeed such a construction can be used to overwinter ALL hardy species. A simple construction can be built to provide overhead cover using timber and plastic sheeting. Open on all sides, or possibly closed off to the north will provide shelter from rain which aids in keeping roots dry which will increase their cold tolerance. Just make sure your shelter is capable of supporting the weight of snow that may fall on it. Dropping a half ton of snow on top of bonsai will never end well. There is also an issue in regards to frost actually forming on the leaves of evergreen trees. From personal experience I can say that preventing frost and ice actually forming directly on the leaves of trees will dramatically improve their chances in winter. Fleece is particularly good at this. As an example I have a ceratonia siliqua (carob), this species is not frost hardy. However when kept in our cold greenhouse and with dry soil as described above this tree has survived temperatures down to -7 Celsius. Using an overhead cover has proven to be one of the most successful winter strategies over the last few years of harsh winters. If you have free standing benches simply place trees underneath and draw plastic sheeting or UV Stabilised Bubble Wrap over the top and secure with heavy timber batons. Leave the ends open and you have a very simple and cheap temporary winter shelter that will guarantee your trees emerge in spring hale and healthy.

Overhead cover for bonsai in winter
Overhead cover can be a simple as this.

Using a Cold Greenhouse/Polytunnel
When using a cold greenhouse it’s okay to pack trees very tightly inside. Over winter trees are not going to be growing and so being bunched together will not cause problems. However in this situation it is important to ensure good air circulation either by way of vents and open doors during the day or by use of an electric circulation fan. There are some excellent purpose designed greenhouse fans available that cost virtually nothing to run and can be left on 24 hours a day(ours uses 13w). Circulating air will also help to raise the temperature. Remember the coldest nights are those without any wind. Another very useful strategy with any cold greenhouse is to insulate the inside with UV Stabilised Bubble Wrap. We use a double layer stapled together loosely around the edges, smooth side outwards. This can easily be attached to the inside of your greenhouse using wire twists. This will help retain daytime heat and also the small amount of heat that rises from the ground. The insulation can easily be removed in spring, rolled up and stored for the following years use. When overwintering in a cold greenhouse take care when watering. Many trees will not need watering much at all. Trees like olives can go six weeks or more without watering and will actually benefit from the dryness. Evergreens will obviously need more water. When watering trees in winter quarters use a Long Spout Watering Can and apply water sparingly to the soil only. The less water there is in the winter store the less likelihood there is of having fungal problems. Another aspect of overwintering in a cold greenhouse is to ensure cleanliness, again to reduce the risk of fungal problems. Don’t leave dead leaves etc’ lying around, these will decay and be a haven for fungal and pest activity.

Greenhouse ventilation fan

Ten years old and still going strong our little greenhouse fan.

Bonsai trees in winter storageWinter storage of Bonsai Trees

Unheated poly tunnel with bonsai in winter storage.

If you decide it is necessary to provide an element of heating in winter, either because you expect extremely low temperatures or because you have tropical trees consider the following. By far the most convenient and economical form of greenhouse heating is electrical. There are a lot of alternatives available. Paraffin heaters are very expensive to run and do produce masses of water vapour which can cause problems, they also need a source of fresh air in order to burn cleanly and this is just going to bring cold in and use more fuel to heat. To the best of my knowledge thermostatically controlled paraffin heaters are not available. In that case if the weather warms overnight you may be burning fuel unnecessarily. Paraffin heaters also require daily refuelling and wick adjustment. There are solar and wind powered systems available for greenhouse heating but the outlay is large and for the limited use required it is unlikely to be worth the initial investment unless you have a very large busy greenhouse. Electrical heating via a form of fan heater has the advantage of producing dry air, perfect for our needs. Secondly an electric heater will run it’s fan 24 hours a day with the heat kicking in as required via a thermostat which is both economical (heat on demand) and circulates the air nicely preventing fungal problems. There are a lot of heaters available for greenhouses. Speaking from experience the small fan heaters available from garden centres are not very good. They are prohibitively expensive to run (normally 2Kw) and do not move sufficient air. This means that even a small greenhouse will have hot spots and cold areas. We also found they can fail without warning when running for extended periods of time thanks to their thermal cut outs. Heaters controlled by a thermostat are a great asset for overwintering bonsai trees. Get a good maximum/minimum thermometer and hang it in the middle of your space just above your trees. This will help you to record temperatures as you fine-tune your set up. Heaters can be adjusted to maintain any temperature you choose. Either keeping the temperature above freezing or preventing downward spikes of very low temperatures. For most hardy trees a maximum low temperature of -2 to -4 Celsius is perfect. This will keep the trees dormant but will prevent cold damage. Here at Kaizen Bonsai we have a small number of tropical species (Indoor Bonsai) that we overwinter in a closed off section of our large polytunnel that is approx’ 12’ x 16’ x 8’ high. I am sure our experiences will be valuable when considering your own set up. Trees are placed on 3ft high slatted benches and over the past 6 years we have slowly refined the set up. This is how it went….
1st year
Doors closed and a 2Kw fan heater thrown on the floor. Results were poor, some trees furthest from the heat source were lost to cold. Some fungal problems were experienced. The electricity bill was over £800 for the winter period.
2nd year
As above but we insulated the area with two sealed layers of UV Stabilised Bubble Wrap. The most tender trees were placed in the centre of the area. Results were better, no losses and fewer fungal problems but the electricity bill was still over £600
3rd year
We invested in a Bio Green heater. This is a massive 3.2kw! But it cost less to run than the 2Kw job we had before. The heater consists of a large heater/fan housing that mounts at head height. Air is drawn into the unit high in the space and is ejected via a duct that lays along the floor. This has massive advantages. Firstly the warmest air in the space rests ABOVE the trees. This is drawn into the heater via it’s high level intake. Because the air ejection system lies along the floor the cold air that settles there is picked up and circulated resulting in very even temperature throughout the space. Also because the warm air is ejected via a duct along the floor that has lots of small holes the system can be set up to ensure there are no cold spots. Lastly because of the air output system working at floor level along the entire length of the space air circulation is almost perfect.
Result? Perfect heating and incredibly precise temperature control. Temperatures only varied by 1 degree over the entire winter. Also perfect air circulation meant no losses and no fungal problems.
The downside? Cost of setting up including wiring went to nearly £600. However the electricity bill went down to below £500
4th year
We kept the bubble wrap insulation on the doors at the end of our facility but replaced the main areas with 5 wall polycarbonate. The areas below the benches were insulated with 2” polystyrene as light transmission is not important in that area. We positioned 5 full sheets of poly’ very precisely to the sides and top of the space. This reduced the light transmission very slightly but being winter this proved not to be a problem. 

Result? Perfect heating and ventilation from our fan set up and, absolutely amazing insulation from the poly’. It cost over £500 to buy and install the insulation (which should last more than 20 years). In fact the insulation meant that when we did get snow it stayed lying on top of the heated section of our tunnel! The electricity bill for the winter (which was cold) was below £200. I doubt we will be able to improve on this!

Heated winter quarters for tender bonsai trees

Our heated winter quarters for tender tree varieties

Greenhouse heaterSimple heater ducting

Greenhouse heater & ducting still going strong after 10 years.

Carefull greenhouse insulating is important

Insulate carefully with heavy duty UV stabilised bubble film.

Winters in northern Europe can be dark gloomy affairs. Many people suffer because of this and many evergreen plants certainly do. Over the past couple of years we have been experimenting with lighting in conjunction with winter storage. Lighting is not essential if your storage solution is exposed to good natural light however it can be a useful tool to have at your disposal. Many of the trees we use for bonsai are a long way from home. Many climates are very cold over winter but, unlike the U.K, they can be very dry. This results in crystal clear cold air with very little moisture. This is particularly the case at altitude. Many evergreen trees suffer from a lack of light over winter and end up looking sad and discoloured. Supplemental lighting for plants needs to be blue wavelength. Using standard household lighting is useless because it’s predominantly yellow wavelength. There are a lot of what are termed ‘grow-light’ options available from a simple fluorescent tube used by aquarists to monster setups used by Dutch flower producers. Most bonsai hobbyists will only need a small amount of lighting for a select few trees over winter. Kaizen Bonsai researched the grow-light market and settled on the Lightwave Fluorescent Tube units as the perfect solution. Super compact units that are energy efficient, maintenance free and have a long life. As an example of the benefits lighting can bring. A couple of years ago we bought a very expensive Japanese five needle pine (commonly known as seedling pine). The tree arrived in autumn and was a quite alarming yellow colour thanks to having been kept too wet for a long period. This variety is not so strong as grafted Japanese pines and always has a slightly yellow colour but this example was extreme. Upon arrival the tree was placed in our polytunnel to dry out. Two months later this had happened and the tree had certainly improved. Upon the recommendation of a good friend (hydroponics expert) we bought in a lighting unit and positioned it close to the top of the tree, set up on a timer to burn 18 hours a day. The result was almost too good to be true and even though I saw it with my own eyes it is still a little hard to believe. Within one week the tree had gone from pale washed out yellow to fresh green with pronounced white markings on the needles (which it should have) and the needles went from being slightly droopy to being fully erect and reaching towards the light. Since we have been selling Lightwave Fluorescent Tube we have had a lot of similar tales from our customers of dramatic transformations. Lights can be used to benefit weak areas of trees such as lower branches where the improvements can be almost miraculous. Whilst lighting may not be an essential for overwintering bonsai, when used correctly it can make spectacular improvements to many evergreens.


After The Winter
As winter begins to fade everyone in bonsai gets very excited at the prospect of a new season. Trees in winter shelter will often show signs of life before those kept outside. However just because buds show a little swelling doesn’t mean leaves are imminent. Winter can drag on and so do not be tempted into action too early. If you have been sheltering native trees over winter it will pay to move them outside once you are confident the very worst weather is past. As the weather improves gradually move out more tender trees. In general it is not wise to allow deciduous trees to leaf out in their winter quarters. Leaves grow to suit the environment in which they develop. If you allow this to happen inside a sheltered greenhouse the leaves will not be suitably tough to deal with outdoor conditions. When you see buds beginning to split open and just see green under the bud scales move outside. Before buds actually open a winter wash is of great use to prevent P&D problems later in the year. There are several good products available to fruit tree growers that can be used. Even if you do not winter wash give trees a spray with Rose Clear Ultra prior to putting outside. Once empty give your winter storage area a good clean and disinfect before storage or returning it to summer use.


There really are no hard and fast rules when it comes to overwintering bonsai trees. As I stated at the beginning there really is no substitute for careful observation of your localised climate and individual trees. No two people care for bonsai in the same way and so every situation is different. Hopefully this article will give you a few pointers and provide a good starting point in preparing for winter. Trying to second-guess the weather is a pointless exercise but being prepared can save you a lot of pain, heartache and expense. In the U.K winters have become increasingly inconsistent and here at Kaizen Bonsai we have been caught short resulting in the loss of thousands of pounds worth of trees. Taking a gung-ho attitude will cost you a great deal and will put a severe crimp in your bonsai experience. Don’t take the risk, trust me it’s not worth it!

© Graham Potter 12/2017

The above is intended as a guide only. You need to satisfy yourself that acting on any of the information supplied is suitable for your own situation.
When using electrical appliances outside always have instillations done by a competent certified electrician. There is not much point in bringing your bonsai through a hard winter if you are dead.
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