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Overwintering Bonsai

It’s the 21st of November and it’s a clear, crisp morning. We have had freezing temperatures overnight and the average day temperature is around three to four degrees Celsius. In Southern England we don’t have very harsh winters, however on two/three nights of the year temperatures can drop to around -10 degrees. Some of our trees are not acclimatised to the British winter and so require protection during these cold months. Many of our trees our quite hardy such as the Scotts pine or the large maples, but we protect some of them just to be on the safe side. If you look around here, the trees that have lost their leaves are all large trees, so we won’t put them away until the middle to end of December as the worst of the winter is in January.
large winter bonsai
If you look around we have left a few of the trees outside here on the benches. We have gradually begun to take trees away from these benches and put them into their winter quarters.
medium bonsai
As a general rule of thumb, the larger the tree and the larger pot, the better they are able to withstand the cold. Larger pots won’t freeze as easily as some small pots. Some of these large trees will stay here until the end of December, then we will start putting them away.
large bonsai in large pot
These are imported Japanese white pines which came from Japan around 20 or 30 years ago. As these trees have acclimatised to the British winter they are fairly hardy, so we are not too worried about them. For trees which have been newly imported from Japan we need to be more careful, as they have not yet become acclimatised to their new environment.
japanese white pine bonsai
I will now show you what we do with the trees which require protection during winter. If you were to take a walk around our nursery you will see we have greenhouses and shade tunnels. Let’s take a look at some of these shade tunnels now.

The shade tunnel which you see behind me, is exclusively for our satsuki azaleas. Satsuki azaleas are quite hardy during British winters. They will lose some of their leaves as they are not permanently evergreen. The overhead shade will provide enough protection for our satsuki azaleas during the British wintertime.
bonsai shade tunnel
If we look inside the shade tunnel, you will also see that the benches for our satsuki azaleas are at least 60cm to 90cm off the ground. The purpose of having these benches is twofold: they provide a very useful display and added winter protection for trees which we place underneath them.
bonsai bench
bonsai bench 2
You can also place your satsuki azalea bonsai inside an unheated greenhouse during the winter. However if you do this, you must make sure the greenhouse does not get too hot. If it does, you will confuse your tree into thinking it is spring, causing it to break its dormancy.

On our nursery we have two large polycarbonate greenhouse complexes around 30mx30m. In those greenhouses we keep a lot of trees which are being developed. We also have one large shade tunnel where we keep our evergreen plants and a couple of polytunnels which we use for overwintering other types of plants such as the trident Maple.
shade tunnels
Let’s have a look inside some of our shade tunnels…

When the first winter frost forms on the trees we call it the ‘frost blush’. All these brown junipers have been exposed to about 10 to 15 days of hard frost during the end of October and very early November. We call this exposure frost blush, because, like sunburn, the frost burns the foliage and tints it brown. If we look underneath the brown foliage we can see the rest of the tree is still green. Frost blush is not something to be concerned about. In fact, if you were to visit any of the famous bonsai nurseries in Japan during the winter you would see trees which look very similar to these. Come the Spring, these trees will turn green again.
frost blush on bonsai
frost blush on bonsai
frost blush on bonsai
This shade tunnel provides enough protection for our juniper rigidas and itoigawa juniper trees during winter. The temperature in these tunnels will go down to -5. If the frost becomes too harsh we will place the trees underneath the benches for additional protection.
bonsai under bench
I know that most amateur bonsai growers will not have the space or the need for a large structure such as this. However, if you were to make a small wooden structure and cover with netting or plastic sheeting that would provide enough protection for your bonsai during winter.

This is one of our white polytunnels. We protect most of our trident maples in this structure. The trident maple is not as hardy as the ordinary Japanese maple and so requires a little more protection during winter.
bonsai in polytunnel
We always make sure to ventilate the tunnels. This ventilation helps to prevent the trees from developing fungal infections. On very cold days we will close our vents, but will never close them permanently for the winter.

Here you can see all our deciduous trees including Korean hornbeams and pyracantha. Different plants have different degrees of hardiness and so it’s important to consider the harshness of your climate and protect the plants accordingly. For example, the pyracantha is not as hardy as the cotoneaster.
european hornbeam bonsai in shade tunnel
pyracantha bonsai in shade tunnel
Tip: Incurve pots (pots which have an inward curving outside edge) are particularly venerable to frost. The pot’s shape has no room for frozen soil to expand and so will burst under pressure. Be particularly careful with incurve pots and take precautions to make sure the soil doesn’t freeze.
incurve bonsai pot
Most Japanese bonsai pots are completely frost hardy, however, in recent years we have found that some of the glazed pots which come from China are not as hardy. In hard frost the glaze on the pots can sometimes crack and flake off. Chinese unglazed pots are very frost hardy as are Japanese ceramic pots (especially those from tokoname).
glazed bonsai pot
ceramic bonsai pot
This is another one of our tunnel structures where we keep some of our smaller sized Japanese Maples. This is where they will stay until they are ready for re-potting in February. We will then bring the Maples out into the open around March.
japanese maples
We also protect some of the more delicate trees in this tunnel such as the Japanese Holly. Japanese Holly are generally outdoor trees, although some people grow them indoors in cool rooms. We protect them in here during the winter as they are not entirely hardy in the European climate. We also protect our olive trees which come from a Mediterranean climate.
japanese holly garden bonsai
garden olive tree
Here is an example of an ordinary English Yew. You will see that the frost has made outer foliage brown but the inside is still green. By spring this yew will turn vivid, bright green.
english yew bonsai
This is our greenhouse where we keep our indoor trees. This greenhouse is not heated in the winter. Although it is a glasshouse the temperature does still drop to freezing but not below, it tends to hover around the 0 celclius mark. Many of the trees wouldn’t be happy at 0 celcius so we cover them with horticultural fleece. I have known bonsai growers to use net curtains instead of fleece which is also an adequate covering. You could also use a plastic sheet.
horticultural fleece
As I mentioned before, we find our bonsai benches very useful as we can place trees which require additional protection underneath them. We add additional protection to our trees using a plastic sheet like this.
protective plastic sheet
We also sometimes place sheets at the side of the benches to protect the plants from strong winds. We secure the sheets with breeze blocks or nail them to the benches. This sort of structure is ideal for winter protection. During milder weather or particularly sunny days it’s important to air it out so that mould and fungal diseases cannot form and spread.
plastic sheet on side bench
My preferred side, however, is this next one. Although there are a couple of cuts on this side, they will heal over nicely. I could also carve this side and make it a hollow trunk tree. As you can see there is already a nice taper forming also.
Tip: wrap pots with bubble wrap to help protect the roots and prevent the root ball from freezing too hard.
bonsai pot bubble wrap
If you take care of your trees in winter, they should flourish again in the spring.