Working Out How To Over Winter Bonsai Trees

Working out how to look after your outdoor trees during winter can seem daunting, but with a bit of pre-planning it’s very simple.

If you follow the 4 simple steps below you should be able to put a plan in place, no matter what your winter temperatures are like and make sure your bonsai survive the winter.

Step 1 – Assess The Weather Your Bonsai Will Be Exposed To

You need to have an understanding of your weather before you plan anything for your trees.

Even if you’ve lived in the same house for years it’s still good to look up the weather data for your area. You might be surprised by what the actual figures are.

So, go to your preferred internet search engine and type your town name in followed by “climate”. You should be able to get the data for the average temperatures over 12 months, for your area and it should show the high and low temperatures throughout the year.

A good place to look is wikipedia. Most pages about cities and towns have a “climate” section that will give you what you need.

You can also search for your areas “hardiness zone” which will give you the average low temperature that you are likely to experience. (I will talk about this a little more further down)

Once you have this, you can now see how cold your average winters are actually getting and have an idea of what the extremes might be. Keep a note of these numbers as this will be important to see if your bonsai can survive over winter outside.

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Step 2 – Work Out What Bonsai Trees You Have

You need to make a list of all your tree species.

It’s probably best to do this on a spreadsheet then you will have it saved for years to come and can edit it easily.

I’d suggest you group your trees into different categories such as Tropical/ Sub Tropical/ Native and you can then also further divide this up into deciduous or evergreen as you will find trees that fall into these groups will need similar care.

Now you will have a clear picture of what bonsai trees you have you and will have them grouped into what care/protection they are going to need.

(For more info read – How To Identify Your Tree)

Step 3- Find Out The Hardiness Of Your Bonsai

Once you’ve got your list of species, you need to check how hardy each species is.

You can normally search the name of the tree species followed by “hardiness” or “temperature” and you should be able to find many pages that tell you the minimum temperate that they can survive in and maybe a little extra information about what the tree can handle.

(If it is a tree species I own, I have made a Care Guide, which should mention how hardy the tree is, so you can start by looking there)

Write down the minimum temperature that each tree species can handle next to its name on the list.

All though this information isn’t precise and it may not be exactly perfect, you should at least now have a rough idea of what cold temperatures your bonsai trees can generally handle in winter.

(You can read more about – What Are Cold Hardy Bonsai Trees?)

Step 4 – Putting It All Together

Now all you need to do is check if your local temperatures are lower than what the trees can handle.

• For example. My minimum winter temperate is around -2°C/28°F. My ficus (a tropical tree) will probably start to struggle once it goes below 5°C/41°F and will most likely die if exposed to below freezing (below 0°C /32F° )

So simply put – If I leave my Ficus bonsai tree outside all winter, it will die, as my climate is colder than it can handle.

I can see from my simple spreadsheet that this tree can not handle the weather here, and it needs protection.

• On the other hand my juniper can handle temperatures down to -20°C/-4°F. So my juniper isn’t going to be too upset living in my -2°C/28°F garden. It won’t even be fazed in the slightest, it can handle this “cold”.

By spending a short amount of time to put all this data together it should really help you work out what trees are going to need protection and what ones are going to be totally fine being left alone all winter.

Having this information will allow you to decide how much protection you need to give each bonsai that you own, in your own garden.

I am lucky enough to live in a very mild climate and all of my trees can handle much lower temperatures than what I am likely to actually get throughout the winter. This means I am able to leave my trees outside all winter. I do not need to protect them. If I moved to somewhere colder than here, I would need to start considering cold frames, burying trees, covering with mulching or storing them in the garage.

Having this information also means if there is a sudden drastic dip below normal, I can react by protecting any bonsai trees that may now be at risk.

If your trees cant handle your weather, then you are going to need to read – How To Protect Your Bonsai Trees Over Winter – to make sure you know how to actually look after them.

Other Factors To Consider For Over Wintering A Bonsai

The temperature is not all you need to look at. you should also look at pot size, soil composition and where your trees are positioned, as this will all affect their ability to handle cold temperatures over winter, so you need tho think about this as well.

Bonsai Pot Size

So as I said above my juniper bonsai can survive temperatures as low as -20°C/-4°F.

However, if I have a really small root ball, in a really small and shallow bonsai pot, this will effect its hardiness. It will probably still be able to go pretty low, but probably not down to -20°C/-4°F.

If it was in the ground and growing strong, it would be able to handle -20°C/-4°F with no problems. When a tree is in a bonsai pot. Just be careful and use your judgment as it will be as hardy as you think.

In my climate of -2°C/28°F, a juniper, even in a small pot, can handle this no problem. However, if my weather dropped to -10°C/14°F . I might start thinking about sheltering the tree. In theory the juniper can also handle this temperature, it is hardy down to -20°C/-4°F but I would just be careful. The small pot can really change how much it can handle.

Calculating Hardiness Zones For Bonsai

You will sometimes see trees described as “hardy to zone 4” or something like this. Basically the United States Department of Agriculture (USDA) have made a map of America showing where the different low temperatures are and marked these into hardiness zones.

(This is only showing the LOW temperature, so they can work out what plants will survive in the area during the winter)

If you cant find what temperature a tree can handle, try searching to find its “hardiness level”, then you can use the image chart below to see how that translates in to temperature.

By USDA-ARS and Oregon State University (OSU) – USDA Agricultural Research Service (USDA-ARS), Public Domain, Link

Adding Two Zones Of Hardiness For Bonsai

I have also been told by a lot of bonsai professionals that as a general rule, they will consider any potted tree to be the equivalent of 2 zone levels higher than a tree planted in the ground.

I obliviously have no way of measuring how accurate this is and maybe this is over the top and being too cautious?

However, when I said above “When a tree is in a bonsai pot. Just be careful and use your judgment.” This is an excellent way of quantifying this “judgment”. It may be over cautious, but it is often better to be safe than sorry. If you get really bad winters, you might want to take more interest in this.

If you are going to use this method. Just carry out Step 3 above as normal, but mark the trees low temperature as “in the ground” and then make a second column where you add the “hardiness +2” which is the figure you will actually use to calculate what your tree can handle compared to your local climate.

For example – My juniper can handle -20°C/-4°F which means it can survive in Zone 6 …if it is in the ground.

If we want to be cautious with our pot grown juniper, we will add +2 hardiness zones to calculate what it can handle.

This means: Zone 6+ 2 = Zone 8.

Zone 8 on the chart is lows of -12.2°C/10°F – This is the figure I would now use as my trees “low” temperature.

As I said, My climate is mild, I’m actually in Zone 9. Which means lows of -6.7°C/20°F. When we compare that to the -12.2°C/10°F that the tree can handle, it means that even on the most extreme days, my weather is not going to bother my juniper, even if it is in a bonsai pot. So I still don’t need to worry about any form of protection.

(I appreciate trying to work this out can seem like a mind melt, just take your time and you should be able to make sense of it, this is also why you make a spread sheet, its just easier to see all written down)

The Position Of Your Bonsai Tree

The same consideration is needed for the position of your tree.

It seems counter intuitive, but the ground is actually warmer than the air above it. When you check the weather, the temperature you see is the air temperature at about head height or around 6ft. So what you read on your weather app, might not actually be how cold it is for your trees if they are sat on the ground.

Then to confuse things further, the bare ground, such as dirt can be warmer than concrete.

This is important to bare in mind if you keep your trees on the ground, concrete, or on benches. The difference between these positions may only be subtle, but for some people this may make the difference between a tree being able to handle the temperature or not.

(For more info read – Where To Place Your Bonsai)

Monitoring Your Bonsai Throughout The Winter

Although these calculations should be done long in advance of the winter, you should still keep an eye on the weather every day.

You should be aware of your areas lowest temperature. From my research and as I live in Zone 9, I know it shouldn’t get any lower than -5°C/23°F.

However, I will still check the weather daily. If I see any temperatures getting down towards this, I will double check my spreadsheet and just see if any of my trees might be close to this limit, which luckily they are not.

Obviously if had a freak cold spell that was way below normal, I would be checking my spreadsheet again and would probably need to take some action and protect some of my trees.

Conclusion

Hopefully now you can work which of your bonsai trees might need protection during the winter and how to continually assess if you need to change anything.

Hopefully you are as lucky as me and have mild winters and you don’t need to protect your trees much, but I know some of you get a lot more cold than I cold ever handle. I know some of this is hard to figure out, but just go through the figures slowly and you will start to make sense of it.

(You can read more about – Do Bonsai Trees Die In Winter?