How To Build A Bonsai Cold Frame In 9 Easy Steps

Bonsai Cold Frames

One of the things that I struggled with when looking after my bonsai collection last year was winterizing my bonsai trees. It always seemed like the hard work I had put in over the summer months had just gone to waste with my plants almost dying during the cold winter. Luckily I discovered bonsai cold frames, which save my plants during winter. 

To build a bonsai cold frame, invest in two rows of small shelving units large enough to support your bonsai collection. Drill or glue these on top of each other. Cover in insulation with a removable panel to access your plants. Regulate the temperature with a fan and heater and place it indoors. 

So what exactly are bonsai cold frames? And what are the step-by-step tips you can follow and the equipment you will need to build a bonsai cold frame? Keep reading to find out more!

Just a quick heads up, over the past three years of running Plantpaladin, hundreds of people have asked for product recommendations. As such, You can find my favorite indoor bonsai tree here (link takes you to Bonsaiboy), my favorite outdoor bonsai tree (link takes you to Bonsaiboy), or have a look at all the products I recommend here

Blueprint out your design

The first thing then that we need to figure out when it comes to making a bonsai cold frame is exactly how we want it to look. 

After all, it’s all well and good buying the materials but if we don’t have a blueprint on what it will look like then you’re going to waste about as much time as I did when I first made mine. 

To come up with a blueprint then do the following: 

  1. Make a note of how many bonsai you will keep in your bonsai cold frame – the more plants you have and need to store the more space you will need 
  2. Ensure your storage space can fit multiple sizes – one of the best things about bonsai trees is that they vary in size with some being more vertical than horizontal – ensure then that you have enough room 
  3. Draw out a rough sketch on paper of what you want your bonsai cold frame to look like – I like to do this on a rough piece of paper to get a rough idea 
  4. Finish off your blueprint by using Canva or free interior design software 

Now I know this can be incredibly daunting (I’ll be honest everything bonsai still is to me) but to hopefully help, I’ve enclosed some images below of my rough drawing followed by my more in-depth image on Canva.  

bonsai cold frame blueprint

blueprint for a bonsai cold frame
Decide on the equipment you will need

Firstly, we’re going to need to do is an inventory check of the gear we’re going to need to build a bonsai cold frame. 

Now bonsai cold frames come in a variety of sizes from entire purpose build sheds to small cooler boxes. 

For our bonsai cold frames, we are going to take a universal approach that should work for pretty much any bonsai collection large or small, regardless of species. 

Our cold frame will have two rows and can fit either 4 large bonsais or 8 small bonsais. (see the above image for reference

To build your bonsai cold frame you will need to invest in the following equipment: 

  • Shelving units – This will be the structure that we will use to hold our bonsai 
  • Insulation foam panels – Easily the most important part of a bonsai cold frame, needed to winterize our bonsai 
  • 2×3 wood pieces of varying length – These will be sued to help us create a door and a frame if needed for our cold panel 
  • Door hinges – Again used to have an easy to open door in our cold frame 
  • A fan – Used to temperature regulate our cold frame and provide circulation
  • An electronic thermostat – To help monitor the temperature of our bonsai cold frame 
  • An electric heater – To help increase the temperature if needed
  • A saw –  Used to cut holes in your bonsai cold frames if you have larger bonsai 
  • A drillUsed to drill in excess holes in your bonsai cold frame for ventilation purposes. 

Decide where you will place your bonsai cold frame 

Whilst it can be super tempting to start building your bonsai cold frame as soon as we have the material, the truth is the whole purpose of this is to protect our plants in winter. 

It’s important before you start assembling anything, to decide where you will keep your bonsai cold frame during the cold winter months. 

Whilst some people opt for keeping them outside,  my recommendation has to keep them in a garage. 

This will maintain a similar temperature as outside whilst avoiding any severe weather damage. 

Glue your shelving unit together

Now the fun stuff. 

Start building your Bonsai cold frame by gluing one of your shelving units on top of the other. 

Ensure that each segment of your shelving unit has enough room for each bonsai. 

Now if your shelving unit does not have enough room for your larger bonsai to stand vertically without damaging themselves then you may need to saw a hole in between your units. 

For my recommendation, I’d have to go with an Ikea Kallax Bookcase (link takes you to Amazon) is easily a shelf unit that works best for bonsai which is durable and can be adjusted for a variety of sizes depending on your own needs. 

glueing bonsai cold frame

Cover your bonsai cold frame in insulation foam

Now your structure is complete the next step is to cover your cold frame unit in insulation foam. 

I found that Owens Corning pink Insulation panels (amazon link) work best for this. Mainly because they come in a variety of sizes and avoid you having to cut and trim these to size. 

Alternatively, I found that soundproof panels (link takes you to Amazon), also are a fantastic option to keep your bonsai insulated.  

Regardless of the insulation option you go for, you want to staple or glue this to the backsides and top of your bookshelf. 

A trick I found is to double insulate the foam on the left and right sides of your structure, to avoid any cold air hitting your cold frame that might come through your garage door. 

Avoid however getting any insulation to the front for now

Build a door

Whilst this step is optional for some, you will still need to water and check in on your bonsais during the winter months…this becomes a lot easier when you can open and close your bonsai cold frame. 

To add a door then to your bonsai cold frame, get a panel of thin MDF wood or another lightweight wood, cut it to size so it fits the front of your cold frame. 

We want to choose a lightweight material as anything too heavy will take a lot of effort to move consistently. 

Then attach another plank of wood to the top of your shelving unit and attach a hinge to it. so your door can open and close.  

You then want to cover your door in two layers of insulation. 

Don’t forget about the holes 

One step that I have forgotten about, way too much in the process of making my own bonsai cold frame is to ensure you have enough holes in the back of your cold frame for ventilation and wiring (for a fan, heater, and thermostat) 

I like to drill or cut around 7 to 8 holes at various points in my cold frame to ensure I’m covered. 

Place your plants

Congratulation, your bonsai cold frame has now been build!

The next step is a simple one. 

Once any glue staples or other furnishing have had a few hours to set, simply wipe down the inside of your bonsai cold frame and then delicately place your plants where you would like them to go. 

Manage the temperature

The final step then of setting up a bonsai cold frame is to manage the temperature. 

After all, it’s all well and good building a cold frame but if it’s colder in there than outdoors then you would have gone through a lot of effort for nothing. 

The best way to do this then is to invest in a small heater, fan, and thermostat. 

The heater will help heat your plants if it gets too cold. 

The fan will help cool things down if they get too humid, and the thermometer will help regulate the temperature of your cold frame interacting with both the heater and the fan. 

My choice for a digital thermometer has to be the Govee temperature monitor (link takes you to Amazon), which not only monitors the temperature of your cold frame but also the humidity and wetness. 

This thermostat also connects to your phone meaning you don’t even have to go to your garage on a cold winter morning to check the temperature of your bonsai cold frame. 

If your bonsai cold frame does drop below the optimal temperature you need then simply turn on your heater – my choice would be a small electric heater like the Homechoice personal ceramic which should be large enough for most bonsai collections and doesn’t break the bank when it comes to your electricity bills. 

Finally, if your bonsai cold frame gets too warm, the thermometer will tell you, turn on your fan – for me, a smaller fan works best which is why I would opt for a desk fan like the Giatop desk fan (link takes you to Amazon) 

What is cold frame for bonsai trees? 

Bonsai cold frames are box structures that bonsai owners use to keep their bonsai safe from cold winters in harsh climates. These are either kept outdoors or indoors (most commonly in garages) and are usually covered in foam insulation to keep plants warm. 

More advanced bonsai cold frames may also contain a thermometer, heater, and fan to regulate temperature. 

What do bonsai cold frames do?

Bonsai cold frames protect bonsai trees from harsh winter conditions, protecting plants and trees from bad weather, predators, and general decay. They do this by creating a protective box to store bonsais in which are usually insulated and help create a sub climate where bonsai will be protected. 

What equipment you will need?

A box or shelving unit, insulation panels, plywood, thermometer, heater, and fan are all required to create a bonsai cold frame from scratch. Consider using items around the house like old cardboard boxes instead of shelving units to save time and money if on a budget 

Can bonsai be left outside during winter?

Bonsai trees can be left outside during winter in most climates however you must protect them from the dehydration associated with the winter months. The best option would be to create a bonsai cold frame to protect your trees. 

Failingthen to look after your bonsai can potentially kill your plant in a, particularly harsh winter.

What do I do with my Bonsai during winter? 

In the winter, ensure you protect your bonsai from dehydration and pests. The best option would be to winterize your bonsai by investing or building a bonsai cold frame, moving your plants into a garage, or using tarp and mulch to insulate your bonsai

To find out more about this, check out my post on winterizing your bonsai

How To Protect Bonsai In Winter infographic

How cold is too cold for bonsai? 

Temperature below -10 degrees celsius (14 degrees Fahrenheit) is not recommended for bonsai trees, with most species struggling to survive in those temperatures for long periods. Some tropical species should be kept indoors near 0 degrees celsius.

Now if you’re interested in finding out more about how the winter affects bonsai, there are some fantastic studies from ABC ScienceThe BBC and Science Direct.

My top picks for the gear you will need!

So like I mentioned earlier, over the past three years of running PlantPaladin, hundreds of people have asked me for my recommendations on the best bonsai gear on the market. 

Having spent thousands of dollars on bonsai items these past few years and tested at least 100 bonsai-specific products, I’ve listed my favorite products below – All of which I highly recommend and think you can get great value. 

They can purchase directly by clicking the link to take them to Amazon. 

Bonsai Tool Set: One of the significant challenges I’ve had is finding a toolset that was not only durable but didn’t break the bank. SOLIGT has recently developed a fantastic bonsai tool set that covers all the tools you need to trim, prune, and repot your trees. – You can grab it here

Complete Bonsai Set: Many of you will want to grow your bonsai trees entirely from scratch, but finding the varicose seeds, pots, and other items in one place can be challenging. Leaves and Sole then have created a complete bonsai set that I’ve personally used that ticks all the boxes. You can grab it here

Bonsai wire: The number of times I’ve run out of wire for my bonsai or purchased cheap bonsai wire that doesn’t do the job is embarrassing for me to admit. After a lot of trial and error, I found that using Hotop’s aluminum bonsai wire is one of the best options on the market. This can easily be used for both indoor and outdoor bonsai. You can grab it here.

Fehed Nicass

Fehed Nicass has been passionate about all things bonsai and botany focused for the past 3 years. What started out as a hobby has developed as a passion and he is now on a mission to teach and learn.

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