By Celina Blandford
My first year starting bonsai, winter care seemed the most intimidating aspect of keeping trees in Minnesota.
Unless you only keep tropicals, outdoor winter protection for your trees is a necessary part of caring for bonsai in Minnesota. My first year of bonsai I stored my tree with Lionel Flood (he offers this great service, lionelflood2@gmail.com) for the winter, but I knew I had to build a cold frame if I wanted to expand my collection. I toured a lot of members cold frames at the member tour and got a lot of great tips from experienced members. I hope this article helps and you find that winter care isn’t scary or intimidating at all.
Left Image: Newly constructed cold frame. Wood bench and shelving line the back wall. 14″ deep wire shelving (not seen) runs along the long wall opposite the bench (behind the shovels).
Building your cold frame
My first cold frame was 4′ x 4′ x 8′ pink foam room in my garage (similar to this) which worked great, and my trees made it through the winter! After that first successful winter, I acquired more trees that year, lots of trees, and I quickly realized that my small cold frame was not going to cut it anymore.
I then decided to build a larger, permanent structure at the back corner of my garage that can double as a small workshop in the summer months. It is an insulated 6′ x 12′ x 9′ room, built with 2 x 4’s and slat board on the exterior. My advice would be to build it larger than you think you need it.
Image Right: Newly added LED lighting for conifers and peg board was added to back of the bench to store tools.
Below are the items I use to keep my cold frame warm for the winter.
1. Temperature Control There are many options online, but I found this Inkbird temperature gauge. The prices fluctuate, but I purchased this one for under $30.00 (Inkbird Itc-308, 2-stage, 1100w, w/Sensor)
I keep the setting at 36F with a 1 degree difference. This keeps the hardier trees closer to the floor at 34F and everything higher on the main bench at 36-38F. I like that the temperature gauge is waterproof with a 4 foot cord. I have the gauge placed in the exact center of my cold frame, about 3 feet above the ground.
I also have the heater placed 5 feet away from it, and the heat directed away from the gauge. I don’t use the cooling side of the outlet, but it could be attached to an exhaust fan to bring cool outside air in the room when the temperatures are too warm and you don’t want your trees to come out of dormancy.
Update: In 2019 Inkbird released a Wi-Fi version (ITC-308 Wifi) that you can control your cold frame temperature from an app on your phone. It will also send alarms to your phone if your cold frame goes out of your desired temperature. Highly Recommend!
2. Heat: I opted for a milkhouse heater, my biggest concern (and nightmare) was burning down my garage and having to explain to our insurance company what I was doing in my garage. So this seemed to be the safest option for me.
I keep a 2 feet clear space around the heater and keep it at the lowest setting. I also don’t have it blowing heat at any of my trees. The Inkbird (above) has a 1100w max, so choose a heater that doesn’t go over that setting if you purchase the Inkbird.
Tip: Oil filled heaters work great since they don’t blow hot air. The hot dry air can kill trees if pointed directly at them.
3. Fans for air circulation:
This is key for air movement to prevent mold and fungus. This will also keep the air temperature more consistent throughout your cold frame too. I use the AC INFINITY circulating fans. They have 10 speeds, from gentle breezes to a powerful airflow. I find gentle breezes is perfect. I currently have two but plan to add more. You can also connect them to an app for control from your phone.
Tip: Ensure you remove all dead leaves from the trees and container surface before placing them in the cold frame. You can also spray your cold frame with a lime sulphur before bringing your trees in. (See #7)
4. Temperature Sensors
This gives some peace of mind knowing that your trees aren’t freezing or getting too hot.
I added these temperature gauges around my cold frame, it is crazy how much it differs and can kill a non-hardy tree if it is in a cold corner close to the floor. If you purchase the Wi-Fi version of the Inkbird temperature control, you can add these to the same app. It will also send you additional alarms if there is an issue in other parts of your cold frame.
I originally used bluetooth, only because my Wifi didn’t reach my garage. But I got an outdoor WiFi extender and now I have completely switched over to Wifi devices. If you need one, ensure it is a dual band since most of these devices on use the 2.4ghz signal.
The extender is rated for outdoors, but I keep it under a covered area. These are good to -20F. Indoor versions are only good to 32F.
5. Water
Watering your trees over the winter is still necessary. You don’t want them to dry out but you don’t want them soaked all winter, you still need the balance of water and oxygen.
On average, it seems that watering once a week is fine, but that changes on how often my heater kicks on. I keep a few 1 gallon jugs constantly filled plus two of these 5 gallon jugs for quick watering with a watering can.
Tip: Ensure your water is acclimated to the cold frame temperature before watering any of your trees. For better water quality – I try and save as much rain water I can in the fall to use over the winter.
Using boot trays under your trees will keep your area clean and prevent water dripping down on the trees below. I purchased these from IKEA.
6. Lighting
LED Strip light – 4500kI recently upgraded my cold frame and added LED lighting. I have lots of conifers and have heard they slowly seem to suffer without the light. Deciduous technically don’t need light since the foliage is gone for the winter, but I have heard they do better with light over the winter. Update: My conifers are much healthier and vibrant green after coming out of the cold frame with lights on them.
I use a manual timer to slowly decrease the light until December and increase it slowly until April, hoping this will transition them better when I bring them outside in the Spring. Make sure to put them in the shade for a few weeks when you bring them out in the Spring!
Note: These lights usually cast a weird purple light that makes your trees hard to read, but the new ones are a warm white light! I highly recommend so you can see our trees better. You can also purchase LED florescent lighting too, anything to give your trees some light over the winter.
7. Putting your trees away for the winter
Typically in early September I start adding systemic fungicide and pesticide so the tree has time to suck it up before going dormant.
But once the temperatures go down to the mid 20’s at night, I start moving my trees in. Everyone has different thresholds they are comfortable with, so there isn’t a rule to follow, this is just what I do. Note: I do not have tropicals, and I believe they should be brought it once it drops below 50F.
Ensure you remove all your fertilizer, debris and all leaves on your deciduous trees. You want to keep you cold frame clean to avoid any fungus from spreading in your cold frame.
2022 Update: I now spray the trees with lime sulphur (previously copper) before putting them away for the winter. I even spray my cold frame walls with lime sulphur before putting anything in. Spray all your trees with it. I use 4 TBSP/Gallon, but follow the directions, it depends on concentration. Tip: Veterinary lime sulphur is much cheaper!
In the spring, be careful taking them out and keep them in the shade for a couple weeks. The foliage will also be sensitive to frosts, so you will have to bring them back in if frost is in the forecast.
Please take advantage of the classes MBS offers, each class covers seasonal care for your trees and all my information above was collected from long-time members and classes.
That’s it! I hope this helps you get started building your cold frame. Comments are now closed.
Thankyou so much! The info is so simple but perfect for us northerners. I lost a lot of trees last winter and I resolve to eliminate that this winter.
Great article Celina!