Bonsai Winter Care
You read a lot of information out there on how to prepare a bonsai for winter. The only thing is most of these people that write those articles on Bonsai winter care do not live in our lovely Zone 4 but in warmer zones. You have to take this into account when reading and following their direction.
My goal with this page is to tell you what I do and to get others to tell us and share what they do. So please if you have images, videos or even want to write up something on what you do for the winter (Zone 4 or colder), please contact me.
Autumn is when trees start going dormant. The light levels start to drop outside and the temperature starts to do the same. Both of these help trigger dormancy in our bonsai. If you own any tropical trees you are going to want to get them inside or into a greenhouse when the temperature at night starts to hit 50-55 degrees consistently. Tropical bonsai cannot withstand freezing temperatures. Your hardy trees can stay outside at this point.
Before your trees are ready for winter storage (whatever your method is) your hardy bonsai need to experience a few good touches of frost. Just don’t let your bonsai roots go unprotected from temperature under 15 degrees. Brent Walston says:
No winter protection is needed for temperate climate woody plants until the temperature falls below 15F.
When the night temperatures start to hit the low 30’s consistently you will want to start preparing your bonsai. You need to clean off all the remaining dead leaves and pluck all of the dead, diseased or deformed needles from your bonsai. You will want to remove all debris from the soil surface including dead leaves, needles and the such.
Below I have included a quick 40-second video of my winter storage for my bonsai.
After losing all of my best trees to an overly long and ridiculously cold winter (2013-2014) I built this storage area (above video) in my garage. It’s 5 feet deep x 5 feet long x 8 feet tall. it is insulated with R-13 and a layer of plastic on the outside and inside. Shelving is the plastic 4 tier shelves you can buy at Menards or other home repair stores. This particular shelving allows for you to make it as many shelves as you want. I bought 3 shelving kits and made them 3 height and in turn got 4 shelves out of them. I placed boot mats on the shelves to place my trees on top of and these collect the water that runs through the trees. I use a “Milk House” space heater with a thermostat and an oscillating tower fan. I have a two bulb shop light in there so I can see what I am doing.
After the temperature has put my bonsai through a number of hard frosts I spray them with a 3 in 1 product and place them inside “the box”. I set my thermostat to 36 degrees. This triggers the heater to come on at 34 and turn off at 38 degrees. The optimal “chilling” temperature after a tree has gone fully dormant is 35-40 degrees. The trees are fully dormant by the time I bring them in so they do not need any light. I check the soil of my bonsai every couple days to make sure the soil is not dry. If the soil is drying to much I water them. I use a 1-gallon pump sprayer to water the trees in storage. The trees are dormant so you do not need to water as much as you do during the growing season but it is still important to keep the roots damp.
I think this year I am going to set my thermostat a bit lower to try and extend the dormant period and hold off the “chill” time.
Spring is Here
After your bonsai have gone dormant they each have a minimum amount of “chill” hours that they go through. As long as these minimum chill hours have been reached and the temperatures are favorable to induce growth, your bonsai will wake up. When the temperatures are consistently in the 40’s you will want to start thinking about getting your bonsai back outside. Your bonsai are going to start growing and will need light.
If your bonsai were stored inside a box like mine or in a cold frame and they have started to grow you need to protect them from frosts and cold winds.
If you buried your bonsai for the winter I hope you have kept them covered with snow all winter long. The soil in your pots are probably still frozen and the roots are unable to absorb water. As the snow melts from around your trees the foliage of your conifers are exposed to wind and sun and will dehydrate as the roots are not able to replenish the water.
Things to do in Fall
- Tropical trees are moved inside when the night temperatures are consistently 50-55 degrees.
- Hardy trees are left unprotected until temperatures are consistently in the low 2o’s. No lower than 15 degrees.
- Clean remaining dead leaves off of the trees.
- Pluck all of the dead, diseased and deformed needles from your bonsai.
- Remove any debris from the top of the soil.
- If going into a cold frame or other storage you will probably want to spray with a 3 in 1 product.
It’s getting colder and the leaves have fallen from your deciduous trees. Its been in the lower 30’s upper 20’s overnight for a little while now and you have removed all of the dead leaves and needles from your bonsai and the tops of the soil. Now you need to find a place to store your bonsai for winter before it gets too cold. Once you have found your spot and the temperature has been in the very upper teens and very lower 20’s (above 15 degrees) for a while and your bonsai have experienced a number of these day/nights, it’s probably time to start thinking about moving your trees in for winter.
If you are going to store your bonsai outside for the winter you will want to find a place that gets little sun and wind. Some people wrap the pot in burlap and bury the pot in the ground and cover with mulch up to the first branch and then cover the rest with snow. In my neck of the city, we are not able to rely on enough snow early enough.
I have seen cold frames where you dig up an area 5 or 6 inches deep and as long as you need. Place insulation board around the inside of your hole as walls. Brace the inside of your walls so they don’t fall in and crush your bonsai. Build up a layer of gravel, mulch, leaves or whatever else you would like to act as insulation on your box on the outside. Put a top/lid on it using insulation board and make it so you can open and close the lid.
Some people just store their bonsai in the garage whether it’s attached or unattached, but please do not store your bonsai on the cement floor. Put something under them. You will need to protect them from flash freezes or long exposures of cold when you open the garage door, especially if you forget to close the door.
There are a number of cold frame blueprints and ideas on the internet.
Terry White Duluth, MN
Other Reading On This Site
- Upon Us Comes the Wrath of Winter (Minnesota Bonsai Society)
- Over Wintering Bonsai (Evergreen Gardenworks)
- Winter dormancy: it’s more work than it sounds like (Friends of the Mississippi River)
- Winter Storage image gallery (Minnesota Bonsai Society)