Ginko Biloba in autumn colors. Photo: Mactographer, Wikimedia.
There are many species of bonsai, so how do I prepare for winter?
Many species, such as junipers and certain types of spruce and pine, can be very hardy. Several can even withstand being outdoors during the winter.
Then we have many of our favorite deciduous trees such as apple, Japanese maples, hawthorns, and others that don’t like the cold as much. Plants whose crowns can withstand many degrees below freezing while their roots are a bit more sensitive and cannot tolerate many degrees below zero.
In addition to these, we have Mediterranean trees such as olive, citrus, and others that need to stay above freezing and only, in exceptional cases, withstand occasional frosty days.
Finally, we have indoor trees, which usually endure indoor temperatures year-round, but shed a lot of leaves if they don’t receive supplemental lighting. Others tolerate reduced light but would also appreciate a cooler winter down to the 10-degree mark or so.
I cannot provide you with exact directives for your specific species, but here are some suggested solutions that I or some of our customers use…
I gather the hardiest plants together in a sheltered part of the garden under some trees, surrounded by bushes. I place all the pots close together, with the least sensitive ones on the outer edges. Then, I spread dry leaves around them, or preferably spruce branches if I have them available. Spruce branches do not absorb water and do not promote fungus in the same way as leaves do.
One of the simplest methods, especially for less sensitive trees, is to either bury the pot in the ground or take your bonsai out of the pot and plant it directly in the soil. Perhaps add some extra leaves for additional insulation. Note that you should not cover the crown of the tree as it may be harmed. I have successfully used this method on junipers and pines. When the snow arrives, I shovel fluffy snow on top as extra insulation.
Another method is to build a small box with glass or plastic covering. This protects against drying winds (which are particularly troublesome for evergreen trees and maples). It also guards against the most extreme cold. Just ensure that ventilation is good, so humidity doesn’t become an issue. An unheated greenhouse might be even better due to the slightly larger air volume. If you have a greenhouse full of tomatoes and melons in the summer, it might be empty come winter. I usually let the trees experience frost first before bringing them in, so they have a chance to feel that winter is approaching.
I cover the most sensitive species in the greenhouse with bubble wrap to protect them. However, note that it needs to be removed on warmer, humid days.
Be careful with the water balance! Do not water your trees when there is a risk of frost. When there are a few warm days, just ensure that the soil is slightly moist, perhaps by lightly spraying the surface. Many trees perish due to dehydration during the time they cannot take up water because of the frost. It is therefore important that the tree has the opportunity to absorb moisture during warmer days. The root growth of many species begins at temperatures as low as 5-7 degrees Celsius. And, even root growth requires water.
One method to protect the trees can be through heat mats or heating cables. Note that if the temperature is consistently above plus five degrees for an extended period, it can no longer be considered winter dormancy, and you risk exhausting the tree. It’s better to maintain a few degrees below freezing during the coldest parts of the year. This way, the tree gets proper rest.
Once again. These are examples. You should base your decisions on the type of plant you have, your local climate, and the size of the pots. But perhaps it helps you in some way.
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