All living things require a rest period. For humans its eight hours of sleep per day. For trees, including bonsai, its wintertime. This resting period is called “dormancy” and refers to that time frame in which the tree’s growth either slows down greatly or comes to a complete halt. Like a bear hibernating for winter, the bonsai’s vascular and cellular activity is reduced as it begins to store up energy for the coming spring.

It can be a confusing and troubling time for bonsai enthusiasts… but it need not be so. Understanding what is happening to the plant is the key to success.

Beginners think that allowing a bonsai to freeze will kill it, (and in the case of tropicals, it’s true), but the vast majority actually need the cold in order to put them into “sleep mode” for the winter. If they don’t get it, they are like marathon runners who never rest. They begin to slow down and gradually lose vigor. Eventually over time, they simply run out of energy and die. This is what happens to temperate bonsai which are brought inside the house during the winter months because their owners fear that exposure to cold weather will kill them. Providing correct winter dormant conditions for your bonsai will allow them the rest which is essential in order to remain healthy and happy.

Depending upon what part of the world you live in the provisions which you make for wintering your bonsai will vary. Here’s how it works.

Some trees can withstand more cold than others. That is to say, one variety is more “cold hardy” (or capable of withstanding lower temperatures) than another. It’s common sense. There is a reason that Japanese maples don’t grow in Miami Beach and Palm Trees don’t grow in Montreal. If you don’t know how much cold your bonsai is capable of withstanding, then the first thing you should do is find out. Head for the library or the internet and make a list of the hardiness ratings of all the trees in your collection.

The next thing to do is determine the minimum temperature range for the location in which you live. The US Department of Agriculture has a chart prepared which divides each section of the country into climate hardiness zones, based on the historical low temperatures for that region. This information is also readily available with a little research. How much winter protection you need to give your bonsai will be based on the individual trees cold hardiness and on the climate extremes in which you live.

Insofar as cold hardiness is concerned, all trees can be divided into one of four categories:

Tropicals – Those plants which must be protected from freezing.

Semi-Tropicals – Those plants which can withstand cooler temperatures and a moderate frost.

Temperate – Those plants which can withstand extended periods in a cold or frozen condition without cellular damage.

Hardy – Those plants which can withstand extremes of cold (i.e. – sub zero temps) without incurring cellular damage.


The transition of a bonsai from an active growing condition into a dormant state MUST be a gradual one. The process takes place normally and naturally as the cycle of the seasons progress. In summer the tree grows and develops. As fall approaches, the trees growth and vascular activity gradually begin to slow down and in winter the tree becomes inactive. As springtime approaches and temperatures begin to improve, the tree gradually awakens from its dormant state and begins to grow again. In the case of temperate and hardy bonsai this gradual transition into dormancy is primarily activated by temperature. In the case of tropical and semitropical bonsai, the dormant period is primarily activated by light because winter days are shorter and winter sunshine is less intense than summer.

Tropical and Semi-Tropical Winter Protection

Trees which fall into this category (i.e.- ficus, fukien tea, pomegranate, serissa, etc.) will need to be provided with winter protection when nighttime temperatures begin reaching the 50 degree range. In no case, should they be allowed to freeze. Most can withstand a light frost, but extended exposure to such cold will certainly result in the severe damage or death of the bonsai. Unlike temperate and hardy plants, the growth of a tropical bonsai does not come to a complete halt. They continue to grow during the winter months, but their rate of growth is greatly reduced. Because of this, you must provide a wintertime environment in which your tropicals can receive the proper amount moisture, humidity and light.


The obvious answer to this problem is a heated greenhouse with a climate controlled environment. It is also outside the budget of most bonsai practitioners. Most growers of tropical and semitropical bonsai bring their trees inside the house for the winter when temperatures start falling below 50 degrees. It is the simplest and most cost effective solution to over wintering, but it comes with a few problems which must be addressed.


Light is the first. Tropicals must be placed in the sunniest location possible. This might include a heated porch or a south facing window. Care must be taken to rotate the trees on a regular basis so that all sides of the plant receive an equal amount of winter sunshine.

Moisture and humidity are the next two issues. We keep our homes dry in the wintertime because it is comfortable, but if you were to attempt to provided the amount of humidity in the air that your tropical trees would prefer, you would have moss growing on your walls. Keeping the trees in a spare room along with a humidifier will certainly help. If this is not possible, place the trees on top of plastic drainage trays which are filled with pea gravel and fill the tray with water. As the water evaporates it will help to keep an envelope of moisture around the trees.

Take care not to set the pot directly into the water, as this would keep the soil too moist and encourage root rot. Keep a spray bottle of room temperature water nearby and mist the bark and foliage of the bonsai several times a day.

Likewise, if you cannot provide a sunny window, you must provide artificial lighting to compensate. Special and often expensive grow lights are not usually necessary for bonsai. Plain 40 watt fluorescent lighting will do the trick, so long as the bulbs are located within 3 or 4 inches of the plant and are on for about 15 to 18 hours each day. Be on the lookout for insect problems as well. A variety of multilegged critters are fond of the dry conditions in our homes in the winter time and will happily eat your bonsai if you are not attentive.


Certainly maintaining bonsai in the house during the wintertime can be more troublesome than wintering temperate trees outside, but tropical owners also have the added pleasure of being able to enjoy their bonsai all year round.

One of the most innovative solutions to maintaining bonsai indoors we have seen came from a friend who used the tub in a spare bathroom to overwinter tropicals. He placed a long narrow bench in the center of the tub and hung fluorescent light on a timer above it. He kept about 2 inches of water in the bottom of the tub to provide humidity and was able to close the bathroom door so that the increased humidity and light did not disturb the rest of the house. The commode, with the lid closed, turned out to be the perfect wintertime workbench and the sink was a ready source of water for the trees. The shower head could also be used to flush away insect problems if necessary or just give the trees a wintertime thundershower.

Necessity is the mother of invention and many solutions to wintering bonsai indoors are possible, if you just keep in mind that the plants requirements for light and water must be met.


Overwintering Temperate And Hardy Bonsai

In over wintering temperate and hardy bonsai the objective is not to shield trees from the cold, but to ameliorate some of the more extremes of winter. They NEED the cold in order to emerge healthy and happy in the springtime. At the same time, nothing on earth is particularly fond of 20 degrees below zero or minus 40 below wind chill factors.

Most people are not aware that the root system of a plant is much more sensitive to cold than the top of the tree. It makes sense, when you think about it, but it also puts bonsai at somewhat of a disadvantage compared to trees growing in the ground. Cold can only attack landscape plants from directly above. However, a bonsai is in a pot, sitting on top of a growing bench. When the cold comes it will not only attack the roots from above, but also from all four sides of the pot. Because the pot is elevated on feet, it attacks the underside as well. Because of this, a certain amount of winter protection is prudent even though the bonsai is considered completely hardy in your climate zone.

At the beginning of cold weather, temperate and hardy trees should be left on their benches and allowed to freeze once or twice. This will cause them to shift into dormancy. Deciduous trees will drop their leaves and evergreens will shift from the bright green they show in summer to a steely blue green in winter. When you are reasonably sure that they have shut down for the winter, they need to be moved into a protected location. At this point they should be relocated off their growing benches and moved into an area of the garden especially prepared to provide winter protection. In northern climates this will occur sometime just before Halloween. In southern climates they will need to be put away around Thanksgiving

Exposure to the winter sun and wind can wither delicate branches and dry out buds. Bonsai should be moved to a shaded location out of the winter wind and sun. They can be nestled close together on the ground and then bark chips or oak leaf mulch can be placed in-between the pots. They may even be buried directly in the ground, pot and all. This creates an environment in which the cold (as it does with landscape plants) can only attack the root mass from directly above.

The objective is not to prevent freezing, but to reduce the daily repeated cycle of freezing at night and thawing during the day were it to be simply left on the growing bench. Once trees are “tucked in” for the winter, all you need do is water the trees and bark mulch thoroughly and check ever week or two to make sure they are not drying out. Since they are dormant and in the shade, they will use much less water than usual.

There are a number of variations on this system. Some growers place them under an elevated deck at the back of the house. Some keep them in an unheated garage. Just remember that the objective is to soften the blows mother nature gives us in the winter…. not to prevent them.


Winterization In Extreme Climates

For those of you who live in more northerly climates the overwintering procedure described above will only be useful for plants that you have determined are hardy to your climate zone. For temperate, but non hardy plants, you will have to create an environment which provides sufficiently for cold storage but will not permit the extremes of cold that would normally kill that particular species. Such a device is usually referred to as a “cold frame” and can take a variety of forms.

Remember that if your trees are dormant, they are not conducting photosynthesis and do NOT need light. Yes, this even applies to the evergreens. Keeping them in the dark for the winter is not going to hurt them. A cold frame is essentially a box designed to accommodate your trees. It can be buried in the ground a couple of feet or sat on the ground surface. It can be an enclosed area under an elevated porch; attached to the side of the house; built inside an unheated garage or a box on a shaded apartment balcony. It may be constructed out of brick, cinder block, plywood or plastic, as you choose. Temperate but non-hardy trees should be placed inside along with a heat source and a circulating fan. The objective is to keep this airtight space as close to freezing as possible all winter long and to circulate air just enough to prevent mold and fungus from forming in the damp environment.

If your cold frame is fairly large, an inexpensive space heater will do the job. If it is a small space, the warmth from a light bulb may be sufficient. Make sure the breeze from your circulating fan does not blow directly on the plant material and desiccate it. Its a wise idea to purchase a high/low thermometer so that you can see what the temperature variance is over a 24 hour period. The objective is to keep the interior of the cold frame consistently just above the freezing mark.

The main thing to guard against in using a cold frame is a box which gets so heated up when the sun is shining on it that it will cause temperatures inside the box begin to creep into the growing range. This would cause trees to come out of dormancy too early and undoubtedly result in a disaster. In extremely cold climates such as Minnesota and Maine, all the moisture freezes out of the air during the wintertime. If you are operating a cold frame in such an environment you would be well advised to mist the interior of the cold frame regularly and perhaps to keep a container or two of water in the box as well.



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