Winter Care

Outdoor, or hardy, bonsai must remain outdoors during the Winter. This allows them to experience a dormant period. Without this period of dormancy, the tree will lose vigour and become susceptible to disease. It is important that outdoor bonsai go through the seasonal changes of Spring, Summer, Fall, and Winter.

The freezing of an outdoor bonsai's trunk and branches is not a problem unless the tree has already broken dormancy and put out tender Spring growth. With that said, it is equally important to note that bonsai are more vulnerable than their cousins growing in the ground. This vulnerability comes mainly due to the fact that the tree is housed in a relatively small container. The root system requires some protection during Winter, such as mulching, a cold frame, or an unheated garage. These methods are described in more detail below.

Most quality bonsai pots are made to withstand freezing. Therefore, it is not necessary to remove your bonsai from its pot for winter protection.

Watering during winter

Your bonsai's need for water will diminish in winter, mainly because it is either losing leaves or because transpiration has slowed. However, you must not let the soil dry completely, or the roots will dessicate and die. It is a good idea to water your bonsai before a hard freeze, as frozen water acts as insulation.

Check before you water

This does not mean that you should water daily! Check on your trees every few days and water only when necessary. Just like in summer, some will dry out faster than others.

Mulching

One way you can protect your bonsai is by mounding mulch 1-2 inches above the pot and around the base of the trunk. This mulch will insulate the roots and keep the tree and pot intact. This is a common method in areas that do not experience hard ground freezes. Do not mulch on the sunny side of your house or next to a wall that receives winter sun. This type of microclimate is very likely to experience early warming that will fool your tree into leafing out too eary, only to be caught by a freeze later on. Mulch on the north side or in a shaded area. Avoid low-lying areas if possible, because cold air accumulates there and therefore freezes are harder.

Unheated garage

An unheated garage or other room can house your dormant bonsai during the worst of the weather if you feel that mulching will not suffice. Be sure that your trees do not get too warm, or they may think it is spring and start leafing out. This should be avoided because if the tender leaves experience freezing temperatures, they may die and the tree with have to begin again – a very stressful process.

Cold frame

This solution is part-way between mulching and an unheated garage. Usually the tree is mulched in a raised bed with a glass covering or transparent or white plastic. White plastic is used by many nurseries because it reflects some sunlight and this protects somewhat against overheating.This method has the advantages of being both enclosed and being outside. The drawback is that it must be monitored to make sure the temperature inside does not rise too high for too long. It is higher maintenance, but can extend your growing season and protect early spring foliage from unexpected frosts.

Tropicals (Indoor bonsai)

Tropical Bonsai do best when placed outside for the Summer. They should be brought indoors when the temperatures are down around 40°. If you are sure that the temperatures will not drop below 37°, you may wish to consider leaving your tropical bonsai outside year-round. To be on the safe side, Tropicals should be brought indoors when nightly temps get down to 40-45° farenheit.

Treat your tropical bonsai for any insect infestation before bringing it in for the witner. An infestation in the middle of winter is hard to deal with because you can't very well spray toxic chemicals in your house, and you can't take the trees outside due to the cold. If you do experience an infestation at a time when you are unable to take your tree outside, don't despair. Get a clean spray bottle that has never been used for toxic chemicals and fill it with a weak soap solution. Use 1 tablespoon of soap per gallon of water. This will control aphid and scale. Spray it on your trees (non-infested, too!), covering all surfaces of leaves and branches. Repeat every week until it clears up.

Tropicals will need as much light as possible, preferably a southern exposure, close to a window. And misting with water to keep the humidity higher doesn't hurt.