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Over Wintering Bonsai

By Brent Walston


Bonsai need protection from killing cold temperatures in winter. The degree of protection depends upon the severity of the winter in your area and the species that you grow. Protection can be as simple as bringing them inside the house a few times on the coldest nights, or it can be a complex scheme to store your entire bonsai collection for the whole winter.
Do I Need Winter Protection?

If you grow only temperate climate plants (those that freeze in the winter in their native habitats) and you live in USDA Zones 8 and above, you will rarely need freeze protection. The rule of thumb I use is:
No winter protection is needed for temperate climate woody plants until the temperature falls below 15 degrees F (-10 degrees C).

Below this point, some kind of freeze protection is needed. I live in USDA Zone 8 and each year I prepare all of my container plant areas for freeze protection by programming the irrigation system to come on if I expect the low temperature to approach 15 degrees F. When water freezes, it releases a good deal of heat. The temperature of the ice does not fall until all this heat is released and radiated into the surroundings. In addition, the ice forming on the plant can insulate it somewhat, protecting it from falling air temperatures. This form of frost protection is widespread in the orchards and vineyards of our area.

Tropical bonsai, in general, need protection from freezing. This means bringing them in the house in the winter or keeping them in a greenhouse. Subtropical species that are tender but can tolerate some freezing must be protected from temperatures that will kill their small branches or roots. This varies by species and you must do some research to find out how low a temperature they can tolerate. In general, these species have shallow or no dormancy requirements and can be brought into the house if a suitable environment can be established.

Other simple, temporary freeze protection measures include placing a tarp or plastic film over the plants for the night and removing it during the day, or moving the plants to an unheated, but protected area. These are methods for minimal freeze protection.

If you live in zones colder than 8 and experience winter temperatures below 15 degrees F, then you need more protection.

Zone Numbers for Species to Determine Freeze Protection Levels

The numbers given in the USDA zone system are for mature top growth. This is fine for landscape purposes, but it doesn't really work for container plants, including bonsai. Roots do not undergo the cold hardiness acquisition the top of the plant experiences. In general, they are much more tender and susceptible to freezing temperatures. This is not much of a problem when the roots have the great insulating capacity of the earth, but in container plants the roots experience the same freezing temperatures as the top of the plant. This is the reason I use the 15 degree F rule for all temperate plants in containers. Some species can indeed survive root temperatures lower than this, but it makes a good guideline for the majority of temperate species.

Why Can't I Just Bring Them in the House?

Temperate climate woody plants must go through a period of cold dormancy to survive. This is not just a good idea, it is a matter of necessity. If you do not give them this cold dormant period they will die.

Serious Freeze Protection in Cold Areas

In areas colder than zone 8, you must prepare your plants for winter protection. This begins in the fall by allowing your plants to experience the full brunt of the early freezes to trigger their cold hardiness mechanisms. Let them stay outside and unprotected until the low temperatures start to fall below about 20 degrees F. At this point, begin your cold protection plan.

In areas of heavy snowfall, nature will do most of the work for you. Make sure that your plants are well watered before the freezes and snows hit. Before the earth freezes, heel your plants into the ground and cover the tops of the pots and lower stems with a mulch such as pine, cedar, or fir bark. After the first snowfall, cover your plants with snow and make sure that they stay covered with snow all winter. It's as easy as that.

If you don't get a lot of snowfall, or you can't depend on a regular snowfall, then other measures must be taken. One method is to build a cold frame to house your plants in the winter. There are many plans for cold frames and all of them will work as long as they keep the temperature from falling below 15 degrees F. I strongly recommend that you place a minimum-maximum thermometer in the cold frame to monitor the temperature. Cold frames do have one serious disadvantage--they can heat up if the sun shines through a transparent housing. For this reason, it is best to place it out of direct sunlight or construct the covering with translucent, not transparent, materials. You do not want the temperature to rise above 40 degrees F for any appreciable period of time. You can monitor this with your min-max thermometer. In very cold areas, heating cables can be installed in the floor of the cold frame.

Another method of freeze protection is to house your plants in an unheated garage, basement, or other structure for the winter. Again, it should be monitored to make sure that the temperature does not fall below 15 degrees F. Small space heaters can be installed with a good low-reading thermostat to heat the enclosure when the temperature starts to fall below 20 degrees F. High temperatures should not rise above 40 degrees F for more than a few days at a time.

A more bizarre, but perfectly acceptable, method of over-wintering is to keep your plants in the refrigerator during their dormant period. This works well if you have only a few plants that require a dormant period and you plan to return them to an indoor environment in late winter. The constant 35 to 40 degree F temperature of most refrigerators is ideal for winter storage.

What About the Need for Light?

Dormant deciduous trees have no leaves and do not need light until they begin growing again. Dormant evergreens do not need light as long as the temperature does not rise above 40 degrees F for very long. Evergreens stored in the dark at temperatures around or below freezing (32 degrees F) will survive the winter nicely.

Do I Need to Water?

Many areas will have warm spells during the winter that will warm the earth and your bonsai above freezing. This is a good opportunity to uncover them and check for dryness. If they need water, give them a good soaking and replace the mulch and coverings. Always check to make sure that the bonsai itself is getting water. Mulch has a way of shedding water and it may not get through to your plants.

When Should I Bring Them Out?

After dormancy requirements have been satisfied and the temperature is allowed to rise above 40 degrees F for more than a few days, temperate climate plants will begin to grow. This will occur even in total darkness. Therefore, you should plan to have your plants introduced to sunlight when temperatures reach this level. In some areas, spring weather can fluctuate wildly and you must be prepared to protect plants from sudden freezes.

And Finally...

Bonsai is a wonderful and diverse art. Trees can be grown indoors and outdoors, stored for the winter, or manipulated to give them dormant periods. There is no reason to let your natural environment limit your possibilities. But you must pay attention to the needs of the individual species and the limitations of your space.

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