by Earl Cormney

This article first appeared in our newsletter in 2000. Since we are having a ‘Winterizing Your Bonsai' meeting on Nov 22, I thought this would be a good time to dust it off and run it again to refresh everyones thinking on putting your bonsai to sleep for the winter - ED.

Much is written about how to care for bonsai during the growing season, but little is written about winter care. Pruning, wiring and proper watering can all be lost if the tree is not cared for during the cold months. With proper wintering techniques, and if mother nature is not too severe, our cold months are a time for us and our trees to relax and prepare for Spring. The winter care I am referring to deals with only outdoor bonsai. Indoor or tropical bonsai need constant growing conditions such as warm temperatures, water and light.

One reason not a great deal is written about winter care is because of the differences in climates throughout the world. In Japan where mild winters are the norm, most trees remain on their stands all year. Unfortunately our cold months are more severe. There are several different methods which work well for wintering bonsai. All methods have a few similarities. In all techniques the ideal situation is a cool environment with minimal temperature fluctuations. A factor that is not required for winter care is light. Deciduous trees, and even pines and junipers do not require light while they are in a dormant state. Bonsai will still need water but at a much reduced rate.

One technique for wintering trees is using a wooden box without top or bottom. Place your potted trees inside the box on the ground and add wet mulch up to the first branch on your evergreens and as high as you want on deciduous trees. Be sure that you have mulch between your pots so that each pot is covered and surrounded by wet mulch. The sides of the box will shelter the trees from the wind and hopefully any varmints that may wish to make a snack of your trees. By having an open top rain and snow will help provide moisture to your trees. Even though they are dormant bonsai still should not completely dry out nor should they stay soaked. Watering is not needed unless you experience a drought during winter and make sure the pot drains well before temperatures fall below freezing. If you do water choose a warm day and water early so by nightfall all excess water is out of the pot. Too much water in a pot during cold temperatures can cause a pot to freeze and burst. If possible locate your box in a shaded spot so that the sun will not warm your trees daily. It's the cold-warm, cold-warm conditions we try to avoid. We need our trees to go into dormancy and stay that way until spring. In especially cold conditions, prolonged temperatures below 20° F, a lid can be used along with a heater (a light bulb on a thermostat works well). I have never needed to use a heater for my trees since our last few Winters have been mild with daytime temperatures usually climbing into the 20's or more. With mulch acting as a blanket and the trees being on the ground, temperatures have to be really cold for a prolonged period to cause problems for our trees.

Another wintering technique that is often used involves a storage shed or a detached garage. Trees can be placed in an unheated building for winter but this has certain advantages and disadvantages. On the upside you can utilize a shelter without taking up any additional yard space, using existing structures. Since temperatures in a shelter will stay on the warm side by being protected from the wind and frost, mulch is not needed. By the trees being easily accessible, pruning and wiring can be accomplished before the Spring rush. Cascade style bonsai will conveniently rest on a shelf which can be more difficult to care for if you choose a different wintering technique. On the downside, about once a month the trees will need to be watered as they dry out without the benefit of rain or snow. Temperatures inside a shed will probably not drop too severely. A heater or light bulb can easily be hooked up and utilized if drastic weather persists. On warm or sunny days temperatures may rise more than is needed. An open door or window will help but may not be enough. If temperatures stay warm inside long enough buds will swell and your bonsai may come out of dormancy earlier than expected. When the trees think that Spring is here and buds pop, they will need light and warm conditions which may create some problems if this occurs too soon.

If you have a small collection, both in number of trees and size, a refrigerator works well. Inside a refrigerator your trees will have constant temperatures but will occasionally need water. This technique works best if wintering only a few trees and if you live alone.

A cold frame can create an excellent method for caring for your trees. A cold frame is usually constructed a few feet below the ground with insulated walls forming the sides. A lid is attached but can be raised or lowered to adjust for temperatures outside. A light bulb can add additional warmth but is rarely needed for our climate since the trees are actually below ground level. The trees will not require a layer of mulch so they can be removed for wiring and pruning in your spare time. If you decide to construct a cold frame, the bottom can be dirt with a thin layer of sand or gravel for drainage. Use insulated walls and a lid to keep temperatures constant inside. Also make your cold frame large enough to accommodate your tallest tree in the pot and allow for any additional growth you expect in the future. You may also wish to allow room for additional trees as your collection grows. Cold frames are used by many bonsai enthusiasts in much colder climates.

Whichever technique you choose for wintering, there are a few key points that will help provide success.

• Winter healthy trees. Healthy trees survive better. Don't use a high Nitrogen fertilizer after early August as this will promote leggy growth and not prepare the root system for winter.
• Have good draining soil. Poor draining soil will choke roots. If too much water is in the soil during freezing temperatures roots and even the pot can burst.
• Don't allow the tree to dry out completely. The trees will still require water but at a much reduced rate. If your trees don't receive rain or snow then they will need to be watered approximately once a month.
• Moderate temperatures without quick fluctuations. Trees should stay between 25-50 °F with 35-45 °F being the best.
• Certain mulches work better. I use cypress mulch but hardwood mulch also works. Mulch made from leaves or pine needles may work but not as well since they don't hold temperatures or moisture as constant. An old pair of pantyhose around your pot can save you time cleaning your pots in the spring.

Wintering our trees does not mean putting our bonsai away until Spring. The colder months will allow for preparation for the growing season to come. There are always tools to sharpen, wire to anneal, pots to scrub, soil to sift, stands to build.