Overwintering bonsai
From BonsaiWIKI
Most native tree species are well adapted to handle the change of seasons. We enjoy the fall color and winter silhouettes of trees in our city parks and gardens without any doubt that they will burst forth in spring with beautiful blossoms and luxurious growth. It would be tempting to believe that these same native plants which thrive in our urban landscapes should have no trouble overwintering as bonsai. In fact, native species generally present the fewer problems than exotics when grown as bonsai. But, occasionally an unpleasant Spring Surprise is in store: the loss of a favorite tree or a well-shaped branch that enters a dreaded state of "permanent dormancy."
Let's look at some of the reasons for the difference between our bonsai and the trees that grow in our yards and nearby landscapes. Understanding the differences will help us avoid problems and help ensure your trees survive the challenges of winter
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SIZE DIFFERENCES
Even small landscape plants are larger than our biggest bonsai. This size provides them better insulation and resiliency than bonsai, which are small and finely twigged. If a landscape plant loses a few inches of growth due to a cold snap, it bounces right back. On the other hand, with a bonsai, a few inches is all the plant we have!
DIFFERENCE IN LOCATION
Landscape plants are rooted firmly in the ground. This keeps their roots protected in the earth, where a few inches beneath the frostline, the soil remains unfrozen. By moving trees into bonsai pots and lifting those pots several feet off the earth onto growing benches, we expose their roots to more extreme ranges of temperature than they would encounter in the ground. Pots lose heat to the air, are more exposed to wind chill and frost, too. When a bonsai's root ball becomes frozen throughout, the tree is unable to take up moisture. If this condition persists, the plant may "freeze-dry" as the soil dessicates, or there may be less severe symptoms - branches wither and die.
SPECIES VARIATIONS
As plant enthusiasts, we also tend to like to grow plants that are not native to our areas, and are perhaps less adapted and more prone to frost damage than plants commonly seen in our urban landscapes. Before you buy that exotic species, be sure to educate yourself as to whether it's adapted to your climate.
For these reasons, it's important to take steps to ensure our trees' survival! The steps you take to overwinter bonsai are dependent on both your climate and the frost tolerance of the plants you are cultivating. In any case, you should be prepared for emergencies, even in relatively mild climates. Discuss overwintering practices with experienced bonsai enthusiasts and professionals in your area.
Some steps to take
Depending on the severity of winter in your area might include the following.
LEARN ABOUT YOUR TREES
Identify your most susceptible plants using a botanical guide, or the US Department of Agriculture's Plant Hardiness Zone Map. Don't confuse normal plant response with trouble signs. Deciduous trees drop their leaves. Conifers shed lots of needles in very late Fall. You may notice color changes in conifers like juniper, false cypress,cryptomeria or pine - some turn from their normal vibrant greens to gold or rust-tinted hues. Broadleaf evergreens like azalea may turn dark red or even crimson.
KEEP NOTES
Keep notes this year and use them to refine your practice next year. Some plants, especially conifers like pines and junipers, can shrug off the worst cold most anywhere. Even a tropical, like Bougainvillea or a fig or Citrus bonsai in Southern California or Florida might present no problems. In colder climates, they're on the watch-list!
MILD WINTER CARE
Move frost sensitive plants closer to your home, or to sheltered areas of the yard. Move plants from the benches to the ground - under the bonsai benches, to a cool garage, or if it's appropriate, indoors. My ficus benjamina bonsai thrives outdoors 3 seasons each year. But at first sign of frost, it becomes my only "indoor bonsai!"
MODERATE WINTER CARE
With the trees beneath, cover bonsai benches with poly tarp - white is generally recommended because it helps reflect light that could create too much heat. Any opaque color should do. Most deciduous trees can tolerate frost. They do not enjoy having the root ball frozen, however, so if you can prevent this, they will have a higher probability of surviving winter.
SEVERE WINTER
Soil heating cables may be used to keep the plant warm. These can be buried in the pot directly. I've even seen minature christmas tree lights used on larger bonsai to keep them warm! Protect your bonsai in unheated greenhouse enclosures, sheds, polyethelene covered structures (poly tunnels) or cold frames. In the most severe climates, trees can be brought into heated enclosures or deciduous species can be buried below soil level in a mulch that will protect them until spring.
WHAT I DO
Winters are usually very moderate near my home about 60 miles south of San Francisco. Occasionally, we have had deep or extended freezes lasting up to two weeks. It pays to be prepared.
I have an unheated greenhouse that I use to protect trees in midwinter. I have found that I can store dozens of leafless trees in this relatively small space by packing them in densely and nesting the trees' pots carefully. I check the plants every few days and water anything that appears to be drying out. The trees go into the structure in late December and emerge in February. I have found that the smaller buds on the maples and elms, particularly on the older, more refined bonsai do much better if they are protected for the 4-6 weeks that make up the worst of our winter in this mild climate.
If cold spells are predicted, I leave a couple incandescent 100-watt lights burning overnight on a 24-hour timer, or use a thermostatically controlled electric heater to buffer the temperature. Always be especially careful when you have electricity anywhere in the vicinity of water! Ground-fault outlets are a great idea, or you can buy a Ground fault adapters to interrupt current in the event of a short circuit. You can buy an extension cord with a Ground Circuit Fault Interruption (GCFI) adapter at your neighborhood hardware store or home improvement center.
FINAL THOUGHTS
Idle winter time is a great for getting ready for the spring growing season. Clean, sharpen and oil tools, prepare bonsai pots for repotting, sift and mix soils, enjoy deciduous trees' leafless silhouettes, and take advantage of this quiet time to make your plans for the coming year. In December we will be getting prepared for spring grafting, which will need to be undertaken fairly soon.
