Your Plants Need a Vacation-Dormancy

Over time our plants have become so accustomed to a dormant period, that to delete it can harm the plant. Whether the plant goes dormant to escape the heat, as in the South, or to escape the cold here in Michigan, these periods of rest are crucial to the survival of the plant. They become particularly tricky when we deal with bonsai. Temperate climate woody plants must go through a period of cold dormancy to survive, it is a matter of necessity, if you do not give them this cold dormant period they will die.

Evergreens/Deciduous (those that drop their leaves)

While deciduous plants get a total rest, dropping all their leaves allowing the tree to send all its sugars to

 the roots where they are stored (that’s why we tap maples for sugar after leaves drop), so the roots still need to be maintained.  The poor evergreens do not get the winter entirely off, still plugging along, albeit at half speed. They must continue to photosynthesize and resperate as they always do, just at a much lower level than in the growing season.  

The zone numbers given in the USDA Zone system are for mature top growth.  This is fine for landscape purposes, but it does not really work for container plants, including bonsai.  Roots do not undergo the cold hardiness acquisition that the top of the plant experiences.  In general they are much more tender and susceptible to freezing temperatures.  Since Bonsai are in containers, the roots experience the same freezing temperatures as the top of the plant. Use the 15 degree rule of thumb for temperate plants in containers.  http://www.absbonsai.org/articles/overwinter.html

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 40F for very long. Evergreens stored in the dark at temperatures around or below freezing (32F) 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. If you have your bonsai stored out of doors, 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. 

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 (see the article Freeze Damage in Woody Plants noted at the end of this article). Let them stay outside and unprotected until the low temperatures are around 25 to 35 degrees at night. 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 it begins to freeze and snow.  Optimum placement would be a location sheltered from wind near the foundation of the home, or under the cover of a larger tree.  Do not mulch in on the sunny side of the 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 early, only to be caught by a freeze later on. Avoid low lying area if possible, because cold air accumulates there and freezes are harder.   Make sure you have not picked a spot where your plants might be damaged by falling ice.   Next, dig a hole deep enough so that the pot, when placed inside, will be slightly lower than the soil line. Wet pine bark mulch should be filled in around the sides of the pot inside the hole. After, a top layer of pine bark mulch should be placed on the surface of the bonsai pot's soil to make it even with the surrounding soil line. The pine bark serves a few purposes; it insulates, keeps the tree watered, and makes it easier to take out of the ground later. After the first snowfall, cover your plants with snow and make sure that they stay covered with snow all winter.  Lastly, a plastic sheet can be placed over the bonsai as further protection if freezing precipitation is forecast.  Also, trees with fleshy roots can be protected by wrapping the pot in several layers of burlap, which allows air and water to pass through, but provides adequate insulation.

Another option, building a tent or some such structure over the benches you display on will usually work, or under a wooden deck.  Some folks un-pot their trees in early fall and plant them back in the ground on top of a piece of slate or the like (to prevent deep rooting).

Some have greenhouses that they partition, leaving part barely heated for the dormant trees. An unheated porch area, the corner of the garage away from the fumes, even an unheated room in the house can be enough cold to provide dormancy (three months at or below 50 degrees will treat that Japanese garden juniper nicely). You can even fool trees into thinking they’ve had a full dormancy by leaving them out for the first two months (watch for hard weather) and then bringing them in (I don’t recommend doing that too many years in a row).

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 15F. 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 can shine 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 40F for any appreciable period of time. This you can monitor 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 15F. Small space heaters can be installed with a good low reading thermostat to heat the enclosure when the temperature starts to fall below 20F. High temperatures should not rise above 40F for more than a few days at a time.  If the temperature rises too much the tree may come out of winter dormancy too early. If this method is used, it helps the roots to keep the soil more dry than wet, but not completely dried out. Additionally, inspect the tree to make sure there are no lingering insects that may try to make a meal of your tree.

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 only have a few plants and you need to give them the required dormant period and plan to return them to an indoor environment in late winter. The constant 35 to 40F temperature of most refrigerators is ideal for winter storage.  Take them out weekly and check to see if they need watering.  At a minimum keep them in the refrigerator for 6 weeks, longer is fine.  After that time they will have the 1000 hours of chill considered for most temperate climate plants.

This means watering is still our primary duty; although the requirements have certainly dropped, we still need to check the pots and water if they are going dry (I mean dry, over watering at this point can be deadly if you are in a freeze thaw cycle; the constant up and down of waterlogged soil freezing and refreezing will break roots).

When Should I Bring Them Out?

After dormancy requirements have been satisfied and the temperature is allowed to rise above 40F for more than a few days, temperate climate plants will begin to grow. This will occur in the total dark. 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. They 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.

Dormancy is a brief respite from the rigorous needs of our trees, not a vacation. But armed with the right knowledge and tools you are prepared to keep your tree until you both reach old age.

This article has been prepared with information from various sources, but mainly the authors below.http://www.evergreengardenworks.com/frzekill.htm

by Andy Walch

 Over Wintering Bonsai

by Brent Walston