Outdoor bonsai must live outside because they need a dormant period in order to survive. These trees have a pigment, called phytochrome, which resides in the leaves and bark of the tree. It works as a timer, sensing the amount of light it receives during the day, to get the tree ready to shut down. As a tree prepares for winter, it absorbs the nutrients from the dying leaves and stockpiles them for spring growth. The trees sit and wait, counting the days until spring arrives.
It is important to note that you MUST KEEP THE BOTTOM OF YOUR BONSAI POT IN CONTACT WITH THE GROUND ALL WINTER. If you keep your bonsai on a display stand all winter, the wind hitting the pot will drop the soil temperature drastically. This can lead to the roots getting too cold and they will die. Another note is to keep the dirt moist at all times. Most beginners hold back watering during the winter because they are afraid of the dirt freezing. When the dirt is wet, it can freeze, and it takes a lot to get ice below 32 degrees. Ice acts as insulation and helps keep the roots warmer than it is outside if temperatures drop below freezing.
Common practice is to bury your bonsai in the pot in a hole for the winter. Bury the pot all the way into the dirt and place mulch up to the first branch of your tree. Being below the ground will help keep the bonsai warm throughout the winter. The mulch will act as insulation and also help trap moisture. We do a trench every year and line the pots up side by side down the trench. After placing the pots in the trench, we fill in around the pots with the soil that we removed. We then place an inch of mulch over the top of the soil. Using this method, we have never lost a bonsai to the hard Boston winters.
Once spring arrives, the phytochrome sends the signal that it is time to get growing again. The tree uses the stockpiled nutrients to form and open new buds. Once the buds are open and the tree begins to produce energy on its own, you will see the new growth begin to form.