Overwintering bonsai outdoors

Twenty-two years ago, I was walking trough a commercial wholesale nursery searching for just the right trees for a special project I was working on.

Along with several other plants, I purchased a small tree that would later become my bonsai. While I was pondering whether or not to buy it, a worker at the nursery passed by and told me he had seen it in that same spot for the past seven years.

That little tree I brought home is a "Chamaecyparis obtusa," commonly known as Hinoki Falsecypress, Japanese Cypress or simply Falsecypress 

In their natural state, the species typically grow to be 70 feet tall, but many dwarf varieties have been developed over the years. Mine is most likely one of those dwarf varieties.

Hinoki Falsecypress are a favorite species among bonsai artists in Japan. They are also traditionally considered a sacred tree in the Shinto faith

When planted directly into the ground, these evergreens are hardy to zone 4 and as such, they require exposure to winter temperatures for a period of time as part of their lifecycle. One way to make that happen with potted bonsai is to keep them outdoors but in a sheltered spot.

Since zone 4 low temperatures can dip to -30 degrees, that temperature is really not an issue for these plants in my area in zone 6. Instead, bright sunshine and drying winds are what we need to watch out for. 

Winter winds can evaporate water from evergreen leaves, and if the soil is frozen, the plant will not be able to move water up to the leaves to replace lost moisture. As a result, leaves become desiccated and eventually die. Bright sunshine can exacerbate the problem.

To make sure bonsai trees have enough moisture for the winter, they must be watered well before being tucked away for winter. 

Alternate freezing and thawing can damage roots from “heaving”. The wide temperature fluctuation causes water to expand when frozen into ice, putting pressure on the roots. As the ice thaws the pressure is released and liquid water moves into any empty voids present in the soil. Additional freezing causes pressure again and once more presses against the roots. If these freeze-thaw cycles happen often enough, roots will be damaged permanently. Sometimes the plant can be pushed right out of its pot and often the pot itself is cracked open. 

I deal with this problem by overwintering my bonsai under a group of white pine trees. I’ve tried other areas such as under wood chip piles, under straw bales, in a partially heated hoop house, andunder homemade teepees, but finally settled on this spot.

The pine boughs shade the area and very effectively slow down air movement from the wind. 

I also partially bury my trees to take advantage of the insulating effect of the soil. The surrounding soil freezes much more slowly than a small pot of soil and conversely, thaws out very slowly too. That effectively eliminates the freeze-thaw problem. 

As you can imagine, the first time I dug the hole for storage, there were lots of pine tree roots to contend with. Over the years, the roots have been pruned so much in that spot that only a small amount of fine roots make their way into the hole, which makes for easy digging.

White pines drop needles every year, and they make a superb mulch to blanket my plants with. They combine with the soil to provide even more protective insulation for the trees. I just rake up however many pine needles I need from the immediate area. They work better than most types of mulch because they tend to stay drier and fluffed up more all winter. Many other kinds of mulch can become waterlogged and matted over time. 

Because of the mild late fall and early winter, this year was the latest in the season I put away my bonsai. There was no point in putting them away any earlier with relatively mild temperatures and frequent rain so I left them out to enjoy an extended autumn as long as possible.

With single-digit temperatures sure to come our way, my bonsai will make it through another Michigan winter securely tucked away in their sheltered winter home.