Wintering Bonsai

by Mike Roussel

I would say that the most important aspect of bonsai cultivation is simply keeping the trees alive. It really is pointless to learn how to style your trees if they are just going to die over the winter. I have had my share of "permanent dormancy" so I spend a lot of time making sure my trees are adequately protected during winter. I like to err on the safe side. I have read many articles on wintering bonsai and there are tons of great ideas out there so instead of reiterating what others have said I will simply share with you what I do with my trees, providing some tips and tricks I have learned on the way.

We are very lucky here in Eastern Ontario because we can grow virtually any species suitable for bonsai cultivation. It is important however to note that not all trees can be treated equally because of the different native climates in which they normally grow. To make things simple, I group my trees into three categories: tropical/indoor, tender, and hardy.

These days, if I'm in doubt, I look up the tree on the internet (use the full Latin name).

Tropical trees are typically grown in a frost-free environment. Below are some examples: fig, fukien tea, olive, jasmine orange, pomegranate, tree of a thousand stars.

These trees will drop their leaves and die if they are hit by any significant frost and most cannot stand even cold temperatures (<10C). As a result, these trees must be brought inside in the early fall. Keeping tropical trees happy and healthy requires regular watering and feeding all winter (although diminished), as there is no true dormancy period. You must avoid any nasty drafts from an open window or door. Typically you will have to put these trees in a greenhouse, a sunny window (south facing is the best) or you will have to provide artificial light. Keep in mind that you must continuously turn the tree so it receives light on all sides since you now have directional light and as we know, plants grow towards the light source.

Be very careful to check for bugs. Spider mites really like the warm, dry environment of a home and will multiply quickly if left unchecked. One thing that I do periodically is to take my trees into the shower and hose them down. You can use a sprayer but be sure to get the undersides of the leaves wet, where those nasty critters hide. The trees love it; the bugs hate it. If you use insecticide make sure you wash it off at some point to avoid the buildup. Dust is an issue too.

Don't be too much in a hurry to take your tropicals outside in the spring. Typically it is safest to keep them in the house until all chances of cold evenings are over such as late May or June. It is possible to acclimatize them earlier by putting them out in the morning and bringing them in at night.

Tender is a relative term and basically means that if your tree is exposed to the worst cold possible in your area, it will die or be severely damaged. If you live in extreme southern Ontario you might categorize certain trees as hardy whereas someone living in northern Ontario would see it as tender. The best way to determine what is tender is to check the plant hardiness zone map for your area and the trees in question. In the Toronto area where I live, I categorize the following trees as tender: Japanese maples, trident maple, cedar, hornbeam - Korean and Japanese, Chinese quince, cryptomeria, crape myrtle, pine - tender varieties like Japanese black and Japanese white, azalea, zelkova, juniper - tender varieties like "San Jose" and "Hollywood"

I have read about and heard of some really fancy and then low-tech ways of taking care of these types of trees. I sectioned off a part of my garage and insulated it so I can control the temperature better. I bring my trees into my "cold room" when all the leaves have fallen or we start getting hard frost, when I pick them off (this does not apply to evergreens of course). I clear any other debris before I put them on shelves. I leave the moss sometimes, but it is better to take it off and treat it as hardy.

I like to keep the temperature hovering around 0C (plus or minus 5 degrees) and have had little or no dieback. I use an oil heater instead of a heater and fan combo because I want to avoid directing heat on any trees in particular. I keep an even temperature in the room by using a separate fan hanging from the ceiling, pointing down and to one side. It is on the ceiling to force the warmer air downward and to the side to create a cyclonic circulation of air. Air circulation is important to avoid fungus problems and provide a "wind" to the trees. Since my trees are not frozen constantly I must ensure that I check for dryness periodically and water when needed (typically bi-weekly or monthly). Since I don't have a drain, I sometimes bring them outside on warmer days so I can soak the soil properly. Make sure the water is cold so you do not wake up your tree and make sure you allow the tree to dry somewhat before watering again to avoid fungus problems and root rot.

The tricky part with tender trees treated in this way is that they can wake up early. In late winter/early spring I turn off the heater if it starts to get close to 0 Celsius. If the buds break and start showing their leaves, I keep the trees in a window in the main garage. When we have warm sets of days in April and I bring them outside during the day and then inside during the evening until the threat of frost is over (frost may kill the new leaves). Realistically, I often keep them in the garage for days on end in April when there is frost at night. Alternatively, some types may be brought inside the house at this time but then must be treated as tropicals. Other things to consider is getting some mousetraps and buying a thermometer because most heaters come with a low-high setting, not a specific temperature setting.

The hardy type are the easiest to care for and allows for long winter holidays :) These trees will survive the worst winter temperatures (in your area) so they don't need to come inside or even be protected, other than planting them in the ground (pot and all). You will find that locally collected trees and those you can find at the nursery (outside section) are hardy. One of the first bonsai-beginner mistakes is to keep a hardy tree inside for the winter (even worse, in the basement with intermittent light). The plant's nature is such that if it isn't exposed to a severe winter, it will weaken and die. Examples are: birch, American hornbeam, hawthorn, beech, maidenhair tree, juniper, larch, apple, pine - mugo and scots, and our three local varieties, white cedar, hemlock.

I winter these trees either exposed to the elements or in a shelter that I build in my garden. I started building a shelter each year when one of my trees lost an important branch due to a heavy, wet snowfall. Basically, I build a "post and lintel" structure with a solid roof. I do this by first digging in 4 posts, and then I hammer on the lintels (the side pieces), making sure it is all level. To finish the shelter, I add the roof boards. Since this is a temporary structure I make sure I do not hammer in the nails fully so it is easier to remove them in the spring. Lastly, I wrap it all with plastic (white is best). Mind the wind by weighing down or stapling the plastic to the structure well and keep it taught. You will need a door. I suggest stapling a wrap around piece of the plastic wrap to a board the length of a post and temporarily nailing it to the post.

When it gets really cold, around the same time I take in my tender trees, I start burying my hardy trees. I wrap the tree pots in fabric (something breathable and not a coarse weave) and dig them into the ground slightly up the trunk. Wrapping them means that I can dig them out of the ground in the spring, take off the fabric and it is like they were never in the ground! It also keeps the ground soil out of the pot. Note that pots that have a lip that turn inward or aren't high-fired may break so it is best to take the tree from the pot and wrap it. I put the big trees on a board so I don't disturb the roots too much. One thing to be sure of is leveling the soil properly so you don't end up with excessive ice buildup around the tree due to bad drainage. Lastly, I spread mothballs over the ground to discourage rodents, and then I close up the structure. The only time I open it up again is when we get that special "fluffy snow" which I shovel in to add more insulation and moisture. Make sure you spray it well so you don't damage your trees and use common sense with the amount. Be mindful to open up the structure when it heats up in the spring so the trees don't break dormancy too early with frozen roots!

Conclusion

Now you are armed with some information on how to over-winter your bonsai trees. Luckily the weather has been mild this year so you still have time to get it done but you better hurry!