I’ve received quite a few emails from people asking about overwintering hardy trees outdoors. While there is lots of good information out there on the internet, most of it is region specific. What applies in England, Japan, or North Carolina does not apply in Toronto. With that in mind, this information is specific to zone 5-6 and is for people who have no special facilities (heated garage, polyhouses, etc.).

Reiner Goebel of rgbonsai.com has a good article about overwintering trees in the Toronto area. Have a read here. What I do is very similar.

• Use tough species to make life simple. Semi-hardy species (Trident maples etc.) require more attentive care in this climate. My collection consists of Eastern White Cedar, Larch, Juniper, Yew, Siberian Elm, Potentilla, Ginkgo, and  few others. All fully hardy.

• Don’t be afraid to leave them on the bench through a few late autumn frosts. They can take it. I usually put mine away in late October or early November. If the ground is frozen, you’ve waited too long

• Bury the pots in the ground in your garden and water everything really well. This insulates the roots. The risk is not the rootball freezing. It can and should freeze solid. The tree will be fine. The risk is frequent freeze-thaw cycles. This is why unusually warm winters are more of a concern to us northern bonsai growers than unusually cold ones. Burying the pot helps ensure that once the rootball freezes, it will stay frozen until around March, even if there are some freakishly warm days in January (it happens). If your tree is in a massive training box but is a hardy species, just sit it on the ground and mulch around it. Just having contact with the ground will help buffer the rootball temperatures.

• Snow is your friend. As soon as fluffy snow hits the ground, shovel it onto your trees (don’t shovel on heavy wet snow!). Sure, three feet of snow can bend branches but I feel most comfortable when my trees are buried in snow all winter. This protects from -20 degree air temperatures and drying winds. But when the snow starts to melt, brush it away as this heavy slushy mess can really cause damage to branches and fine ramification.

• Try and winter them in a shady spot in the garden protected from the wind (i.e. a north facing corner). This provides a bit of insurance against wind and sun if it is a snow-less winter (like the infamous winter of 2011). It also helps keep the tree dormant as far into the spring as possible. This is important. If your trees break dormancy too early, the tender new growth will be exposed to the unpredictable early spring weather.

• Rabbit-proof your garden as best as possible. When rabbits get hungry in the winter, bonsai bark apparently looks quite appetizing. Even  a couple of exploratory nibbles can ruin the image of a tree. Try to board up any gaps under the fence.

• If you have the energy, fence in your trees with hardware cloth to provide further protection from pests. Burying the fence a couple of inches provides me some reassurance against mice, although if mice want to get anywhere bad enough, they will find a way.

• Don’t worry about watering! If you put them away late enough, they should not need any water the entire winter.

• Again, keep them dormant as long as possible. Don’t take them out of the ground at the first sign of spring in March. You know that in our part of the world it is not impossible to have freezing nights in May, and these spring freezes can kill off tender new deciduous growth.

This approach has worked well for me for 8 seasons. There are many ways to do it, but I find this to be the simplest and most foolproof approach. It takes some time to prep the trees for winter, but once they are in the ground you can literally forget about them, sit back, and wait for repotting season to come! Unfortunately, since the trees are frozen solid in the ground, you cannot really work on them in the heart of winter.

Korean Hornbeam need to be cut back later than maples, otherwise they won’t backbud well. Usually this mean late spring, once the new growth has hardened off. This is also a good time to send through some thread grafts as defoliated branches will bounce back quickly, and smaller holes can be drilled since winter buds have not yet formed.Thread grafts can also be sent through in the late winter, but larger holes need to be drilled to get the branches through without rubbing off the large winter buds.

Healthy spring growth. I was planning on repotting this year, but never got around to it.

Leaves have just hardened off in the last week.

After cutting back. The tree will be moved to a sunnier location and fertilized heavily to encourage backbudding.

This branch was thread grafted a few years ago.

This is the first branch I threadgrafted on this tree. Funny that it is now supplying the whip for this year’s thread graft (note small branch wired inward)! Ahh… the circle of life…

This years little threadgraft, growing well. Note the size of the hole. This is the disadvantage to sending threadgrafts through in the winter. Larger hole = longer time period before the scion is large enough to start fusing with the trunk.

Some fruiting structures (catkins?) were spotted and removed. The branches that these grown on weaken dramatically. I try to remove the conspicuous flowers in the spring, but usually miss a few.