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  • Ian Evans

Bonsai Care During Cold Weather

As hard as it is to believe that fall is upon us, it also comes as kind of a relief to me, because I know the growing season is coming to a close. We're now entering into a phase of bonsai work to advance our trees’ design and then we can set about preparing our bonsai and their environment for winter. For some of you, this might be your First Bonsai Winter, and that’s a big deal! So, today I am providing you with some insight into the autumn metabolic processes taking place within our trees, some thoughts about the possible scope of autumn bonsai work, and finally a few tips for protecting your trees from Old Man Winter himself.

Let’s start by talking about where we are right now.

What’s Happening Inside My Bonsai? Autumn Metabolic Processes.

Autumn is a critical time for a few bonsai care and development processes, including fall fertilization, pruning, potting, and protecting our trees from pathogens. The trees themselves are also very active this time of year even though visible growth has slowed or stopped entirely because autumn brings about big changes in the metabolisms of our bonsai.

For all species, vascular growth ramps up, while foliar production backs off. All outdoor trees do require a period of “dormancy” during colder months (I put “dormancy” in quotes because while we say dormancy, the tree is certainly not dormant, as you will see). The true trigger for dormancy is now said to be the changes in daylight length, as opposed to temperature, as commonly believed. This makes sense when we consider that there are deciduous trees in warmer regions of North America that lose their leaves every year, temperatures notwithstanding. Most trees will require at least 45 days of temperatures below 45 F to undergo the process. The tree will begin to prepare itself for the cold and darkness by turning its powers inward as soon as September and strengthens itself in ways we can’t see right away. This metabolic change occurs within the branches and trunk, and also the roots, as the tree switches gears to start revving up its cold tolerance in preparation for the deep cold of January and beyond.

Chlorophyll is withdrawn from deciduous leaves and broken down into sugars and starches. These individual components are then used internally by the tree to stave off cold damage. This process is why deciduous trees’ leaves turn color and fall off. By late autumn, they are basically just disconnected solar panels. Deciduous trees use all of that energy to gain strength and get set to push out new leaves in spring.

For coniferous bonsai, this process takes place on a smaller scale, as these trees will keep their needles year-round and continue to photosynthesize during winter. The same goes for broadleaf evergreen bonsai, like boxwood. These internal changes in coniferous and broadleaf evergreen trees show up as a burnished, tan, or bronze color, and even striking purples and blues on some species as the tree breaks down chlorophyll to defend its branches, trunk, and roots from the intensifying cold. These changes will reverse themselves when the warmth and longer daylight of spring come along, and the new growth season commences. Just monitor your trees regularly to make sure you aren’t seeing foliage death due to extreme cold or wind.

Out In The Cold (Where We Like It)

Cold tolerance is something that is developed slowly over a couple of months from roughly October to December. By January, the trees are at their peak (or trough?) of cold tolerance and have hardened themselves off against the worst stress or possible damage due to all of that internal metabolic action they engaged in during the autumn season. Keep this in mind on the occasional, unseasonably warm days in the dead of winter- a bonsai could be shocked by putting it out in the full sun on a 60 F day in January, February, or March. Trying to avoid the unnecessary stress of abrupt temperature changes is key during winter, so also try to be aware of the sun, and the ambient warmth it can create on that strangely warm winter day when it gets up to the 50F mark. Best to keep your bonsai mostly hidden from the sun during days like that. We want our trees to keep their cold tolerance as long as possible, so be aware that sudden warming can actually change the level of cold tolerance your tree has built up, and make it susceptible to root-system weakening, and decreased or delayed spring growth.

This cold tolerance begins to taper off sometime in late January or February, as the days lengthen and temperatures slowly creep up. This cycle makes our bonsai strong and healthy, and nothing can replace this natural process. Let them be cold, but not too cold.

Our Autumn Scope Of Work

For some species, Autumn can be a good time to perform certain modes of bonsai work. Remember: knowing the needs and wants of each species is an important prerequisite before proceeding.

Elongating species can be structurally pruned and styled at this time, and in some circumstances, they can be repotted. Junipers and pines can handle some pruning and shaping to allow light and air to penetrate into the inner branches. Sometimes, and if done without doing too much other work, these trees can be potted. In that case, I would advise performing minimal pruning or wiring at the same time as potting. Boxwoods (a broadleaf evergreen) can handle all this kind of work, too. As long as the weather doesn’t get too cold too quickly, these operations are safe and healthy, so check the weather forecast as always! If you are aware of the weather and have confidence in your “after-care” regimen, you can proceed with any of these steps of bonsai design advancement.

Winter Is Coming

As temperatures cool down toward freezing, it’s important not to assume that your trees aren’t thirsty. They won’t be as thirsty as they were back in July or August, but they still need your attention on a daily basis at a minimum. Check them at least three times every day if possible, and water as needed according to species, age, pot-size, etc. (For more watering tips, check out this blog post)

Now is the best time to start planning your strategies for winter protection. Affordable, small greenhouses and greenhouse/cold frame set-ups are available on the internet. These definitely work as designed, but let’s dig a bit deeper into a few other ideas in the realm of cold protection for your bonsai.

First, concern yourself primarily with protecting the roots of the tree, and your trees will stay healthy. Methods for covering the pot/root system are temporary and can be used in conjunction with a cold frame or greenhouse set-up when temperatures plummet. You can do this with bubble wrap, towels, blankets, etc. You can avoid mildew and dampness that can result from these methods by checking the trees regularly, and removing the blankets or plastic, and taking them out, and giving them plenty of fresh air when conditions permit. Don’t leave them battened-down if they can have fresh air and sunshine, even if it’s still cold. This regular moving of bonsai to place them in optimal conditions is known as “The Bonsai Shuffle”. Do the dance. Move your trees in and out of full-cover (and darkness if using my moving blanket method during the deepest cold) as often as possible.

Last winter (2020-21) was very cold. I used a combination of DIY cold frames, benches covered in plastic (and even moving blankets on all the benches and frames when temps dropped to -25F overnight!), and a four-shelf, plastic-covered, upright greenhouse for my small shohin-sized bonsai. Anything that raises the temperature around the bonsai by 10 to 20F should suffice. That means that when it was -25F at night, the bonsai were in an environment of roughly -5F. This is still quite cold, but it will suffice. The obvious added benefit here is that the bonsai are also protected from that horrible winter wind, which is crucial to preventing excessive dryness.

In Conclusion

Using methods like these, and being aware of the changes our bonsai experience, will help you figure out how best to protect your trees from the deadly and drying winter winds, which can quickly desiccate and even kill even the healthiest bonsai. This risk presents itself because bonsai live in tiny pots and therefore their roots have minimal protection against temperature extremes, both hot and cold. Practically speaking, the shallower and smaller the pot, the more at-risk the bonsai. One solution would be to use far larger pots, but to many, that doesn’t fit with the aesthetic concerns of bonsai as a true art form, complete with its own traditional guideposts regarding proportion, perspective, and dimension.

The best way to proceed is often the middle path, so use this information to figure out what could work for you and what you want from your bonsai collection this year and beyond.

I hope you'll use these tips and methods to increase your confidence in your ability to steward your trees through winter and into abundant spring growth. If you have any questions, concerns, or good ideas, just send us a message!

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