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How to care for bonsai in winter

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Dormancy: Your Plants May Need a Vacation
Here 's how to accomodate those hard working plants

While I have covered plant dormancy briefly elsewhere on this site, we continue to get a lot of questions concerning the topic, so I will cover some more ground here. This is not a simple topic; it is of great concern to the scientific community, even more so now with global warming. In fact there is to be a world wide symposium in May on just this topic (in Holland I believe), so with that thought in mind, let’s start with a scientific explanation.

Plant dormancy defined
Plant dormancy is a form of phenotypic plasticity that minimizes exposure to seasonally stressful conditions. Let’s break that down. Phenotypes are observable characteristics of an organism, either from genetic make-up or outside influence. The scientific use of plasticity denotes two things, both of which apply in this case. In physics, something plastic is capable of undergoing continuous deformation without rupture or relaxation; in biology it denotes the ability to form new tissue. So what we have is an observable phenomenon of a plant reacting to outside stimuli to protect itself from harm, usually either too much heat or too much cold, in a manner that does not harm the plant.

As plants develop these traits over millennia, they become so accustomed to this dormant period that to delete it can harm the plant, and that is the main thrust of this article. Whether the plant goes dormant to escape the heat (those of you in the South are familiar with Virginia bluebells and their disappearing act in late spring) or to escape the cold (we in the New England area derive much pleasure and not insignificant income from the foliage change as winter dormancy begins), these periods of rest are crucial to the survival of the plant. They become particularly tricky when we deal with bonsai; although what I will discuss here applies to all plants, we will be paying particular attention to bonsai dormancy requirements.

Dormant Evergreens?
“But my plant is an evergreen; it doesn’t drop its leaves so it’s not going dormant.” Au contraire, mon ami, the fact that it is an evergreen plant makes the dormant period even more crucial. While deciduous plants get a total rest, dropping all their leaves allows the tree to send all its sugars to the roots where they are stored (that’s why we tap maples for sugar after leaves drop), the poor evergreens do not get the winter entirely off. They must continue to photosynthesize and respirate as they always do, just at a much lower level than in the growing season.

So when you force that potted juniper (the most common bonsai is the Japanese garden juniper) to grow through the winter, it gets tired. Sure, Japanese garden junipers are a Zone 8 plant, so they’ll handle it for the first year, even the second. I just answered a post from a lady who had cared for her beautiful bonsai for 9 years and couldn’t figure out why it was slipping away this season. Even I was surprised to find that she had never given the plant any sort of cold period; usually they won’t take that kind of sleeplessness for more than 4 years, so she must have done everything else perfectly. Unfortunately, that won’t save the plant now.

Giving our plants the rest they need
So what can we do to give our plants the rest they need? If you are in a milder part of the country, and hard freezes are a rarity, simply leaving the plants out of doors will work (remember not all plants go dormant, so any tropicals in the collection should still go inside unless you live in the tropics (Florida or Hawaii, even San Diego had hail this year!)

For those with a slightly cooler clime, building a tent or some such structure over the benches you display on will usually work, I have a friend in New Jersey who sticks his trees under his wooden deck and finds that works fine. For those of us up here and further north, special allowances must be made. Many of the folks in my bonsai group un-pot their trees in early fall and plant them back in the ground on top of a piece of slate or the like (to prevent deep rooting). Some dig sun pits (2-3’ deep hole with a greenhouse structure over it; this is very similar to the Chinese method, see Peter Valder's Gardens in China for an explanation and beautiful illustrations.

The plants are placed in the pit, the pots are mulched in and the cover sealed down. Some have greenhouses that they partition, leaving part barely heated for the dormant trees. An unheated porch area, the corner of the garage away from the fumes, even an unheated room in the house can be enough cold to provide dormancy (three months at or below 50 degrees will treat that Japanese garden juniper nicely). You can even fool trees into thinking they’ve had a full dormancy by leaving them out for the first two months (watch for hard weather) and then bringing them in (I don’t recommend doing that too many years in a row).

“So we can dump the trees in our chosen locale and forget about them until spring?” Non, mon ami, remember that our deciduous trees are moving sugar to the roots so the roots still need to be maintained, and our evergreens are still plugging along, albeit at half speed, so they both need most everything they usually do (the deciduous trees don’t need the light for photosynthesis; I know someone who over-winters his maple in a paper box for insulation, closed).

This means watering is still our primary duty; although the requirements have certainly dropped, we still need to check the pots and water if they are going dry (I mean dry, over watering at this point can be deadly if you are in a freeze thaw cycle; the constant up and down of waterlogged soil freezing and refreezing will break roots). Rodent damage is another consideration. I’m no fan of rodenticide, be it traps or chemical, but if it’s them or my trees, then by whatever means necessary. Look for chewed bark at the base of the trunk, and if you find it take whatever action you deem appropriate.

Dormancy is a brief respite from the rigorous needs of our trees, not a vacation. But armed with the right knowledge and tools you are prepared to keep your tree until you both reach old age.

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