sunnuntai 11. lokakuuta 2015

Overwintering bonsai in Finland

This is for Tom, really. First of all, there is a good amount of information available, some of it even quite usefull. Evergreengarderworks , for example. Those would be the best advice one could get, IF we lived in zone 8. Southernmost Finland is borderline zone 6, zone 5 being very close. Anything that is written of overwintering bonsai in zone 5 is of interest.

I like that map, because it puts me to zone 6. There are other maps that are not so nice :-). Maps made in other parts of the world are not too accurate in Scandinavia.

For the purpose of discussion, I would first divide trees into 4 categories. Naturally I have not invented these classifications, I just modified general knowledge.
  1. Tropicals
  2. Tender temperate 
  3. Somewhat hardy trees
  4. Domestic trees (and other fully hardy trees)
First and last groups are easiest. Tropicals you can keep inside all year round (given you can provide adequate light and humidity). Domestic trees require only minimal protection, because they are potted and not growing in the ground.

Following remarks are generalisations. Species is one thing, origin is another. Your mileage will vary.

1. Tropicals

The main difference between Finland and lets say Central Europe (or most parts of the US) is the length of winter. Generel advice tells to take the trees outside when temperatures allow. In my experience this is mostly futile. 

Ficus for example can take temperatures below 10°C, but they wont much grow. Growth takes place at nigth time, mostly, and my experience is, one should take Ficus indoors when night time temperatures fall below 15°C. We may get temperatures like this in May, but usually not before half of the June is gone. Night time temperatures can get (constantly) below 15 again at August. For tropicals, the time one can keep tropicals happy outside is about 6-10 weeks. Sure they can survive a longer period, but will not get any better. Whats the use of keeping bonsai in conditions where there will be no development? 

If you have the time, one could take trees inside for night and back out at full sun when temperatures allow. This will give you several more weeks of nice sunshine every year. Still, you will have to grow tropicals indoors for 3/4 of the year. 

This means, one will have to find out a way to grow trees indoors so that they not only survive the winter, but get better too. This is not an easy task, and many Finnish bonsai hobbyists have found it too time, space and money consuming to be worth it. They concentrate on outdoor trees. 

2. Tender temperate

These are most common trees we see in shops. Species include Chinese elm (borderline hardy temperate) and Ligustrum sinensis. These are possibly the most difficult to grow in Finland. They want a dormant period, or at least can take some cold, but they really dont wan't anything that we would call a winter. They can take occasional slight freeze, but not much more than that. 

How do you keep trees in this category alive? I have found only one way - those that can be grown as tropicals, are fine. I keep Ligustrum sinensis outdoors until night time temperatures are about 5°C. Some have been fully indoors for many years and seem to be ok. I keep Carmonas indoors all year round. I have yet to find out a way to keep chinese elm alive for more than 3-5 years. 

These trees should have been inside for about a week now. They can be taken outside after danger of last frost is over, perhaps April - May. That means they will have to be grown indoors half of the year. Same thing as with tropicals, it is not enough to keep them alive, they should develope too indoors. I have got this far with a chinese elm (just to add a picture): 

3. Somewhat hardy trees

These include most species commonly grown as bonsai. Japanese maples, Juniperus chinensis, white and black pine etc. This is the second most difficult group. One can not suppose these plants can take our winter, year after year, and they can certainly not be grown indoors. 

Cold frame is my answer, well insulated box with a translucent insulating cover. When it gets really cold, I use additional styrofoan covers. Coldest temperature last winter was -21°C, and it did not get below -7°C in the cold frame. All zone 7/8 trees should be ok. One has to open the cover when temperatures are over freezing point, or molds and fungus will attack the trees. 

More south, usual way to overwinter trees in this category is in the garage or other unheated space. I know of only one person who does this in Finland, not surprisingly coming from UK. Period for protection is long, 5 to 6 moths, and it will be difficult to keep trees between -9 and +4°C (Brent Walston's article above) for so long. From February on daytime temperatures may be quite high, and trees will probably think its Spring already. This is the most dangerous time, because night-time temperatures will be well below zero. It is usually said, its not the cold, its the freeze-thaw cycle that brakes the roots and kills the trees. New soft growth is also very tender. This period could last for 2-3 months. 

In garage one has to take good care, that there are no rodents. 

Moisture is often considered to be the worst enemy of the trees in cold frame. I don't yet know how this is. To prevent too early spring I have allways covered my (domestic) trees with lots of snow. In the spring this will thaw and freeze, but trees under the snow and ice are relatively well protected. With hardy temperate trees in cold frame, advice is often to keep things rather dry, meaning no snow insulation. Last winter I let it snow inside the cold frame, but did not add any snow other than that. Japanese maples and wine maple were ok and did not start the Spring too early. 

4. Domestic trees

This is by far the easiest and most rewarding group, when it comes to over-wintering. My tables second as a wintering boxes. There is a steel net all over, to keep rodents out. This is necessary only for decidious trees, I suppose. 

I put trees inside the boxes after temperatures are constantly below freezing. I is common to get -20°C BEFORE we get a snow cover, and even domestic trees will suffer when in pots. If there is a long very cold period, I cover my trees with broken cardboard boxes. If so, I left the cardboard under the snow for rest of the winter. I have not lost any domestic trees due to winter for a long time. 

I do not bother to protect pines (P. sylvestis). They are mostly just on the ground, covered with snow when we get some.

2 kommenttia:

  1. Very interesting article! According to this map I live in zone 5. When I try to explain to people about overwintering I usually divide my trees in three categories (your number 2 and 3 together in a "middle-group"). -But then again I don't think I have any specimen of the "tender temperate" - no chinese elm, no ligustrum. I don't know where you would put myrtus, that is a sort I have tried, but can't keep, I killed it. The same with fuchsia, which some of my friends in Sweden use as bonsai. I couldn't manage to keep it alive. I use the garage for "somewhat hardy trees" - AND have had problems with rodents! My tropicals are indoors all year round. Summer temperature is too unreliable! And all domestic trees are put in the ground for winter. I have done this about one week ago, and will take them up in May? or so.

  2. Hi Anna! Zones are only part of the story, we both have the added disadvantage of short and unpredictable summer. We have already had about 10 nights with temperatures way below zero.

    Classifications are difficult. I would put myrtus to group 2, Fuchsia would probably be borderline 2 - 3, if not 3.

    Apart from rodents, in the garage, how do you prevent trees don't start Spring too early?

    Perhaps in the future we could have good scandinavian wiki for bonsai, especially for zones 6-4. I am sure a lot can be done here too, but many times not with the usual species. We could use Amur maple instead of Japanese maple, for example.

    There is a Finnish arboretum called Mustila, and they have about 100 years of experience of foreign trees. They locate in USDA zone 5. They have dozens of maples, azaleas, conifers, etc, that originate from mountains all over the world. Do you have something like that in Sweden?