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Wintering Bonsai
Next to watering, successfully wintering bonsai
is the most important factor in keeping them alive.
Trees (and other plants) native to temperate zones have adapted
to ensure survival during that time of the year that is inhospitable
to growth. They go dormant.
Dormancy is a fairly complicated process. It is initiated by the
shortening day-length of late summer and fall, and is accelerated
by the decrease in average temperatures that go hand in hand with
the shortening days. This process is only reversible after the plant
has spent a certain time in the dormant state, during which it has
experienced chilling temperatures. The length of time required varies
from plant to plant, hardier plants generally requiring a longer
time. Since plants take their cues from conditions in their environment,
it is important to let them experience those conditions and not
put them into shelter prematurely. In Fall, I have had my bonsai
exposed to temperatures as low as –10ºC (about 14ºF)
without any damage.
The ‘hardiness acquisition’ that takes place in fall
applies only to the above ground growth. Plants growing in the ground
do not need to make their root systems hardier, because the soil
does not get anywhere near as cold as the air above it, and the
roots are designed to be able to withstand those temperatures .
The first thing to do in a successful wintering program is therefore
to make sure that bonsai are given the opportunity to build up their
natural defenses, which in most cases will be good enough to assure
the survival of the top growth.
The roots of bonsai are a different matter, however. Since bonsai
are usually grown above ground and in shallow containers, their
roots, if left unprotected, will at one point be just as cold as
the air around the pot, and that is much too cold to assure survival
of the roots, and therefore the plant. However, cold hardiness of
roots varies from plant to plant. It is safe to assume, that the
colder it gets in a plants natural growing range, the cold hardier
its roots will be. I am not aware of any publication that gives
these values, so I would err on the side of caution, taking the
–10ºC threshold as a general guide. These remarks, of
course, do not apply to tropical species.
It follows from the above that root balls have to be insulated
against low temperatures.
The best way to accomplish that, in my view, is to bury the root
balls, still in their pots, in the ground up to the rim of the pot,
and to cover the pots with a mulch of dead leaves, ideally from
oaks, but I find any old leaves will do just fine. If you live in
an area of abundant snowfall and a reasonably consistent snow cover,
you may do without the mulch and rely on the snow for insulation,
but here too I would err on the side of caution, that is I would
mulch and treat the snow as bonus insulation.
Wintering bonsai in the ground has the advantage that they will
come out of dormancy in step with outside conditions. That is often
not the case with some of the other wintering methods, such as unheated
garages or sheds, cold frames dug into the ground, window wells
or cold rooms in basements. While these methods are very convenient
– no digging in, no mulching, and no digging out – the
facilities tend to warm up quickly in spring, and since resumption
of growth is determined solely by warmer temperatures, the trees
will start to grow and require light when outside conditions are
not yet ideal.
Regardless of which method is used, the root balls should be well
moistened before the trees are put away, and they should be checked
regularly – say weekly – to make sure that they are
not about to dry out. If so, the trees need to be watered. Also,
the wintering spot should ideally be in shade for most of the day,
and preferred positions to face would be either north or east.
It is important to note, that the trees, which withstood –10ºC
without damage in November, will not be able to withstand that temperature
once they have begun to grow. Just as trees became gradually ever
more frost resistant in fall, they will get progressively less frost
resistant in spring. The closed buds, although swollen, will still
be able to survive short periods of mild frosts, but once the buds
have opened and the young leaves start to unfold, frost can cause
serious damage, and trees should be returned to frost-free shelter
when it threatens.
More wintering information can be found at:
http://www.evergreengardenworks.com/frzekill.htm
http://www.evergreengardenworks.com/overwint.htm
I have in the past used all of the wintering methods mentioned
above, but now use only the ground and a cold frame. In the following,
some pictures that might illustrate the points made.
Below: A view of some trees immediately after digging into
the ground. No mulch has been applied: it’s still attached
to the big trees higher up.
Below: Unfortunately, an exposed portion of my
wintering bed faces south. By stacking three shelf boards on top
of each other, I provide shade. The space between the chimney flues
provides convenient storage space for shelving materials.
Below: Cold frame in the dead of winter. The lids
are closed because the inside temperature is cold enough.
Below: The cold frame with lids up. The cold frame
measures 1.98m x 1.73m x 1.62m deep (78” x 68” x 64”
deep) Since it floods regularly, bottom shelves lift the trees up
by about 20cm (8”). A sump pump pumps the water out automatically.
Below: I use the cold frame almost exclusively
for Japanese maples and azaleas.
Below: When it snows, opening the lids makes for
easy watering of the contents.
Below: Wintering bonsai in an elevated flower bed
makes the procedure
a lot more comfortable.
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