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WINTER QUARTERS AND PROVIDING ADEQUATE PROTECTION FOR YOUR
BONSAI
The temperature of the bonsai soil should be stopped from dropping
by placing the tree and pot in a covered shelter. An unheated
outhouse such as a shed or garage provide the best protection,
but well insulated cold-frames or covers can also be adequate.
Soil temperature can also be protected by mulching the pot or
putting the tree back into the ground, though this is not generally
as effective. These measures may not stop the water content of
the pot from freezing, but will reduce the drop in temperature
and stop it falling below the point where root damage occurs.
The most reliable way to measure the effectiveness of your winter
quarters is to place a thermometer alongside your trees to measure
the degree of protection afforded against the outside temperature.
My experience of placing trees in an unheated garage, is that
although temperatures will occasionally drop below freezing inside
the garage, when outside temperatures dropped to -10°C for three
days during the winter 2000, the temperature inside the garage
never dropped below -4°C; well above the point where damage can
occur. Placing especially vulnerable trees inside a cold frame
inside the garage, reduced this drop to just below freezing.
Winter protection should also include protection from strong winds
and sun during periods when the soil is frozen and the roots are
unable to replenish moisture that would be lost from the effects
of direct sunlight and wind.
The tree should not be placed anywhere that would allow the soil
to warm up too much, placing the tree in a heated room temporarily
for instance could bring it out of dormancy. Trees are brought
out of dormancy when exposed to temperatures above 10°C for a
number of days.
My bonsai after a snow storm in March 2006
HOW WINTER QUARTERS PROTECT TREES FROM LOW TEMPERATURES
Frost is created when the ambient air temperature drops and and
as a result ground-level warmth rises to be replaced by the 'heavier'
colder air. When the air immediately above the ground drops to
0°C, the ground itself freezes. Covering your trees provides a
physical barrier that stops the warmer air surrounding your trees
from rising, which in turn lessens the effect of the outside temperature.
It is not possible for cold air to displace warmer air; if the
warmer air is retained, the cold ambient air is unable to enter
the quarters. Though the air under the covering can and does eventually
drop below 0°C it will still remain warmer than the outside temperature.
The thicker the covering over your trees, or the more layers of
covering you use, the more difficult it is for the warmer air
to escape.
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PREPARING YOUR TREES FOR THEIR WINTER QUARTERS
Fungal spores and small pests can also benefit from your trees'
frost protection by overwintering in and around the tree itself
and the pot. Ensure that all leaves and other debris on the surface
of the pot and in the branches is cleared away. Remove all cobwebs
and look out for small insects, in particular scale insects. Clean
off all algae from the trunk and remove any mosses growing on
the surface of the soil. Moss can be placed outside during the
winter ready for re-applying in Spring.
Pests that are harboured overwinter can rapidly increase in numbers
during warmer periods of the winter and particularly in Spring,
causing damage that can go unnoticed until trees are brought outside
in Spring.
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The branches of a Trident Maple bonsai in Winter
LOCAL MICROCLIMATES
Although the general climate and weather condition cannot be altered,
there are micro-climates around your house and garden that can
be taken advantage of when considering winter protection. It is
possible to have a wide range of variation in microclimates around
your back garden or yard.
Walls and hedges or overhanging plants will of course result in
less light, but will also reduce the amount of rainfall and wind
that a bonsai will be subjected to. The effect and risk of frost
though can be increased or decreased by these factors.
Frost will always drop to the lowest point it can find in the
garden where it will create a frost pocket, physical barriers
to the downward flow, such as walls, will trap the movement of
the cold air and can lead to frost pockets at the base of walls.
On the other hand, walls at the top of slopes and particularly
house walls tend to be slightly warmer places which can be less
affected by frosts and icy conditions.
Careful observation of different areas of your property during
adverse conditions will highlight places that are advantageous
for placing your trees and/or constructing winter quarters.
WATERING DURING THE DORMANT PERIOD
Your bonsai should never be allowed to dry out and this can happen
during the winter when trees are placed in protective quarters
out of the rain. Water consumption is however very low during
the dormant period and the soil should be kept damp but never
sodden. Trees that are left out in the open can be at risk from
overwatering during prolonged periods of rain and also from melting
snow. Trees that are sat in overwet compost all through the winter
can suffer from root-problems associated with overwatering. Always
ensure your soil mix is free-draining to avoid these problems
from the outset, but, if during the winter a tree is found to
be standing in poor-draining or overly wet soil, try to move it
to the side of an outhouse or wall against prevailing winds where
it will receive less rainfall.
REQUIREMENT FOR LIGHT
Deciduous trees have no requirement of light once their leaves
have dropped. There are however many differing views on the requirement
for light during the winter of evergreen species. The requirement
of light in evergreen species is temperature dependant; the rate
of photosynthesis drops as the temperatures approach freezing.
At temperatures below freezing, photosynthesis does continue but
excess light can actually start to cause damage, an effect known
as 'photoinhibition'. In conifers and other evergreens, exposure
to direct sunlight during periods of sub-zero temperatures can
cause damage though this is repaired during the growing season.
A balance has to be struck when overwintering evergreens, light
is still required through the winter as photosynthesis still takes
place but strong light or long periods of direct sunlight should
be avoided during periods where temperatures are below freezing.
During periods of temperatures below -10°C, evergreens can be
stored in near dark with no adverse consequences.
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Snow on my bonsai during the Winter of 2013
REMOVING TREES FROM WINTER QUARTERS IN SPRING
Trees start into growth in the Spring when temperatures rise above
10°C for a period of days. They are not prompted by increases
in light levels. It is important that as trees start to grow they
are placed back outside where they have access to light. However,
late frosts in Spring can devastate tender new growth and on nights
where frosts are forecast, temporary protection must be afforded.
IN SUMMARY
Allow your trees to enter dormancy and harden to the cold.
Store your trees somewhere that will remain above at least -10°C
all winter. Optimally, temperatures will remain between 0°C and
9°C, though temperatures just below freezing down to around -10°C
will cause no damage.
Keep your trees out of direct sunshine and strong winds when the
soil is frozen.
Do not allow the compost to dry out, nor allow to stay overly-wet,
it should always remain evenly damp.
Preferably, in areas where night frosts are rare and seldom drop
below -5°C, it is better that trees are left outside in a position
where they are protected against damage from excess rainfall and
wind. To be cautious, if temperatures threaten to drop below -5°C
during the forthcoming day or night, temporarily use winter quarters
though temperatures down to -10°C could be tolerated.
Related Article: Bonsai in the Snow Gallery
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