Thoughts on winter care of bonsai
Peter Snart – Willowbog Bonsai
In the light of this relatively early onset of harsh winter weather, which in itself follows on from last winter's long and cold spell, I am minded to contribute these thoughts on the subject of getting our bonsai safely through the next months.
Like most things to do with bonsai it can be as simple, or as complicated, as you want to make it ! Consider the problem on a tree by tree basis and take appropriate measures, or don't bother protecting any of them! The likelihood is that either approach will result in most of your collection at least surviving into the spring.
However, I would contend that as bonsai enthusiasts we should be looking to do everything to do with the activity in an optimum sort of way and for winter protection that will require thinking about each tree and acting in the most effective way given your particular circumstance.
We need to bear in mind some basic factors that affect trees during this season. Mostly, and very pertinently as I listen to the weather reports whilst writing this ! we think of ice and snow in this context but temperature, ie. frost, is way more significant here, snow may do a bit of physical damage by way of it's weight on branch structure, but if it comes before the worst of the cold then it can actually be of benefit in providing an insulating layer on top of our pots, I am sure that it was this sequence that led to less damage to roots last winter. If it occurs the other way round then the cold may be, as it were, sealed in by the snow which is much less desirable. But plants respond to more than temperature at this time and day length and light intensity need to be taken into account. For example the fact that trees with no leaves need little or no light might make it much easier for you protect a vulnerable deciduous bonsai in a shed or garage or even a basement. On the other hand the evergreen species will require good light levels even through the winter.
But let's take a step back and consider how to make an appraisal of our bonsai collection, be 5 or 50 in number. First look at it on a species basis, now I hate rules of thumb in bonsai as each plant can differ so much but sometimes a generalisation can help, so think of conifers tending to need less protection than deciduous trees, think of native or long established species needing less than non native or imported bonsai. Unfortunately such simplicity must be qualified, eg Larix or larch are conifers but they are also deciduous and they are also amongst the toughest of the species commonly used for bonsai. Some Japanese conifers may not like being out in our winters though that is likely to be more to do with the persistent rainfall combining with the cold rather than just low temperatures. Most of our native broadleaved or deciduous species will be tougher than, say, an imported maple.
Alongside considerations of species we need to think about age, young trees of any kind are generally more resilient than much older examples.
Next we should think about the sort of growing season that a plant has had prior to the onset of winter. A very strong and healthy plant withstands adverse conditions much better than one that is weak or that has been stressed in any way in preceding months no matter what species.
In thinking about the effects of cold it is predominantly the roots that concern us and another factor that plays into this is the pot or container. Most of our bonsai will be in ceramic containers which though they look very nice, unfortunately, provided very little insulation for the roots whereas roots in plastic training pots or wooden boxes will be slightly better off.
Another very important factor in considering the effects of low temperature on roots is your growing medium. This is where an open, free draining mix that holds very little water and is therefore full of air will provide good insulation. Air is a good insulator, hence it's use in double glazing units. A proportion of porous particles, which will also be full of air when not full of water, will also help here. On the other hand you should be aware that in using a growing medium with a significant content of either peat or loam the potential retention of water in the mix can lead to physical damage of roots due to the water turning to ice in low temperatures as well as at least partial exclusion of air which in itself leads to a much greater risk of root rot due to the anaerobic fungus that cause this.
Also, if you are proponent of autumn re-potting then if the procedure was not done early enough to allow for the full recovery of damaged or pruned roots then this can make the plant more vulnerable to early hard winter conditions. In this, the pattern of weather following the re-potting must be considered when looking at the risk. Autumn re-potting will in most instances require a greater thought being given to onset of bad weather protection if unnecessary root damage is to be avoided . In my view a very good reason to concentrate on spring as the season for root work.
Finally, when considering risk factors, the work carried out on the bonsai in the preceding months should be borne in mind, a tree that has had major wiring and styling work carried out is going to be at least to a degree more vulnerable to winter conditions, likewise a bonsai that has had major structural pruning, especially if carried out towards the end of the growing season.
In looking at how protection might be afforded, an understanding of where and how you keep your bonsai is important. You should be aware that the conditions in your garden may differ significantly from those in a fellow enthusiast's just a street or two away. Exposure to wind, potential frost hollows being just two examples.
We at Willowbog Bonsai are increasingly of the view that for deciduous species such as maple then more or less complete protection from frost is a good idea if possible. A polytunnel or greenhouse will provide just this sort of protection, especially if used with a thermostatically controlled heater to keep temperatures just hovering above freezing. Even without the heat great protection will be afforded the trees, remember, the bonsai do not need to be warm just less cold !! Something to bear in mind here is that, whereas with bonsai outside all winter, little or no watering may be needed due to adequate rainfall, trees kept under cover must have dryness checked regularly, winter sun can have a big affect on conditions inside a building.
With no such building, a degree of protection will be afforded by the simple expedience of wrapping the pot in bubble wrap and sitting this in a bin liner that is lightly tied round the base of the trunk.
Although shelter from wind is something that should be a matter of course for all bonsai at all times, this is even more important at this time of year.
In a bonsai soil, where all the water is bound up by being frozen, then any loss of water by the plant, which can happen even in a tree with no leaves on via openings in the bark called lenticels, cannot be replaced, with resulting adverse affects. This loss will be exaggerated by exposure to air movements.
It is my belief that winter damage to roots is often not recognised for what it is. Poor early growth in a tree may be attributed to other factors when the reality is that it is the plant spending time and energy making good that root loss. We should be looking to maximise the growth we get from all our trees, so that, if even only 10% of any season's growth is by way of compensation for the past winter, it would be better to eliminate the need for this. Think about each of your trees, make plans for each of your trees and you may achieve good results in 5 years rather than 10 ! it can happen.