You may have done all your winter bonsai tasks already but I have prepared a short list of some of those which I think are important and apply to most bonsai growers. Find more winter tips here also.
By now I am sure you all have done whatever pruning you wanted to do, but if not it should be completed before the buds become active.
Remember not to prune branches which are being used as sacrifices or that still need to thicken at the base. Pruning them would be counter-productive. It's also not a good idea to prune large branches in winter, assuming you are wanting back budding (buds which form further back along a branch or trunk which you can use in your design as future branches), its better to do this during the active growing periods.
Be careful of pruning trees such as Maples, especially Chinese maples at the end of dormancy as they will bleed sap profusely in spring - regardless of you sealing the cuts.
Weeding before its too late
Weeds love this time of the year, at least in the Western Cape as we are a winter rainfall area. Be sure to weed as soon as these pesky plants begin to grow, or you might find they get out of hand before you know it. I find its a good idea to walk around with a bucket and throw the weeds into it as you go through your trees. Otherwise these weeds are thrown to the ground and simply grow there and seed, becoming an even bigger problem.
Too much moss
Everyone loves moss, including myself. Moss makes the ideal bonsai tree ground cover for many reasons both aesthetically but it actually serves some important functions such as acting as a form of water filter.
If you live in a wet and cool area, moss will proliferate and envelop the entire surface of the pot and will result in greatly increased water retention, which in turn may lead to root rot or other problems. Keep an eye on this balance of water retention and oxygen (reduced by water logged pots). You may want to reduce moss to a minimum, at least for most of winter, allowing it to develop if you want, towards the end of the season.
It is rather important though that you remove all moss from trunks, this will irreparably damage the bark, as it causes it to rot. Simply paint or carefully spray some undiluted vinegar onto the affected parts of the tree and it will almost immediately kill any moss it contacts. Be careful that you don't spray or treat where you don't want to kill of course and take any steps necessary to reduce the risk.
Its a good time of the year to collect moss though as there should be a lot about. Look in forests, along rivers or fountains, even along the roadside! Try to find different mosses which you can combine around your trees for a little variety, as depicted in the image below.
Image caption. I think there is a perception that moss is grown year round on bonsai trees in Japan. In fact although there may well be some moss which has developed on a tree which has not been potted for a number of years, most of the times when you and I see moss on trees on exhibition, that moss has only days before been placed there.
Select trees for repotting
Identify trees which need to be repotted in the coming spring. A couple telltale signs of a tree which needs a repot:
Some trees may be pushing out of their pots, such as maples or elms. Chinese maples, as shown in the image below can rapidly fill a container with roots. You may decide that all that is needed is to cut off the bottom mass of roots without repotting the entire tree though.
Other trees might be showing signs of weak growth and need to be revitalized. Pruning their roots will often initiate a growth spurt, however this is due to the increase in oxygen around the roots. So first check what the reason is for the weakness and then act accordingly. For example you may find that the surface of your growing medium has compacted over time from watering, fertilizing, surface root development etc. By just carefully raking this top layer off you might find that the medium immediately below is perfectly fine and a complete repot is not necessary. Just top off the surface with more media.
If you intend changing the planting angle of a tree because you are restyling it or have restyled it, then this will require a repot but if the angle change is dramatic you may need to take extra care with the roots especially if they are going to be very much more exposed in one area when compared to before. Its also not advisable to remove ALL the growing media from a conifer or with older deciduous trees.
Image caption. Some species need more frequent repotting than others. Chinese maples can fill a pot in a single season whereas most conifers need not be repotted for many years.
Don't simply repot because it sounds like the right thing to do, or because everyone is repotting and you feel left out. Repotting too frequently results in more bad than good or a tree needs to expend much energy to establish a balance between its roots and foliage again.
Perform the treatment before trees become active as the spray will kill any young leaves, including olive leaves which have not yet hardened off. Inspect your deciduous trees and if the buds are beginning to swell you are too late to spray. Set those trees aside and spray the others.
You SHOULD spray both deciduous and conifer trees.
Do NOT spray deciduous trees which are already active, azaleas, figs or recently wired trees.
You do NOT need to wrap your pots (glazed or unglazed). Yes, they will stain but this stain will disappear after a few months.
Do NOT spray or soak the growing medium "on purpose." You need to drench the tree with the treatment till it is dripping. This excess will go into the soil anyway.
You SHOULD wear old clothes, eye protection and a respirator fitted with a carbon filter/s.
You can use a dilution ratio of 1:15 (Lime sulphur : water) or weaker such as 1 : 20.