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.
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.
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.
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.
Identify trees which need to be repotted in the coming spring. A couple telltale signs of a tree which needs a repot:
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.
Lime Sulphur is the one spray I highly recommended for applying in winter. It is ideal for preventing over wintering of common pests and fungi. It may be used on deciduous and evergreens bonsai trees. It is a highly effective fungicide solution which controls anthracnose (blackened growing tips affecting new leaves on deciduous trees especially maples), Mealy Bug, Mites, Powdery Mildew, Scale and Wooly aphids.
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 can use a dilution ratio of 1:15 (Lime sulphur : water) or weaker such as 1 : 20.
Image caption. Lime sulphur is a very versatile spray to prevent the over-wintering of many pests and fungi.
A great way to use old beverage containers, having to avoid mixing large quantities of spray and to avoid having to clean large sprayers is to use our nifty sprayer attachment.