Hopefully all your bonsai trees are being protected for the winter and here is a quick generic guide to bonsai in January.
Your bonsai trees will still need watering but not as often as their growth rate has slowed right down so their water uptake has also slowed down.
At this time of year if your trees are still outside, they will be getting watered naturally by the rainfall, morning dew and melting frosts. However, monitor the moisture level to ensure your bonsai trees are not water logged as this can cause roots to rot.
If your bonsai trees are under cover check daily and water if required to keep the soil damp / moist but not soaking wet.
Any bonsai being kept indoors should be watered daily if required as they will be loosing moisture due to the higher temperatures inside drying out the soil and the tropical species still tend to grow during the winter period inside.
Ideal time to water is in the morning to allow the water time to dissipate and to reduce the surface water should the temperature cool down overnight or there is a frost.
If the weather is mild repotting can be undertaken but avoid early repotting if you have no protection for the tree like a cold frame or green house if frosts are likely.
Otherwise prepare for repotting by checking soil supplies and also that if you are looking to change the pot that you have the correct pot size to repot into.
Generally feeding in January is not required as broadleaves trees are dormant and conifers or evergreens have slowed down on growth during this period.
However, continue to lightly feed tropical trees that are kept indoors if they are still showing signs of growth to sustain them during this period.
This is a great time of year to review your bonsai trees and check the structural image of the tree as well as the tracery and branch placement. Broadleaved trees are especially easy as they have for most species lost their leaves and are dormant. You can remove unwanted structural branches but remember to cover the wounds with wound paste or sealant as the trees are dormant and will not be able to respond to the pruning as effectively. Otherwise just mark the branch(es) to remove and wait till the Spring to then remove them.
This is not required to either broadleaved or coniferous trees in January. However, you may still need to trim the new shoots off tropical trees that are growing indoors to control the growth and develop ramification.
Carry out root pruning if you do start to re-pot now as part of the re-potting process and check for soil borne pests, root rot and other root problems and address if any issues are found.
Insects and pests
Continue to check your bonsai trees over fully for pests, insects and diseases as some insects can over winter on or in your trees like red spider mite, scale insect and if you have not applied a winter pesticide or removed unwanted pests before now it is a chance to do so. Pests are also likely if the weather is mild so worth checking to avoid disappointment in the spring when buds or leaves fail to flush due to pest attack.
Moss and weed control
Hopefully you carried out pre winter preparation and addressed any weed and moss issues particularly around the trunks of bonsai trees prior to winter. If not this is again a good time to remove moss and weeds to clear the surface of the soil for improved moisture movement as well as prevent bark rot from wet moss. You can use a range of techniques including a diluted vinegar application on the bark which you apply and then around a week of two later the moss removes more easily with a light brush action (tooth brush works well).
If wire is present on your bonsai tree check whether it is still serving a purpose and is taught and not digging into the bark causing wire marks and re-set if necessary or tighten.
Continue to maintain winter protection in whatever form you use and check your trees daily for effects of frost or snow.
This is also a good time to photograph your bonsai trees and update their records for re-potting timings, feeding regimes, soil type etc as it is sometimes hard to keep track of what you do for each tree when you start increasing your collection. Keeping a photographic record does come in very useful over the years to see the trees development and your progress.