By the calendar it is still winter, but there are a lot of jobs that can be done to make it easier for the spring rush when a heck of a lot needs to be done in a short period of time. Firstly we must look at our climate here in Perth (Or depending on where you live) we get winter rains and intense summer heat. With the constant watering, our trees receive 3-4 more times the amount of water than that of our northern hemisphere neighbors.
Our growing season is longer and more intense than most other regions, so when we read that we re-pot only every 1-2 years, we only apply this to "mature" bonsai. This doesn't apply to younger trees that are still developing. Because of our extended growing and watering, this tends to wear out the soil much more quickly. For this reason we have no other option than to re-pot.
In this situation however, do not over fertilize the plant as this only creates more problems, stick to re-potting in Spring and Autumn and the difference will be easy to see.
Now is the time to go outside and see how much work is ahead for you, and how you are going to plan for it. This is a good time to prepare tools i.e. sharpening or replacing old ones. Prepare your soil, and if you need a new home for your trees, now is the time to do so.
Things to do:
For natives, there is a risk that interfering with the roots can be detrimental to the tree. Consider carrying out this work during late August, I have found that this is the best time to work on natives.
For Deciduous trees, August to early September is the best time to carry out this work. You can bare root, sort out the root structure as needed. In the bare branch state, the tree structure is exposed, this is the best time to prune and wire the branches. After pruning, be sure to seal the wound with a recognized sealer to prevent disease entry and minimize unsightly scarring. When wiring in spring, be very careful to not damage the swelling buds on branches and trunks.
For conifers, if you decide to re-pot be sure to add one or two fertilizer cakes or as per package instructions. This will give them a head start heading into the spring season.