The weather is finally getting cooler and winter is on the way, so hopefully your deciduous bonsai will be starting to show some autumn colour. Keep the trees in full sun, if possible, as sunlight combined with cooler nights improve the colour intensity. If you are in the market to purchase more deciduous species, this is a good time to look around, as you can see what autumn display any new trees are likely to give you each year.
Make sure to regularly clean up any fallen leaves on the ground or pot surfaces, as these can harbour fungal disease and insect pests, which may have a negative impact on your trees in the future. Remove old flowers and berries from trees and move tropical species, such as figs, to more protected areas, if possible under cover, so they are not damaged by the cold weather ahead.
If you have azaleas in your collection, lace bug may have been a problem during the growing season (evidenced by mottled silvering on the leaves). Hopefully, you have used a suitable systemic insecticide to deal with them during the warmer months. These pests are not active during the cooler months, but if you need to, spraying should recommence in September.
Gently remove any moss that has grown over the roots of your trees, trying not to damage the bark underneath, and persevere with the removal of weeds.
Tree growth will generally be slowing down, so reduce the use of fertilisers or use a weaker solution. Seasol can be beneficial year round as a tonic, especially after repotting or when trees are stressed in any other way. Native trees will continue to grow in the cooler months while the deciduous species are dormant. As the season progresses, all your trees will benefit from as much exposure to sunlight as possible.
Trees should be rotated year round, so the same side is not always facing direct sun. This helps prevent one side of the tree becoming weak and the side too vigorous and helps to keep the spread of foliage more even. This can be particularly problematic if your bonsai collection is growing against a barrier such as a wall or fence, as light will be completely blocked from that direction. You can tell if your trees need to be turned if all the leaves and branches are facing in one direction. I try to place my trees facing a new direction whenever I have removed them from the bench to work on them to prevent this becoming an issue.