This summer has not been particularly hot in Sydney, because in among some hot days we have experienced much milder days as well. The humidity, though, with the showery weather in February, has meant that I have had plenty of issues on my trees with such pests as scales of various types (white wax scale being the worst on some natives) and the usual caterpillars and fungal infections on susceptible trees. I’m sure I’m not alone though in facing these challenges! The weather usually warm in early autumn, so keep up with the watering and feeding, taking particular care if it’s windy.
Spacing your trees out to improve air circulation can help prevent some fungal diseases and stop hungry insects from migrating from one bonsai tree to another. Try to remove insects manually if the infestation is only small. Some may even be curled up in leaves, so hunt carefully. Try to only use insecticides when absolutely necessary, as there may be beneficial creatures such as praying mantises and ladybird larvae searching for the insect pests we hate for their dinner! I have constant issues with white wax scale on my Acacia howitii, for example, and need to apply granular insecticide with annoying regularity. Additionally, bear in mind that it’s important not to spray any trees which are in flower, as this will be detrimental to foraging bees in the area.
Use a low nitrogen (preferably organic) fertiliser during autumn to help harden late growth and improve flower and fruit production in spring, as plants need to store energy as we approach winter. Remember to use a fertiliser low in phosphorus for your natives. Fertilising is also doubly important, as recent wet weather will have leached nutrients from the potting mix you are growing your trees in.
The foliage on your deciduous bonsai may have suffered during summer, but the trees are now trying to build up energy for the winter months, so try to get as much light as possible on them during the autumn months. Remove any dead leaves and check that the surface of the soil is free of dead foliage, as this may harbour fungal diseases. As the weather becomes cooler, move trees like liquidambar, ginkgo, Chinese pistachio, Manchurian pear, maples and zelkova into sunnier spots to improve their autumn colour. Longer and cooler nights are the plants’ message to slow down, so as much sunshine as possible during the shorter days helps this process. Over this period of time deciduous trees remove chlorophyll and nutrients from their leaves and into storage for the next growing season, and seal off each leaf with a type of cork, until it eventually falls from the tree. Autumn is also a good time to buy deciduous plants, so you know what autumn foliage tones to expect in the future.
Tidy up figs by removing any overly large leaves and cutting back any shoots that are too strong for the rest of the tree. Night temperatures will be starting to drop below 16 degrees soon, so it’s too late for repotting these tropical species. If you do get caught out, though, you may get away with it by moving recently re potted figs indoors each evening for 2-3 weeks in autumn and positioning them in sunshine each day to aid their recovery. I have successfully used this strategy in the past for Ficus species that I missed re potting during summer. I was glad last year to have finished tidying up all the figs in my collection before the end of summer, but this year, there are a couple which I have noted will need to be repotted first next summer, as I have just not had enough time to devote to that this year.
Elms and other trees, particularly evergreens, that have thrown out long branches can be cut back to help develop ramification. Reduce the density of foliage as well, by selectively trimming off unwanted shoots and leaves. Generally, shoots that are opposite each other should be trimmed to one shoot only, and shoots coming from underneath a branch can be removed.
Check your trees for wire that may be cutting in, as branch girth may have increased recently. If branches have not yet set to where you want them, re wire if necessary. If there has been any cutting in, it’s a good idea to rewire in the opposite direction to the last time.
Don’t cut back deciduous flowering trees or plants such as azaleas and camellias, as you don’t want to remove the flower buds that are hopefully forming for next flowering season. I usually spend time during the warmer month of February removing old flowers and much lush foliage from the Bougainvilleas in my collection. They are often neglected during the height of summer, because I have so many figs to attend to, and they are very forgiving. The roots of these plants are very brittle and break easily during the repotting process. Luckily, though, new roots grow back readily in summer, and they are also easy to strike from cuttings at the same time. It’s too late now though (autumn) to re2cpot such tropical species.
Prune pines, if you haven’t already done so, by removing candles once they are one to two centimetres long, unless you want to increase the length of that particular branch. Leave strong candles to develop on weak branches and weak candles to develop on strong branches. Make sure that at any one spot on a branch you only permit one candle (or two candles if additional elongation is wanted from there) to develop and remove any extras. Pines will often throw several whorled shoots from the one position, particulary in spring. If they are all allowed to develop, this can lead to an unsightly reverse taper on the trunk or that branch, and much scarring if the resulting unwanted branches are removed later.
Some people even repot their pines in March, but I prefer to do mine in August.
Autumn is also a good time to start preparing your potting mix, as there will be plenty of use for it later in the season and during the winter months. I try to have plenty of the ingredients I use on hand for when I’ll need them. There’s nothing worse than planning a day of repotting and styling, only to find you first have to go and purchase the requirements you need to make this happen. Start thinking about which trees you want to move to a new pot, or pre-bonsai that may be ready for their first bonsai container, and plan accordingly.