Recent, much appreciated rain has meant that any time saved from the need for constant watering over summer, has now been substituted with the need to weed! It’s important to keep your trees as weed free as possible. Weeds will rob your bonsai of nutrients and moisture, which in the small confines of a bonsai pot can be a particular problem.
Keep an eye out for insect invasions, as the warm days mean that caterpillars and other chewing insects are about. A problematic insect for me this summer, in my collection, has been scale. They love the dry, warm days and spend their time sucking greedily on the sap of stems and fresh new foliage. Olives, citrus and Ficus ‘Little Ruby’ are species which seem to be particularly prone to infestation in my garden. You can pick the scale off and squash them, or spray with pest oil, a non-toxic insecticide, which suffocates them. Also check your pines and junipers for white webbing insect pests hiding in amongst the foliage.
The days are still warm, so keep up with the watering and feeding, taking particular care if the wind returns. Use a low nitrogen organic fertiliser during autumn to 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.
The foliage on your deciduous bonsai may have suffered from the summer heat, 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 disease. If the weather is not too hot, move trees like liquidambar, gingko, Chinese pistachio, Manchurian pear, some maples and zelkova into sunnier spots to improve their autumn colour. Shorter and cooler nights are the plants’ message to slow down, so as much sunshine as possible helps their colour. Autumn is also a good time to buy these plants when they are in colour so you know what 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. 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. 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, rewire if necessary. If there has been cutting in, it’s a good idea to rewire in the opposite direction the next 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 season.
Prune pines, if you haven’t already done so, by removing candles once they are one to two centimetres long. Leave strong candles to develop on weak branches and weak candles to develop on strong branches.
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.