Although we’ve had some hot days this summer, the rain and resulting high humidity have been very prevalent, sometimes making working outside on our trees unpleasant. Some of you may have an indoor area where you can bring trees into the cool of air conditioning for trimming and general tidying up. I have no such space, so at this time of the year, I generally tend to my trees either in the morning or in the early evening, while it’s still light. What a blessing daylight saving can be!

In late summer, it’s worthwhile checking on the conifers in your collection. Black pine branches which have matured since forming in the spring may be cut back by up to half, to encourage more compact budding. But do not cut back to where there are no needles, as existing needles are necessary on each branch to keep the branch alive. Unlike figs and some other species, conifers cannot regenerate if all the foliage is removed.

To tidy up my junipers I wear disposable gloves and gently work through the foliage pads of each branch with my fingers, removing brown needles and any white webbing from caterpillars or the like among the foliage. It’s amazing how such pests can hide themselves from the naked eye, only to be revealed when the foliage is examined carefully! Then I trim each branchlet (if required) by cutting behind a section of growth, but ensuring that what remains has actively growing shoots. In my early days of bonsai we were told to ‘pinch, pinch, pinch’ the growth tips of our junipers. This practice actually leads to a loss of vigour in these species, as the energy for growth is in those very tips. If repeated too energetically, it can lead to the loss of branches and may eventually result in the death of the tree.

Some of the days (and nights) have been quite windy, so, as always, it’s important to keep a close eye on things and not to let your trees dry out. Showers during one day or overnight may not mean you don’t still need to water the next day. Some smaller or shallow pots may require watering more than once a day, and should be kept in a more sheltered position for protection from drying winds and the sun. They can be placed in trays filled with pebbles or sand to maintain humidity around them. Plants such as wisteria, swamp cypress, lilly pilly, melaleucas, callistemon and banksias should be sitting on pebbles in trays of shallow water as well, as they are very thirsty species.

Watering is generally best done in the morning, as early as possible. The stomata (breathing pores) of the leaves on our trees close up for the remainder of the day if the plant is stressed for moisture at any stage. Use a watering can or hose with a fine rose to avoid disturbing the soil surface. If hot, windy weather is forecast, mist all foliage and the surrounding area of trees with a spray bottle, to create a more humid environment, or water more than once per day, if your schedule allows. You can immerse pots in water every so often to help keep plants in good condition, thus preventing the potting media from becoming water repellent, which can occasionally happen if it dries out too much. Adding a seaweed solution to the tub when you do so can give an added boost to stressed trees.

Keep your maples and other deciduous species sheltered from hot, dry winds, and preferably under shade, to prevent burning of their leaves from the strong sunlight at this time of the year. Continue to fertilise your trees through the latter part of summer, but don’t do it on very hot days, to avoid burning the roots. Any deciduous trees which have been defoliated earlier should have been placed in a shaded area until next month, and then only exposed to sun when the temperature is not too high. The new set of leaves will be smaller and colour better in autumn, because they will have a higher concentration of sugars.
There is still time to repot figs and other tropical species, but once the nights start to cool later in the month it will be too late.
Many trees have two growing periods in a year; spring growth, which gives new leaves and lengthens branches (high nitrogen), and the second spurt, autumn growth, which thickens the trunk and branches (storing carbon). This latter spurt is the most important for bonsai growers, as it assists in giving an aged appearance to our trees. This is why using an organic fertiliser in the final months of summer is important to support this process.

Continue to shape branches throughout late summer, with the exception of any sacrifice branches, which should be left to grow unchecked. Rotate your trees regularly so all sides get an equal amount of light, to promote even growth and prevent die back of weaker branches.

If we do get late summer rain, watch for any powdery mildew appearing on crepe myrtles or oaks, and treat appropriately. Eco fungicide, for example, is quite effective and safe, but needs to be re applied every 7-10 days. Fungal disease can weaken a tree over time, so it’s important to address such problems when you first notice them.