This is one of the busiest times of year for all bonsai artists. One needs to keep vigorous trees, particularly deciduous species, trimmed to encourage short internodes. It’s important to keep the balance between pruning and allowing growth just right, though, as trees that are constantly cut back may lose vigour, which is not what you want at all. Branches must be allowed to elongate each time for a period, before they are tidied for the purpose of developing the design of the tree.
Watering is the most important task during summer, because lack of it and hot weather can mean high fatality rates, especially in miniature trees and shallow potted bonsai such as groups and root over rock settings. Plantings of trees on rocks, in particular, can be challenging to maintain during this time of year. They require copious amounts of water throughout periods of high temperatures. If you own any trees planted over rocks, they need more protection over the summer months, because the rock can become very hot in the sun if you’re not careful and decimate roots. Place these trees in a more shaded spot, or you may want to place the bottom of the rock in a tray of water to keep it cooler. Wet cloth can also be used to cover exposed roots. As long as it is not allowed to dry out, the roots will be better protected.
On very hot days it may be necessary to increase watering from once to twice daily, avoiding the middle of the day when it is hottest. Consider using mulch- sphagnum moss will prevent surface roots drying out. Other mosses and pebbles also help keep surface roots cooler.
Miniature trees are best placed in shaded areas in trays containing a layer of pebbles and water. This creates humidity around the tree. Make sure the water level is quite shallow and regularly check that it does not dry out. An alternative solution is to use horticultural sand in the tray and push small pots into the medium. Keep adding water as it evaporates. I make good use of old polystyrene fruit and meat trays for this purpose. I also utilise them to save excess moss from when I collect some or I thin out denser moss on my trees. I lay it in the trays in the shade, and keep it watered, along with my bonsai until I need it for some new project.
Move tender leafed plants, such as maples, into shaded areas to avoid leaf burn. Lots of water is still needed, especially when temperatures are 30 degrees plus, but the problem of excessive transpiration can be solved by removing larger leaves from deciduous species such as oaks, liquidambars and maples. You can even defoliate Trident maples, liquidambars, Chinese elms and Zelkovas with large leaves in December. This should be done only if the tree is in good health. The new leaves which grow after this process will be much smaller, which can make your bonsai look better proportioned overall. Do not repot at this time, as these trees should be repotted during the winter months, while dormant.
The most important trees for repotting during summer are figs. While the weather is hot, you can defoliate (for smaller leaves), prune and repot to your heart’s content. It’s also a good time to strike cuttings of figs, as they will root quickly while the weather is warm. The other species that I usually repot during this time is Bougainvillea, which respond very well to pruning while the weather is hot, as they are a tropical species, like many of the types of figs that we grow. If you want to strike cuttings of Bougainvillea during the repotting process, they will generally strike easily as well.
Over the summer, a high nitrogen fertiliser is beneficial to promote strong growth in young plants. Established trees (such as maples) however will thicken their branches if supplied with too much nitrogen and the overall balance of the tree’s design can be lost. For flowering/ fruiting trees high potassium promotes better flowers and fruit. This type of fertiliser should be used from January on for most of these species, to encourage good flowering next spring. Once again, do not fertilise when the temperature is above 30 degrees. Seasol is a year round tonic for healthier trees.
As always, keep an eye on your trees for any insect attacks and treat as needed. Caterpillars, aphids and other pests are only too happy to feed on the luscious leaves in our collections if we let them! If you are spraying for insect pests, do so in the early morning or the cool of the evening, to prevent damaging the foliage.
Keep your trees as free of weeds as you can, so that all the nutrients and space in your pots is available for your trees’ roots to utilise. Also remember to rotate them occasionally, so all branches receive an appropriate amount of light. This practice is very important in collections such as mine, where there are many trees clustered close together and sometimes shading each other from the sun. Branches that don’t receive enough light may become weak, or even die, forcing you to alter the design you had planned for that bonsai.