Summer is a very busy time for the bonsai enthusiast. For many species, spring growth will continue and one must be mindful of keeping vigorous trees trimmed to encourage short internodes.
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. We seldom see plantings of trees on rocks at exhibitions as these are extremely difficult to maintain. They require copious amounts of water throughout periods of high temperatures. If you own any trees planted over rocks, they need more protection during the summer months because the rock can become very hot and decimate roots. Place these in a more shaded position. 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 constantly 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 it or 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 40 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 are best repotted during the winter months, when 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 bougainvilleas, 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. 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 collection if we let them! If you are spaying for insect pests, do so in the early morning or the cool of the evening, to prevent damaging the foliage.
So far, we have had good rainfall and cooler days this spring, with just the occasional hot day. Very soon, though, the heat is likely to ramp up for the next few months, so preventative steps should be taken now to ensure that your bonsai do not suffer. Clean up your benches and remove dead leaves and any other debris, as these can harbour insects and fungal disease.