The combination of hot weather and rain may cause a lot of confusion about whether your tree needs water or not. Please ensure you do not relax your guard with your tree drying out.
Sometimes a storm may pass but only wet the surface area of your tree. Do not make the mistake that the rain has fully saturated your bonsai.
This is perhaps the most important aspect of our summer work. If we neglect to water our trees the results could be fatal. Also remember my saying "if the tree is dry water: If not sure water it anyway."
On very hot days it may be necessary to water both in the morning and in the evening. It is a good idea to move cool climate deciduous trees such as maples and other sensitive deciduous trees out of the full sun and into a semi-shaded area.
I recommend watering the foliage as well as the root soil area. Foliage watering reduces the dust that could accumulate on the leaves. I dont think it matters what time of day to be honest.
If you have a lot of small trees you could place the trees into a large tray filled
filled with sand, so the bonsai is surrounded by sand. The sand can act as a water absorber and protect from any accelerated drying out.
Remember, it is impossible to overwater your trees in summer. Any signs of wilting or death; usually is related to drying out r insect damage. If the tips of your bonsai are brown or black. IT MEANS YOUR TREE HAS DRIED OUT.
In summer it is one or the other. Ensure the watering is completely saturating your tree. Misting only the foliage with a sprayer is totally inadequate.
If you are going on holidays, it is an idea to show the person looking after your bonsai how much water to give and how you would like to do the watering. Also show them how to tell when it is enough. Most people are unaware of how much water a bonsai needs.
Another option if you have a collection to think seriously about installing an irrigation system. It comes as drippers or sprayers. Also remember that automated and timer systems can fail so it is important to test your system before you leave. Best practice would be to do a run check occasionally.
I highly recommend this option for peace of mind. But I would recommend following up with watering as the drippers only water one section so dont e afraid to move th sprinkler head aorund every week or two.