Watering a Deciduous Bonsai.
The biggest cause of death of Bonsai trees in my experience is inadequate supply of water. It is quite simple: when a tree is growing it needs water.
So first thing to clarify is what we mean by the Growing Season. The oldest myth in the tree world is that spring and summer are the growing season and during autumn and winter the trees hibernate (and this is the dangerous bit) and therefore do not need water.
For deciduous trees the obvious, but wrong, answer is that the growing season starts as the buds begin swelling in the spring. This a good visual clue however at this time the roots will have been growing for about a month before you see any above ground movement, but the amount of water required to service this growth is insignificant. The trees are priming the roots for growth before starting into the full annual growth cycle – the roots service the leaves and the leaves feed the roots, and so forth. If the roots do not get this leap on the leaves then the trees will struggle through the early part of the year and you lose months of the best growing time.
To think the season closes at the time that the deciduous tree drops its last leaf in autumn is also slightly incorrect. The leaves start to change colour and the tree grows a layer of cells at the division point between the leaf and the twig in order to seal over what would otherwise be an open wound. Once this layer of cells is complete the leaf desiccates rapidly and finally falls. The roots are still growing.
To gain an idea of timing of these seasonal events look at a phenology (the science of timing of natural events) website – a really good one that includes native trees is ‘Nature’s Calendar’. It shows the spread across the season between the species and then between north and south. The difference to start of the growing season between an elder in the south and a beech in the far north is several months. And as a quick aside it shows signs that we can all witness that, on average, spring is getting earlier every year, the only logical explanation they draw is that it is climate change in action…
The reason that deciduous trees loose there leaves in winter is thought to be that they would not otherwise be able to survive the time of year when water is in its shortest supply – which surprisingly is the depths of winter. Once you consider that the ground could well be frozen for months. Whatever the reason behind this evolution, deciduous trees do not need winter watering. However it is always prudent to ensure the soil is slightly moist as a desiccated soil can draw moisture reserves out of roots and could lead to ‘freeze drying’ (the same way food in the deep freeze can dry out if the packet is open).