When people find out that I grow bonsai they often respond: “Bonsai trees are very weak, I got an indoor bonsai once; and it died to me. "
No, bonsai are not that weak, they try to cling to life with all their strength. If we help them they will reward our care by showing us their health and beauty for many years. If we help them ...
Bonsai is a potted tree, genetically identical to the ones found in nature, and trees are not adapted to living in a flat. In this environment they need us to provide with what they cannot get for themselves: temperature, light, air, water, and nutrients.
When talking about indoor bonsai, we mean tropical species (minimum temperature 18-20ºC) or subtropical species (minimum temperature 5-12ºC). In the first case we may need hot greenhouse conditions, in the second cold greenhouse ones.
The light of a window may be sufficient if the tree is less than one meter away. Otherwise you have to provide light with culture lamps for about twelve hours a day (seek advice on the most appropriate installation in a grow shop).
Air humidity should be around 50%; if necessary, install a humidifier which will also benefit people living with the tree. The air must be renewed, but avoiding drafts, especially if they are cold.
Once a month give your tree a shower, with warm water, in the bathtub, to clean the dust that prevents it from breathing well.
A serious problem that the cheap trees that we find in the market generally have is the quality of the soil. In many cases they are poted in a kind of very compact silt that prevents the roots from breathing (yes, the roots also need air) and that when dried it is very difficult to water. If we want to give the tree a chance of survival, it is imperative to transplant it to a porous substrate suitable for bonsai, which is easily moistened and allows air to enter.
Watering is critical, because due to the small size of the pot the amount of water available is small, and in hot periods it may be missing in a matter of hours. On the contrary, if we insist on watering when the substrate is saturated and compact we will literally drown the roots causing their rot. The water you use should contain the least amount of calcium possible (ideally 14-16º hardness) and preferably a slightly acidic pH (5.5-6.5).
In the small dimensions of the pot the tree does not find the minerals it needs, they must be provided regularly. I advise you to use liquid fertilizers formulated for bonsai at the doses and frequency recommended by the brand. Someday I will extend in this chapter.
All of the above are generalities, some species may have specific needs that must be considered and, on the other hand, the variety of climates in our country may allow certain subtropical trees to be grown outdoors all year round in some areas.
In Spain, the most appreciated species for bonsai can live perfectly all year outdoors and even appreciate the rest that the low winter temperatures allow them. However, taking into account the fragility of the pot environment and the peculiarities of the climate in each area, it may be necessary to protect some trees from hard frost and others from strong summer sunstroke or strong winds, be these cold, from the North, or hot and dry.
It's really easy to keep a bonsai healthy; My best advice for you is to go with nature, selecting the tree that best suits the environment we have, instead of changing the conditions to adapt them to the needs of the tree.