Many commercial bonsai sold at nurseries are labelled as "indoor bonsai". Whilst some more tropical species find our climate too cold and so need to be kept indoors, the vast majority of bonsai will not thrive in the house. Central heating makes it too warm, the air is too dry and there is not enough light. But also beware of placing bonsai on a bright windowsill; the glass can amplify the heating effects of the sun and at night the temperature next to windows falls dramatically.
Outdoors your bonsai will have a constant supply of sunlight, fresh air and rainwater (this is the UK after all). You will need to monitor the bonsai to ensure that, if they are in prolonged sun exposure during the hotter months, the soil does not dry out or the leaves become scorched. Keeping them in a semi-shaded area will help with this.
You also need to ensure that they are adequately watered, even if it has rained, as the canopy may have prevented any water from reaching the pot. Most trees need to be kept moist at all times and will not tolerate becoming completely dried out. However, more bonsai are lost through over-watering than drought so you also need to avoid allowing the soil to become waterlogged. The only thing you cannot control is the air supply and the temperature.
If temperatures fall below minus 3ºC, bonsai trees will need some protection. They can be put in a conservatory, greenhouse or garage for shelter. The important thing is to protect them from extreme temperature changes and winds, but to allow them a winter dormancy period.
The last requirement of a bonsai is nutrients. As their root system is confined to a pot, they cannot grow out in search of nutrients and so rely on you to provide them. The three main nutrients are nitrogen (N), phosphorus (P) and potassium (K), often marked as NPK on fertiliser packs.
- Nitrogen is needed to promote vigorous growth and to produce strong stems and healthy leaves.
- Phosphate is essential for strong, healthy roots and for prolific fruit production.
- Potassium is important for flower production and disease resistance.
A plant will eventually die from a lack of one of these nutrients. They also need a number of trace elements, such as magnesium or zinc, in order to flourish.
Beware of soil or fertilisers that are sold as "for bonsai". These are often no different from other varieties that are available, but will cost a lot more. For more information go to basic equipment.