See also Obtaining a shakuhachi
To best understand how to care for a shakuhachi, it's important to understand the flute and its materials.
The most fragile part of the shakuhachi is the utaguchi (blowing edge). It is a part of the flute which has been honed down to a fine sliver. Many shakuhachi have an ivory or other material used, but some shakuhachi are one single piece, including that blowing edge. If damaged, repairs are possible but expensive. A capable craftsperson-player would need to be used.
Bamboo is very hard, but it has several fragile qualities. Heat, quick temperature changes, humidity and humidity changes can all damage it. Temperature changes will make the bamboo shrink and grow just a little bit. Humidity is a very significant threat. Moisture changes in the bamboo will often cause cracks to appear. Once a surface crack appears, it /will/ continue to grow until it penetrates through to the bore. Sometimes this can be repaired, but sometimes it will render the flute unplayable. However, if caught early, binding the shakuhachi at key points will halt crack growth.
Because of the problems humidity causes, airtight storage is important. It is also very very important to clean the flute after every usage. Do it gently, making sure to not drag anything against the blowing edge. Be slow and thorough, do it twice. When cleaning, protect the blowing edge by grasping underneath the blowing edge, with your thumb protruding alongside the edge without touching it. Many artists will hold their flutes in this manner when they are idle.
The inner bore is sometimes made "by reduction". This means that the entire inner area was hollowed out by removing material. Many shakuhachi are then "built back up" using other materials. If the expansion and contraction of the flute is extreme, that inner material can buckle, flake or chip. Changes in the inner surface of the flute can distort its sound.
Urushi is sometimes used to lacquer the inside and perhaps the outside. It is a very hard and very durable substance. It's rare, but it's possible it may need a touch-up if there is damage to the bamboo or inner materials.
Do not subject it to heat. No direct sunlight and do not place it near a heater. Don't leave it in your car on a hot day!
- Some people will keep it under their coat if walking with it during winter.
- No humidity. This demands careful cleaning after every use, and storage in an airtight container.
(source) I've seen a recommendation to rub a very light coating of walnut oil on the surface two or three times a year, to prevent drying.
Perhaps the monks of old times would walk rain-or-shine playing their flute, but you may want to have a longer life for yours!