The Twelve Kingdoms Wiki

Riboku (里木) are trees bearing the ranka of humans and hanjyuu. The trees have white branches and trunks made of a hard substance that cannot be cut, even by swords such as the Suiguutou. Each village will have a riboku located at its centre, which people will pray to for livestock, grain, and children. Ranka may only be picked by the person who prayed for it.

Most prayers to the riboku occur on the first seven days of a month. To indicate their prayers, people will tie ribbons to the branches of the riboku. Fowl, such as chickens and ducks, are prayed for on the first day, dogs on the second, sheep and goats on the third, boars and pigs on the fourth, cattle on the fifth, and horses on the sixth. Livestock take one month to ripen and people may tie multiple ribbons, though there is no guarantee that all of them will grow a ranka. Human children may be prayed for on the seventh day, and any day after the ninth of a month. However, only one ribbon may be tied onto the tree at a time. The eighth day is reserved for the ruler of a kingdom to pray for crops.

Human children may only be prayed for by married couples and they will often design their own ribbons to reflect what they wish for their child.