While it is not preferable that a planting material has a high tannin content because tannin shrinks the plant root to suppress growth of the plant, the ligneous part of the plant has a high tannin content, and coconut outer husk also has a high tannin content. Thus, when coconut outer husk is used as the culture medium, a plant cannot su?’iciently grow on account of the in?uence of tannin. Even if coconut outer husk is repeatedly subjected to a water soaking in order to previously remove tannin, tannin seeps little by little for a long time. Thus, a su?icient amount of tannin cannot be removed from coconut outer husk.