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Fuji-Hakone-Izu NP, Kanagawa
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This wetland is a remnant of ancient Lake Sengokuhara, which once filled part of Hakone’s caldera. Volcanic activity reshaped the basin, leaving today’s marsh as a trace of a much larger lake.
When resort development threatened the wetland, a local innkeeper and the village vice-mayor appealed to the government. In 1934, about 0.7 hectares were designated a National Natural Monument.
The 1-centimeter ōruri-hamushi leaf beetle has a metallic-red sheen, despite a Japanese name meaning “large blue leaf beetle.” A Kanagawa Museum account described Sengokuhara as its only known site in the prefecture.
Round-leaved sundew catches insects on sticky leaf hairs. Kanagawa’s 2022 Red Data Book recorded just one confirmed locality for it in the prefecture: Sengokuhara Wetland.
For generations, villagers cut reeds for thatched roofs and fodder, and burned the grass to keep the land open. After the last fire in 1970, alder trees began reclaiming the wetland.
Winter burning alone let tall reeds crowd out shorter marsh plants, so managers added summer reed-cutting. In 2021, they compared flower counts in cut and uncut plots to assess its effects.
Opened in 1976, neighboring Hakone Botanical Garden of Wetlands was Japan’s first garden devoted to wetland plants, created to introduce and help protect this marsh. Its 2-hectare restoration area grows local wetland flora.
One local story says Minamoto no Yoritomo predicted Sengokuhara could yield 1,000 koku of rice if reclaimed; another credits 4 pioneering farm households. These are competing place-name traditions, not documented quotes.
