In Tessei town in western Okayama, there is a wetland called Koi-ga-kubo Shitsugen whose range of rare and interesting flora makes even the difficulty of getting there well worthwhile.

Located in the Chugoku mountain chain that runs through western Honshu, the area is now home to several zanryu-shokubutsu (plants that are remnants from when the archipelago was joined to continental Asia). These plants can only be seen there, at one location in Fukuoka and in North Korea, while many others there are also extremely rare and endemic to Japan.

Altogether, the small, 3.6-hectare shitsugen (wetland) lying 550 meters above sea level contains around 300 species of wild plants and is recognized as an important academic study area that, in 1979, was designated as a national monument.

Long ago, in more superstitious times, locals gave the place a wide berth, fearing it was a bottomless marsh that would swallow them up. Then in 1694 a dam was built to create a pond-reservoir and ensure a steady supply of water for drinking and irrigation, and this was reconstructed between 1854-60.

Now, you will find many of the most interesting plants growing in the marshy area between the water’s edge and surrounding woodland, and the 2.4-km path around the reservoir is just long enough for a pleasant afternoon stroll.

Mid-July through September is the ideal time to visit Koi-ga-kubo Shitsugen and see its wetland perennial flora. One very rare plant — which at first glance looks like a form of nadeshiko (pink) — is the Ogura-senno, a species of catchfly (Silene kiusaiana) whose 60-80-cm stems are slightly branched and bear red flowers with frilly-edged petals. Keep an eye out, too, for the Bitchu furo (Bitchu geranium; Geranium yoshinoi), so-called because Bitchu was the feudal-era name for western Okayama. This perennial, with stems 40-70 cm long and leaves with three to five deep lobes, bears paired, 2-cm-wide light-red flowers with deep-red striations.

When I visited the wetland in mid-July, two species of loosestrife were in flower. One was the yellow-flowered Kusare-dama or ioso (yellow loosestrife; Lysimachia vulgaris var. davurica), a rhizomatous and stoloniferous (creeping-stemmed) perennial whose blooms are borne in terminal panicles, and whose smooth-edged leaves are arranged in groups of three or four on the stem. Oka-tora-no-o (gooseneck loosestrife; Lysimachia clethroides), on the other hand, has white flowers borne on stems 60-100 cm long. Their tendency to nod in the breeze has prompted comparison with a tiger’s tail — in Japanese tora means “tiger.”

Another of the wetland’s attractive perennials is Mikoshi-giku, a wild chrysanthemum (Chrysanthemum lineare) which has daisylike, white flowers from September through November and is here often found along with hankai-so (leopard plant; Ligularia japonica), which can reach 150 cm in height. This perennial’s flowers are orange-yellow and borne in large corymbs, while its lobed, deeply incised leaves are large, up to 35 cm wide and cordate-orbicular in shape.

If you go to the wetland anytime between August and October, look out, too, for the beautiful tsuru-rindo (climbing gentian; Gentiana trinervis), a scandent or even trailing perennial that grows to between 40 and 80 cm and has smooth-edged, ovate-lanceolate leaves 3 to 5 cm long. This gentian’s tube-shaped flowers are 3 cm long and pale-blue to purple in color, with five lobes at the tip.

One plant endemic to Japan that you can find here is sagiso (Pecteilisradiata), a perennial terrestrial orchid that grows to a height of 15-40 cm in sunny, marshy meadows. During its flowering period in August, this orchid’s frilled white petals resemble a sagi (snowy heron or egret) in flight — hence the name.

At Koi-ga-kubo shitsugen, however, aside from the wetland perennial flora there are also many fine tree and shrub specimens. One interesting tree is the nezumi-sashi, also known as nezu and muro (temple juniper; Juniperusrigida). This small conical tree only grows to about 6 meters in height, though it has wide-spreading, arching branchlets that hang gracefully from the main trunk. Nezumi-sashi grow best on hills and ridges in poor soil throughout Honshu, Shikoku and Kyushu, and on the mainland up to northern China. The leaves, always in groups of three, look like awl-shaped needles that are so sharp and stiff that it’s said not even a mouse (nezumi) can pass through them without getting a prick (sashi).

The wood surrounding the wetland and reservoir, however, mainly comprises aka-matsu (Japanese red pine; Pinus densiflora), the deciduous oak konara (Quercus serrata), yamaboshi (kousa dogwood; Cornus kousa), while the shrub layer is composed mainly of asebi (Pieris japonica), dwarf tsuge (Japanese holly; Ilex crenata) and Daisen-mitsuba-tsutsuji (Rhododendron lagopus).

All in all, there’s plenty to enjoy in this Okayama wetland, making it well worth the trouble of getting there — and taking the risk of disappearing in the “bottomless marsh.”

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