This kō's name isn't a metaphor — it describes what educated 17th-century Japan believed was literally happening. Shibukawa Shunkai, the astronomer who led the Edo-period revision of this entire calendar into its Japanese form (already noted at several earlier kō), held that decaying grass and rotting bamboo roots genuinely transformed into fireflies, not as poetic fancy but as documented natural fact.
The belief made a certain kind of sense at the time: firefly larvae do live in damp soil and leaf litter near exactly the kind of decaying vegetation this kō names, and bioluminescence itself wouldn't be scientifically understood for centuries. Two unrelated things sharing a location, at a moment before anyone could test the connection, hardened into a stated cause.
Milder and more delicate than its relative unagi, classically simmered until tender rather than grilled with a heavy sauce.
The earliest fruit of the year is starting to appear, though the true peak of flavor is still weeks away.
Already in full season in Okinawa and Kyūshū well ahead of the rest of the country, its sharp bitterness mellowed by salting and a quick blanch before cooking.
The mature flower bud, distinct from the young shoot (myōgatake) of a few kō back — sharper and more aromatic, classically shredded raw over cold tofu or noodles.
Hotarubukuro's bell-shaped flowers take their name directly from this kō's own subject: children once tucked caught fireflies inside the blossom, its shape and pale color turning the whole flower into a small glowing lantern for the length of an evening.
An evening tradition old enough to appear in Heian-era court life, still practiced today along clean, slow-moving rivers where fireflies remain common. Because firefly larvae need genuinely unpolluted water to survive, a healthy firefly population functions as an informal indicator of a river's condition — their presence or absence over the years quietly tracks environmental change that's much harder to see directly.
This is one of the few kō in the entire calendar where the compiler's own stated belief, not a folk saying or a poetic convention, is preserved directly in the name — most of the 72 kō describe what a season looks or sounds like; this one records what Edo-period Japan's leading astronomer sincerely thought was true.
Fireflies favor unpolluted water with overhanging vegetation — a spot with both is worth returning to at dusk over several evenings during this kō's window.
Artificial light interferes with firefly signaling and makes their own light harder to see — patience in the dark rewards better than any lamp.
"Lost in sorrowful thought, even the fireflies over the marsh look to me like my own soul, wandered loose from my body."
Most species peak for only two or three weeks and are highly sensitive to weather — a warm, still, moonless night shortly after dusk gives the best odds.
The first fruit of the season is rarely as sweet as what's still weeks away — a preview rather than the main event.
Izumi Shikibu, grieving a troubled marriage, looked at fireflies over a marsh and saw her own soul drifting loose from her body — and by legend, the god of the shrine she'd come to pray at answered her poem with one of his own, gently telling her not to let her spirit scatter like that. Shibukawa Shunkai, a working astronomer with no interest in metaphor, looked at the same insect and concluded rotting grass had literally become it. Both were reaching for the most convincing explanation available to them for something genuinely strange to witness. What do you currently believe, in good faith, that someone a generation from now — with information you don't have — might recognize as this same kind of well-intentioned guess?