Sweet Potato Day (Satsumaimo no Hi)

What Is Sweet Potato Day?

October 13 is "Sweet Potato Day" (Satsumaimo no Hi) in Japan. It was established (reportedly in 1987) by the Kawagoe Imo Tomo no Kai, a citizens' group in Kawagoe, Saitama Prefecture. October is sweet potato season, and in the Edo period roasted sweet potatoes were nicknamed "juusan-ri" (thirteen ri) through the pun "kuri yori umai juusan-ri" — "tastier than chestnuts (kuri)," where kuri sounds like "9 ri" and yori like "4 ri," adding up to 13. That is why the 13th was chosen. "Thirteen ri" is also said to refer to the distance of about 13 ri from Edo to Kawagoe.

How the Sweet Potato Spread in Japan

Able to grow in poor soil, the sweet potato saved many people from famine.

• From China to Ryukyu to Satsuma: The sweet potato is said to have been brought from China to Ryukyu (Okinawa) around 1605 by Noguni Sokan. In 1705 Maeda Riemon carried it from Ryukyu to Satsuma (Kagoshima), and cultivation spread. Because it came via Satsuma, it became known as "satsuma-imo."
• Aoki Kon'yo, "Dr. Sweet Potato": After the Kyoho famine in the mid-Edo period, the scholar Aoki Kon'yo argued that sweet potatoes could help prevent famine. In 1735 test planting was carried out on the orders of the eighth shogun, Tokugawa Yoshimune, which helped spread cultivation across the Kanto region.

Tips for Tastier Roasted Sweet Potatoes

• Slow heating makes them sweeter: Sweet potatoes contain an enzyme that turns starch into sugar. Passing slowly through the temperature range where it works best (roughly around 60–70°C) is said to bring out more sweetness — which is why stone-roasted potatoes and slow, low-temperature oven roasting taste so good.
• Stored potatoes are sweeter: Freshly dug sweet potatoes are said to become sweeter after a period of storage, as some of the starch turns into sugar.

Sweet Potato Trivia

• The white liquid: The milky sap that appears when you cut a sweet potato is a substance called jalapin (yarapin).
• Many varieties: From moist, sticky types such as Annou-imo and Beni Haruka to fluffy types like Naruto Kintoki and Beni Azuma, today you can choose varieties by texture and sweetness.
• Kawagoe, the "sweet potato town": Since the Edo period Kawagoe has been known for growing sweet potatoes to ship to Edo, and it is still full of shops selling sweet potato sweets and dishes.
• Nutrition: Sweet potatoes contain dietary fiber and vitamin C. Eating them with the skin gives you more fiber.

Ideas for Sweet Potato Day

• Enjoy sweet potato dishes such as roasted sweet potatoes, daigaku-imo (candied sweet potatoes), or sweet potato cakes.
• Compare different varieties to find your favorite texture.
• Share the "thirteen ri" wordplay as small talk.
• Plan a short trip to Kawagoe or other places associated with sweet potatoes.

A Day to Savor Autumn Flavors

October 13 is an autumnal anniversary born from the clever wordplay of Edo-era people. Growing even in poor soil, the sweet potato was once a dependable crop that saved many from famine. Today, enjoy a warm roasted sweet potato and think about how it spread across Japan.