This is a baby Horsetail, which we call 'Tsukushi (土筆)' in Japanese. It's one of the mountain vegetables that everyone enjoys to eat here in the spring. I found a lot of them growing on the riverbank near Lake Sayama.
Some people make Tsukushi tempura or just boil it and eat it with other veggies. It can be cooked in various ways. I really like Tsukushi rice.
Today is Showa Day here in Japan. It's the birthday of our previous emperor, Emperor Showa, who reigned from 1926 to 1989. It's a beautiful sunny day, a great day to go and pick some Tsukushi for dinner!