It’s called Nazuna in Japanese and Shepherd’s Purse is its common name in English.
The Nazuna is included in a basket pack of the Seven Herbs of Spring (Nanakusa) that is sold at the supermarkets. Rice porridge is cooked using these seven herbs. Eating Nanakusa on the seventh day of the first month of year is said to keep illness away from us (it helps to sooth the stomach after the New Year feast). It used to be thought that the young herbs’ vitality in winter help to exorcise bad spirits. The seven herbs are Seri (dropwort), Nazuna (shepherb’s purse), Gogyou (cotton weed), Hakobera (chickweed), Hotokenoza (cotton sow thistle), Suzuna (turnip), Suzushiro (Japanese white radish). This phrase was in a book called ‘kakaishou’ which was probably written in 1362. People began eating Nanakusa-gayu from Heian period (794-1192). More info in Linda Inoki’s article below:
Haru no nanakusa (Seven Herbs of Spring)
By LINDA INOKI
I also enjoy the seventh day of the first month, when people pluck the young herbs that have sprouted fresh and green beneath the snow. It is amusing to see their excitement when they find such plants growing near the Palace, by no means a spot where one might expect them.
From the 10th-century “Pillow Book of Sei Shonagon,”
Since ancient times, Japanese people have gathered the “Seven Herbs of Spring” on the seventh day of the new year, and eaten them as a healthy start to the year. The herbs, pictured above, are (clockwise from back left): suzuna (turnip), nazuna (shepherd’s purse), suzushiro (daikon), hakobe (chickweed, here trailing over the basket), seri (Japanese parsley), gogyo (cudweed) and hotokenoza (a kind of dandelion; alternatively henbit deadnettle). Today, it is easy to find these fresh herbs in supermarkets, and some flower shops sell decorative versions planted in small baskets. But you are less likely to find them growing in the countryside, because new year on the current calendar is about five weeks earlier than on the old one.