かき根に春の kakine ni haru no
立ちしより tachishi yori
古せにおふる furuse ni ouru
若菜をぞつむ wakana o zo tsu mu
Even at hedges of
spring has come.
Since then, people gather
growing in the ancient field.
In the lunar calendar, the beginning of Spring is New Year's Day, which lands around February 10th. This is the biggest celebration of the year for Japanese people. In Japan, people used to count their age different from how we do today. When they are born, they are one year old, and on New Year's Day everyone becomes one year older. Even if a baby is born on the last day of 12th month, on the next day, the baby becomes two years old. In a sense, New Year's Day is everyone's birthday. Spring, and New Year's Day, comes both to the high-class family and to the humble people's home without discrimination.
"Gathering young greens" refers to the custom of eating rice-gruel with seven kinds of young greens growing during this time of the year, that is around the middle of February in the solar calendar. This custom still continues in Japan.
"The ancient field" is a translation of furu-no (古野) and appeared in Menzan's version of this poem. Furu (古) is old, or ancient. No (野) means field. In the older versions, this word is furu-se (古瀬). Se means a shallow and rapid part of a river. Probably Menzan changed furu-se to furu-no because gathering young greens growing in shallow water does not make sense. Furuse ni could mean by the shallows instead of in the river, or that it is simply the name of a particular place where Dogen composed this poem. Anyway, I think, if we want to interpret this poem as an expression of Dharma, the contrast of young and ancient (old) is important.
In the 15th chapter of the Lotus Sutra, "Spring Up Out of the Earth", there is a parable in which numberless bodhisattvas spring up out of the earth and they are very old. But Shakyamuni Buddha says that those old bodhisattvas are his disciples. People asked a question: those bodhisattvas are much older than Shakyamuni who taught only 40 years or so after his attaining awakening. It is like a father that is younger than his children. How such a thing can be possible? In the following 16th chapter, Shakyamuni explains that his life span is eternal.
In Shobogenzo Hokke-ten-hokke (The Dharma Flowers turn the Dharma Flower), Dogen wrote:
"In general, at the time of the Dharma-flower, without fail the father is young and the son is old. It is not that the son is not the son, or that the father is not the father. We should just learn that truly the son is old and the father is young."
In this case, what Dogen means is that the young father that gives birth is our moment by moment practice, and the old son is the eternal life of the Tathagata. Shakyamuni Buddha said in the Sutra of the Buddha's Final Teaching:
"From now on all of my disciples must continuously practice. Then the Thus Come One's Dharma body will always be present and indestructible."
Dogen's interpretation of this is that our practice at this present moment gives birth (manifests) to the eternal life of Shakyamuni's dharma body.