Now that it is so,
though one would sow the seeds
of the grass of forgetting,
would that even this
were not left to another’s heart.

Meaning

Now that it is so and our bond is ended, one may say to forget and sow the seeds of the grass of forgetting; yet would that even this were not left to your heart’s will.

Commentary

Episode Twenty-One: "If I go forth and leave"

There were a man and a woman who loved one another and had no thought of turning their hearts elsewhere, yet over some trifling matter their married life became burdensome.

Long after the woman had left the house, unable to endure waiting any longer for the man who did not come in search of her, she composed this poem and sent it to him.

Though she herself had departed from him, what did she expect in sending a poem that asks him not to forget?
The "grass of forgetting" is said to be a plant that makes one forget sorrow, yet it is also used to mean a plant that makes one forget a person.

Source

Ise Monogatari

Other

Now that it is so, though one would sow the seeds of the grass of forgetting, would that even this were not left to another’s heart.

Now that it is so,
though one would sow the seeds
of the grass of forgetting,
would that even this
were not left to another’s heart.