I harvest all my Chinese Celery about 2 months ago, except one for the seeds. But this plant just keeps on growing. It's already 3' tall. The leafs are yellowish somewhat. But the whole plant, including stems and leafs, still looks very healthy. It shows zero sign of dying back or even drying out. I see no seed pod at all.

So my question is: how much longer shall I wait till I can collect the seeds?

Thanks.