Hi, Dr. Hill,
This plant is growing in full sun in a semi-moist area adjacent to a pond in the SC fall line and was photographed on October 12. I hope that the sepals, leaves and seed pod will be distinctive enough to identify the plant since there were no flowers still in bloom.
this appears to be Ludwigia alternifolia, usually known as seedbox. The leaves seem a little narrower than usual, but it still fits nicely. It is fairly common at pond margins mostly.
Dr. Steven Hill, Botanist, SCNPS