Another contender for the oldest plant is horsetail. An ancient plant, they're known as living fossils. Like ferns, horsetails reproduce with spores. Unlike ferns, their leaves are small and grow in whorls around the stem.
Horsetails don't mind "wet feet." An area that might drown out other plants might be just perfect for these water loving fellows. Try them by a pond or in a variety of areas with wet soil. From sandy to clay, they'll be okay. Equisetum fluviatile is available in our mail order nursery.