Eryngium aquaticum aka Water Eryngo

Eryngium aquaticum also goes by the name marsh eryngo. It belongs to Apiaceae, the carrot family. This wildflower has soft, not woody, stems. It comes from eastern North America. It grows in coastal wetlands from New Jersey to Florida. Its range extends west to Mississippi. It grows in bogs, marshes, swamps, and wet ditches. It also grows where limestone lies near the surface. Sources call it perennial, meaning it lives for several years. NC State also lists biennial growth, a two-year life cycle.

Plants form coarse clumps about 1.5–4 feet [0.46–1.22 metres] tall. Reported garden widths reach 3–4 feet [0.91–1.22 metres]. The stiff stems stand upright and branch. They have ribs and angled sides. Long leaves grow from the plant's base. These can reach 3 feet [0.91 metres] in length. Some are narrow straps; others are broad, lance-shaped blades. Their edges may be smooth or have a few teeth. Upper leaves can have spiny teeth or deep cuts. The leaves lack hairs and may look green, blue, or gray-silver. NC State calls the leaves evergreen.

Small flowers form round heads near the tops of stems. Each floret, or small flower, has five parts. Most flowers look white; some have blue or purple tones. The heads measure less than 0.08 feet [0.025 metres] across. NC State reports blooms in summer and fall. Missouri Botanical Garden lists blooms from September to October. The flowers draw native bees and other pollinators. Both flowers and leaves suit fresh or dried arrangements.

Grow marsh eryngo in sun or partial shade. It prefers moist to wet soil and accepts shallow standing water. Garden loam also works when it stays moist. NC State lists acidic soil, sand, and soil rich in organic matter. Once plants take root, they can cope with drier soil and drought. Both US sources place it in USDA zones 6–8. Rich soil or excess shade can make tall stems sprawl. Some plants need support. You can raise plants from seeds. The plants dislike any disturbance to their roots.

Use it beside ponds, in rain gardens, or in damp borders. Plant groups in wet low spots or native gardens. Groups also suit gardens for pollinators. Neither US source reports serious pest or disease trouble. Plants For A Future lists no known edible uses or hazards. It records past uses of roots to treat fever and urinary problems. It also notes that large doses can cause vomiting. These records do not prove safety or medical benefit.