Flower:

Dense clusters of short-stalked or stalkless white flowers, 1/8 inch long or less with 5 fused petals, the triangular lobes widely spreading, nearly equilateral, about as long as the floral tube, and with blunt tips. The calyx lobes are broadly triangular to almost round, typically half as long as the floral tube or less. Inside the floral tube is a large, globe shaped ovary with two styles, each about as long as the ovary with a prominent round stigma at the tip (capitate). The ovary is swollen around the base of the styles creating a small ridge called a stylopodium; between the two styles is a slight depressed crease. Hidden inside the floral tube, surrounding the ovary, are fringed scales that are about as long as the floral tube. 5 yellow-tipped stamens are attached to the petal below the base of the sinus between the petal lobes.

Leaves and stems:

Leaves are tiny and scale-like or absent altogether. Stems are hairless and slender, typically yellow to orange, forming wiry masses that twist around and are supported by the host plant. Along the stem are small appendages (haustoria), modified roots that penetrate the host plant and draw moisture and nutrients from it.

Fruit:

Fruit is about 1/8 inch diameter, nearly round (globose) capsule but for the dried stylopodium that creates a thickened ridge around the opening depression at the tip of the capsule, from which the seeds are disseminated.

Notes:

Swamp Dodder is one of nine Cuscuta species either present (6) or historically documented (3) in Minnesota. Like one of its common name implies, it is our most common species, not just in Minnesota but across North America. Swamp Dodder is distinguished from other 5-petaled Dodders by flowers with blunt-tipped triangular lobes, the short calyx, and by the stylopodium which may be most easily seen on the fruits. While most dodders are associated with a broad host range, some are fairly specialized to only a few species and when those species are natives in a dwindling habitat, some of those dodder species can become rare. While this is the most common dodder species across North America, C. gronovii appears (at least in literature) not to be the pest that some other species are capable of. It is noted as an especially serious pest in cultivated cranberry fields and well as in carrot production, but per a personal communication with a large cranberry grower in Warrens, Wisconsin—at the very heart of commercial cranberry production in the US—the problem there is scattered and minimal at worst.

All dodders are obligate parasites, that is they must obtain all their life support from a host species to grow and reproduce. Dodders not only sap energy from their hosts but are also capable of moving diseases from one host to another. When a dodder seedling germinates, it must quickly contact a suitable host upon which it immediately begins to twine around the host plant's stem, invading its tissue via the haustoria, after which the initial seedling root quickly withers away. As the stems grow, they contact and invade more stems, even crossing over and connecting to other suitable host species. All species of dodder are on the federal noxious weed list, except some native species (including Minnesota's natives) as well as a few, now widespread non-native species. Still all dodders, including natives, are "regulated" requiring federal permits for importation or transportation of seed.