Many weavers are found in wetlands and sedges are important elements of many wetlands.
Sedges (family Cyperaceae) are graminoid (grass-like), monocotyledonous flowering plants with over 5500 species globally. Their stems usually have triangular cross-sections and their leaves are spirally arranged in three ranks (compare grasses that have alternate leaves with two ranks). One of the most well-known sedges is the papyrus sedge (Cyperus papyrus).
Weavers use sedges for feeding, roosting, nest material and breeding. This blog will focus on weavers feeding on sedge seeds in Cape Town. Weavers feed on the seeds of sedges. In the Western Cape sedges are especially useful for seedeaters as there is less grass available. The seeds are available in late spring when many juveniles have fledged and adults are starting to get ready to moult.
Cape Weavers

The video shows an adult male feeding on seeds of Schoenoplectus scirpoides (previously Scirpus littoralis; identification confirmed by Cliff Dorse), and then a juvenile struggles for much longer but manages to get some seeds.
Southern Red Bishops
I have observed small flocks of Southern Red Bishops foraging on sedge seeds at Rondevlei (Promerops 268:15) and at Intaka, although I did not obtain good photos (probably Schoenoplectus scirpoides in both cases).

List of sedge species taken by weavers
Cape Weaver – Schoenoplectus scirpoides (see above)
Northern Brown-throated Weaver – Cyperus papyrus (Britton 1978)
Yellow-crowned Bishop – Cyperus dilatatus (Fry 2004)
Southern Red Bishop – Schoenoplectus scirpoides (see above)
Thick-billed Weaver – Scleria schimperiana (Bingham 2012)
Yellow-mantled Widowbird – Cyperaceae sp. (Fry et al. 2004)