The fresh water of the Guadiana favours the growth of swamps with reeds while the brackish water of the Cigüela favours marsh vegetation particularly cladium mariscus, the great fen sedge. The sedge fields are the largest to be found in Western Europe.

In the shallower areas there are large patches of bullrushes, cattails and reeds. One of the most common formations is that of the fields of carofito algae, known locally as ova, which form an almost continuous carpet in flooded areas. Occasionally, on some of the island banks you will find a taraje - a tamarind - the only tree species of this national park.