Juncus effusus grows in large clumps about 1.5 metres (4 ft 11 in) tall at the water’s edge along streams and ditches, but can be invasive anywhere with moist soil. It is commonly found growing in humus-rich areas like marshes, ditches, fens, and beaver dams.
The stems are smooth cylinders with light pith filling. The yellowish inflorescence appears to emerge from one side of the stem about 20 centimetres (8 in) from the top. In fact the stem ends there; the top part is the bract, that continues with only a slight colour-band marking it from the stem. The lower leaves are reduced to a brown sheath at the bottom of the stem.