Shiny ‘v’ shaped leaves and triangular flower stem
Large multiheaded seed heads with three or more long leaves (involucral bracts) spreading out from underneath
Cyperus eragrostis has green/yellow seed heads which have a brown centre while C. congestus has a dark red seed head with a green centre
Commonly found in waste areas, drains, riverbanks and other marshy areas.
Diagnostic differences
Cyperus eragrostis (25-90 cm) is generally much larger than C. congestus (15-40 cm)
Cyperus eragrostis has a green seed head while that of C. congestus is dark red
Individual Cyperus eragrostis seed heads are more compact with individual spikelets originating from a point whereas the spikelets of C. congestus are spaced along the flower stem (rachis).
Biology
Origin
Both species come from the Americas although C. congestus is confined to South America.
Occurence
C. eragrostis is more widely spread, found throughout the North Island and the South Island except Westland, Otago and Southland. C. congestus is found only in the north of both islands plus Gisborne and Canterbury.
Life cycle
Both are perennials with short thick rhizomes although they both have an annual growth habit, dying back completely during winter. Both are also prolific seed producers. They may behave as annuals in areas that are very wet in winter but much dryer in summer such as roadsides and catchment areas.
Impacts
Impacts on pasture
Although these two species are associated with drains and riverbanks, they will grow in poor, damp or marshy pastures. They are unpalatable to livestock and their spread will reduce pasture availability and utilisation.
Impacts on forage crops
Can invade croplands that are very wet in winter and persist through drier months
May clog drains and lead to increased water retention of wet pastures.
Impacts on stock
These sedges are nontoxic but generally avoided by stock anyway so expected to have little impact.
Control
Grazing and cultural management
Improving drainage and strong pastures are the best management tools
Repeat mowing will reduce seed production and sometimes kill the plants
Cultivation followed by discing to chop up the root masses will provide some control.
Chemical control
Few herbicides are effective on sedges
Glufosinate offers partial control, better on seedlings
Halosulfuron (Sempra) is registered for control of nutsedge but may also offer activity on other sedges
Glyphosate/saflufenacil mixtures are reported to be effective on some sedges
These plants are often found colonising areas where herbicides are used to control other weeds.
Further information
Holden P 2020. New Zealand Novachem Agrichemical Manual. Agrimedia Ltd, Christchurch, New Zealand. 924 p.
Popay I, Champion P, James T 2010. An illustrated guide to common weeds of New Zealand. New Zealand Plant Protection Society, Lincoln, New Zealand. 416 p.
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