Weed flora of cereal crops in Canterbury, New Zealand

Weed communities in Canterbury,New Zealand, cereal crops were characterised in the 1990-91, 1991-92, and 1992-93 growing seasons by measuring species population densities and harvest-time biomass in 39 and 45 fields respectively of wheat (Triticum aestivum) and barley (Hordeum vulgare) in the absence of herbicide treatments. A total of >57 species in >49 genera were recorded representing a total of 23 families with Asteraceae, Brassicaceae, Caryophyllaceae, Fabaceae, Poaceae, and Polygonaceae predominating. Annuals were more common (29 taxa) than perennials (12 taxa) whereas biennials (4 taxa) were least frequent. Seven species occurred in >50% of young crops: Trifolium sp. (mainly T. repens) (94% of crops), Capsella bursa-pastoris (85%), Viola arvensis (69%), Stellaria media (69%), Polygonum aviculare (67%), Chenopodium album (67%), and Anagallis arvensis (63%). Weed population densities varied greatly between species but there was less variation between years and crop type. Above harvesting height at crop maturity, most weed species occurred less frequently and just eight species occurred with a frequency >20%; Polygonum convolvulus (34% of crops), P. aviculare (33%), Chenopodium album (28%), grasses (mainly Elytrigia repens) (28%), Crepis sp. (mainly C. capillaris) (26%), Viola arvensis (23%), Sonchus oleraceus (21%), and Vicia sp. (21%). Weeds above harvesting height occurred more frequently in wheat than in barley and their biomass was on average, greater in the wheat crops. The :numbers of weed species per field varied from 6 to 16 and there was a strong trend toward co-dominance by several species.