Fifth survey on weed flora in spring cereals in Finland

Regular weed surveys provide information on changes in weed communities. The composition of weed flora in spring cereal fields was investigated in southern and central Finland during 2020–2022. The survey was conducted in 16 regions, 226 farms and 549 fields. The occurrence of weeds was assessed in the second half of July from 90 organically cropped and 459 conventionally cropped fields. The weed flora was dominated by broad-leaved species, representing 81% of the total number of 138 identified species. The average species number per field was 21 in organic fields and 12 in conventional fields. The most frequent weed species in organic fields were Chenopodium album 99%, Erysimum cheiranthoides 84%, Viola arvensis 83% and Stellaria media 82%. In conventionally cropped fields, the most frequent species were Viola arvensis 83%, Stellaria media 65%, Galeopsis spp. 59% and Galium spurium 59%. The two most common grass species in both production systems were Poa annua and Elymus repens. The frequency and density of Poa annua had increased substantially in conventional cropping since the previous survey in 2007–2009. The average density of weeds was 384 plants m-2 in organic fields and 147 plants m-2 in sprayed conventional fields. The average air-dry biomass of weeds was 678 kg ha-1 and 151 kg ha-1, respectively. Elymus repens produced almost a quarter of the total weed biomass in both production systems. Changes in weed flora were minor in terms of frequency and density of the most common broad-leaved weed species. Increased infestation of Poa annua in conventional cropping calls for special attention. The dominance of Elymus repens decreased in both production systems since the previous survey.