The aim of the study was to compare weed species diversity in organic and conventional farms in Poland. The study was carried out between 2012 and 2014 on production fields located in Lublin province, one of the easternmost regions of Poland. The results showed that diversity and abundance of weeds in spring cereals were generally higher in organic farming system than in conventional one. No significant differences between systems were found in 2013, because local flooding of fields in spring made impossible the execution of agricultural weed management treatment for some farmers. Both above-ground and soil seed bank weed communities were the mainstay of valuable weed species, which is confirmed by the presence of the species threatened with extinction, for example, summer pheasant’s-eye (Adonis aestivalis), poorman’s blue weatherglass (Anagallis foemina) and others in both tested farming systems. Dwarf everlast (Helichrysum arenarium) was the only found species that is subject to partial protection by Polish law. Both organic farms, run under CAP policy and support, and conventional extensive farms of the province of Lublin were the mainstay of biodiversity. Future rural development and CAP should be adapted and implemented regionally. Traditional extensive farms could contribute to the biodiversity and valuable plant species conservation.

The assessment and conservation of agricultural biodiversity is one of the current goals of European Union policies, which are reflected in European Biodiversity Strategy. The biodiversity of soil seed bank and above-ground weed flora were investigated in spring cereals in organic and conventional farming system. The study was carried out between 2012 and 2014. Species of above-ground wild flora were counted on a surface of 1 square m in the field. Soil for seed bank evaluation was taken from the field and evaluated by seedling emergence method in a greenhouse. There were no significant differences in number of species between two communities. Biodiversity of weed community, described by Shannon’s diversity index and Simpson’s dominance index, was significantly better maintained in the soil seed bank than in above-ground flora in both farming systems. A strong, positive correlation between the number of species, Shannon’s diversity index, Simpson’s dominance index of soil seed bank and above-ground weed species was found in both farming systems, while abundance of seeds and above-ground weeds was significantly, positively correlated only in the conventional farming system. Qualitative composition of weed flora depended more on the type of community (soil seed bank or above-ground weed communities) than the farming system.