Abstract
In the years 2009 and 2010, studies were carried out on Polygonum lapathifolium L. subsp. lapathifolium population established in the cultivated fields of the Wigry National Park and its buffer zone. Every year, 30 plants of the Polygonum lapathifolium L. subsp. lapathifolium were picked and collected from each of the three habitats: potato fields, winter and spring cereals. The following traits were examined: plant height, stem length to the first branching, number of internodes and their length, number of first-order branches and their length, number of leaves, number of spikelet-like inflorescences (spikelets) and their average length, the top inflorescence (spikelet) length, seed number per plant and 1000-seed weight. The analysis of the obtained results indicated large morphological variability of plants, which depended on the type of cultivated land. The most favourable conditions for weed growth and development occurred in potato fields, where Polygonum lapathifolium L. subsp. lapathifolium plants were the highest, most branched and productive. The opposite result was noticed in winter cereals, where Polygonum lapathifolium L. subsp. lapathifolium managed to produce the least seeds. Comparisons of the results of morphological measurements for Polygonum lapathifolium L. subsp. lapathifolium taken in the same sites during two growing seasons demonstrated relatively small variation of the analysed traits.