Many Polish cities have central heating systems which consist of a main heat-generating plant and underground pipelines distributing heat to customers. Heating networks heat the ground, thus promoting the development of plant communities of atypical thermal and humidity characteristics. The study presents changes occurring over 2000–2002 in the floristic composition of lawns situated above heating pipelines in Kraków. In the most heated areas two different plant communities appear during a year and there are also periods with no vegetation. In summer, when air and ground temperatures are high and soil humidity is low, thermophilous species of foreign origin predominate; these are small love-grass (Eragrostis minor) and large crabgrass (Digitaria sanguinalis). In autumn and winter, when water is not a limiting factor, a community consists mainly of common chickweeds (Stellaria media) and annual meadow grass (Poa annua). The most prevalent species in both communities are annual species which wait out the unfavourable conditions in the form of seeds. On the largest part of lawn area above heating pipelines the heating effect is quite week and contributes mainly to the prolongation of the vegetation period and the acceleration of phenological phases.

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