The palms are inseparable part of Polish culture and tradition during Easter time. They are made using traditional materials such as: desiccated flowers, grass, corns and herbs.
These substitutes were used in place of real subtropical palms, which were never easy to obtain in Poland.
It is customary to begin Palm Sunday services with a procession, in which a palm-carrying congregation circles the parish church three times.

See this link for more information about Easter Palms in Poland

http://culture.polishsite.us/articles/art274fr.htm