My freinds and I were talking about how the Catholic Church uses the palm fronds from the previous year's palm Sunday for the ashes on ash Wednesday and I began to wonder. What would they have used in medieval Scotland or Poland? Would they have just used any trees they had on hand or what?
Seeing as it has been a while since this was posted and no one of higher standing has an answered I'm going to mention something I heard on NPR a while back. Also, http://en.wikipedia.org/wiki/Palmer_%28pilgrim%29
In the middle ages there were people called Palmers that were for-hire pilgrim essentially. They were hired by wealthier people who didn't feel like making the trek all the way to Palestine to collect a palm leaf as proof they went there. This was usually as part of some penance or something similar to the Muslim Hajj.
Perhaps these same people collect palm leaves en mass for churches as well. Also, this is were the surname Palmer comes from as well.
The simplest explanation is that since in Poland people did not know what a palm was, only that it was used to greet Jesus, they simply thought that this must have been some beautiful tree. They made such decorations from available materials.
http://en.wikipedia.org/wiki/Easter_palm (link to wikipedia to provide CC illustrations).