This is funny, my Archicad pool has weeds growing in it. I tried to google and resolve but did not find a solution. 
Hi @tomppu and welcome to our Forum.
It looks like you have setup a grass material you may want to check via the Enscape Material Editor. Additional information about the usage of Enscape’s grass generally alongside further guidance when it comes to the editor and other materials can also be found in this corresponding knowledge-base article here as well.
Just in case you are unable to resolve this behavior you can also make use of the support report button as detailed here and our dedicated support team will investigate further. Thanks a lot in advance.