I haven't written much about decorating your venue yet, but I am working on a general entry on the subject. Until that's done I had to share this idea with you, that I have seen in several harvest inspired weddings.
These days the grain stands golden and ripe in the landscape dancing in the winds. A drive through the countryside is a true visual pleasure and of course an August medieval wedding could benefit from these inspiring settings.
This is a very simple centerpiece that only requires dried wheat, and a pretty ribbon.
www.drieddecor.com and the pictures in this entry are from that same website. You can also attempt to make them yourself. One good thing about working with dried centerpieces is that you can make them weeks in advance easing the stress in days up to the wedding.
Whether or not the medieval weddings included such decorations is not possible to say, but as the tradition of throwing rice comes from throwing grain for fertility wheat might have been a relevant choice for decorating the halls.