Answer: In a quick search I was able to find a reference to Corylus americana growing successfully under a black walnut, however not specifically this filbert. (Yours may be grafted onto a different rootstock, ie the species C. avellana for example.) Unfortunately, I am not able to find anything specific to the Itea. However, you may find the following helpful or interesting.
There is a certain amount of anecdotal evidence about plants that do or do not thrive under these trees. Having gardened with several myself, I would suggest doing some site analysis that goes beyond the effects of the juglone alone. For instance, many plants have trouble growing in shade, or in competition with a large tree, especially if planted in proximity to the tree roots -- so failure may be related at least in part as much to those aspects as to the juglone from the black walnut tree.
In this case, I would caution that in my experience the Itea does better in moister or even wetter locations. And, in my experience the slow growing Harry Lauder needs ample sun -- about a half day -- except in southern gardens where the summers are excessively long, humid, and very hot.
Having said that, I would probably encourage you to experiment and see how it goes, and be prepared to transplant in a year or so if they fail to thrive. Good luck!
Q&A Library Searching Tips