It looks like you have summarised that well Evan! Headaches.
Yes it is a valuable site, and can be relied on.
There is no result for Astilbe japonica x thunbergii on the RHS but I guess there are crosses. That makes me wonder if it's not possible unless another parentage is involved with which they will cross. For example, A. japonica will only cross with thunbergii if that is already crossed with say chinensis with which it is compatible, but I'm only guessing there.
I searched for the cross and found a site with a few Astilbe hybrids as well as others in Saxifragaceae, Lynn might find the species useful.
http://www.tiptopglobe.com/cel...
On wiki it states "and others" apart from the four species mentioned for x ardenasii.
http://en.wikipedia.org/wiki/A...
I searched out the hybridiser George Arends and found a mention of "ISHS Registrar for Astilbe" on one site so searched that and found it.. you might find the next link interesting too!
http://www.nccpg.com/Info-cent...
http://www.holehirdgardens.org...
So, if you have any queries you know where to get the answer!
