Thanks much Janet. I'll summarize what I've found from further searching of the RHS plantfinder DB.
arendsii group (AG): A. chinensis, A. japonica, A. thunbergii and A. astilboides
A. x arendsii = A. chinensis × A. japonica
http://apps.rhs.org.uk/horticu...
I've not been able to confirm when x arendsii is used vs. japonica hybrid or thunbergii hybrid. That said, given x arendsii parentage I'll assume the following:
x arendsii = hybrids with A. chinensis × A. japonica parentage.
http://apps.rhs.org.uk/rhsplan...
B. x crispa = A. chinensis var. pumila × A. simplicifolia
http://apps.rhs.org.uk/horticu...
x crispa hybrids:
http://apps.rhs.org.uk/rhsplan...
C. japonica hybrid = any A. japonica cross with AG, or AG hybrid, except A. chinensis (or varieties, i.e. x crispa).
http://apps.rhs.org.uk/rhsplan...
D. thunbergii hybrid = any A. thunbergii cross with AG, or AG hybrid, except A. chinensis (or varieties, i.e. x crispa). What the nomenclature is with a japonica x thunbergii I don’t know.
For the purposes of the ATP DB (and correcting duplicate entries) the RHS Plant finder is very valuable.
