After reviewing some Astilbe hybrids at the MOBOT and RHS websites, among others, I've some questions about proper naming.

This listing for Astilbe (x arendsii) 'Federsee' explains the (x arendsii) hybrids as "...crosses between A. chinensis, A. japonica, A. thunbergii and A. astilboides..."
http://www.missouribotanicalga...
RHS DB confirms name as (Astilbe 'Federsee' (× arendsii)) here: http://apps.rhs.org.uk/horticu...

Astilbe (Japonica Hybrid) 'Deutschland' at MOBOT here: http://www.missouribotanicalga...
Does (Japonica Hybrid) indicate a species cross or a hybrid of A. japonica? Why not A. japonica 'Deutschland'?

Our DB is using x arendsii, how should we deal with these Japonica Hybrids?

My guess is they don't know the exact origins of the japonica cross which would be a hybrid.

I checked on the RHS in the plant selector which is different to the plant finder, they only have one Astilbe japonica hybrid listed called Astilbe 'Rheinland' (japonica hybrid). Their preferred name is Astilbe 'Rheinland'.

I would think in cases like this that is the way to go, with Japonica Hybrid only in the extra field.

http://apps.rhs.org.uk/plantse...

Searching for Astilbe japonica hybrid in the plant finder, there's several japonica hybrids listed which are or have at some time been offered by nurseries here participating in the RHS scheme.

http://apps.rhs.org.uk/rhsplan...

The first, Astilbe xardensii: Parentage A. chinensis × A. japonica

Astilbe 'Bonn' : Parentage japonica × unknown

Checking some others it's the same story, one says japonica x unknown and the most say nothing which indicates they don't know. Often a good form is found where nature has done the pollinating.

A search for just Astilbe brings up all the records which might be useful.

http://apps.rhs.org.uk/rhsplan...

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.

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!

Some history on breeding.. a lot of crossing going on!

http://www.plantheritagedevon....

There's links to other National plant collections on the site including Astilbe which is the other National collection holder..

http://www.plantheritagedevon....

This is a great deal to digest Janet. Thank you for the valuable resource material.

Another group I didn't mention earlier are the simplicifolia hybrids. They appear within the searches for japonica, chinensis and x crispa hybrids.
japonica: http://apps.rhs.org.uk/rhsplan...
chinensis: http://apps.rhs.org.uk/rhsplan...
x crispa: http://apps.rhs.org.uk/rhsplan...

Non list parentage but perhaps (simplicifolia hybrid) takes precedence because the A. simplicifolia parent has known lineage or has only simplicifolia lineage. Does that make sense to you?

I couldn't say what takes precedence Evan, but I searched for Astilbe simplicifolia on the RHS and got a good list where most mention (simplicifolia hybrid) .. whether or not it has known lineage above others it's crossed with is something you could maybe contact one of the National Collection holders here?

http://apps.rhs.org.uk/horticu...