I can't figure this out -- why would "Alba" rugosa be own root, but "Blanc Double de Coubert" isn't (from Weeks, anyway)? They're both rugosas and they're both large. Anyone have any insight on this?
The species are often seed grown and form large suckering colonies if left to their own devices. The hybrids - like Blanc Double de Coubert - and named forms are most often grafted to ensure they a) do not sucker aggressively and b) come true to form.
ETA: btw, 'Alba' is not a named form but just a variant of the species. Taxonomically, it should read Rosa rugosa var. alba, not R. rugosa 'Alba', as if it were a defined cultivar.