In article , Spider
writes
I wonder if it has to be an _herbaceous_ perennial? If not, Lavatera would
fit the bill, being substantial in both its form and flowering period.

There are herbaceous Lavateras. Lavatera thurigiaca 'Ice Cool' might fit
the bill. Lavatera x clementii 'Candy Floss' is herbaceous with me,
though I suspect that it might be shrubby in a different climate;
Alnwick Castle has a big specimen (6 feet high by 8 feet across) of
this. I don't know whether 'Sweet Dreams' or 'White Angel' become bulky,
but they are taller than 'Ice Cool'.
--
Stewart Robert Hinsley
http://www.malvaceae.info/