It is a large, strong plant, 3 foot high with maybe 20 leaf spikes but I didn't get a single flower this year. I've had it two years and I didn't prune it at the end of last summer, after flowering. Is this why? ( I didn't know I was supposed to!)

RHS plant care tells me to 'cut flowering spikes to ground level in late summer'.

Should I prune the whole thing down to ground level, then? In that there are no flower spikes??