The text books suggest that the single should indeed be lower growing than the double. The fact that yours is so tall would tend to suggest that it is 'stretching' itself to get more light. You can prune back herbaceous plants but this often mucks up their flowering. If it is troublesome and you don't mind a gamble then maybe try 25% reduction or perhaps you need to lift it and move it to a more suitable location. Both species are ok for root division in the autumn so this would be a good time to move it.
Replied: Friday 14th of June 2013 09:19 AM