Welcome to the world of the 'not-perennial'.
Annual, biennial, perennial - three words which actually represent points on a gradient - from ephemeral to Bob Brown's "bomb-proof". Between 'biennial' and 'perennial' there are a host of short-lived perennials which do generally die after a few years. Their saving grace is that often they self-seed. If they don't its a bit of a con the hort industry telling us they are perennial.
How often do the garden reference books tell you that a perennial is short-lived? Or if it runs about, and if it does, how rapidly or its mechanism of spread. Lots of questions the books don't tell you the answers to.
But go to something like a Hardy Plant Society meeting and the air is thick with anecdotal material on just these kind of questions. So, I thought, why not design a research questionnaire which aims to get experienced gardeners answering carefully targeted questions on just these issues? As it happens there was an EU project: European Regional Development Fund (ERDF), European Territorial Cooperation 2007-2013.Interreg IVB North Sea Region Programme, which I was able to join and get (pretty decent) funding. Hurray for the EU! And of course to the Landscape Department of the University of Sheffield, in particular Nigel Dunnett and Mel Burton.
So, here are the results.
There is the 'lite' version, reprinted from the Hardy Plant Society journal (long-term plant performance), and for those of you who feel up to it, the full report.