Hi littlebucket, your Delphiniums are perennials in any country any zone anywhere, what some people may have, is too cold a zone and loose these plants to frost and the roots being killed off by deep ground freeze, therefore will have to plant new each year, however, with a load of leaf mould or some other mulch in winter, they will survive. when they die back on you, in winter, that is their rest period and as the new shoots come through in spring, they will have managed in that period to make new shoots therefore the plants will bulk up and get bigger but they should not reach anything like invasive, as for self seeders, these will only become a problem IF you dont weed them out, or let them grow on till they are big enough to pot on and increase your stock or give them away to fete or other group trying to get funds for whatever. It is a good idea to lift and split your clumps of Delphiniums say every 4/5 years as if left alone, they die in the middle and all the growth to the outside, you would do this at the end of the season or, when new growth shows in spring., this is a good time to add compost for water retention and also some food granuals to replace all the energy food they have used up while flowering their hearts out over the years. if you do keep the self seeded plants, they may not be as good as the parent as the birds and insects may have helped to croos polinate for you, these seeders will take 2 years to reach good flowering plants, hope this helps you some, they are beauties and dont know anyone who would not be thrilled to have the lovely show that you have and your colour coordination is just great, well done and good luck. WeeNel.
I just started these indoors from seed in hopes of adding them to my butterfly garden later this year. Our area is zip code 44028 in Lorain County, Ohio. Having never started a perennial from seed, I'm a bit nervous about the outcome. The package claims they take 18 to 24 days to germinate so I must be patient. If they thrive, they will make a wonderful addition to my garden!