The medium height delphiniums I grew (no longer remember the variety, but I grew from seed) lasted several years and were nice only until they got terrible mildew every year so I ripped those out. I loved the short bushy variety I grew, 'Blue Butterfly', but I tried it twice and though it bloomed all summer each time, it was effectively an annual and didn't come back the following year.

I guess that I could try sowing it from seed, but I haven't investigated that possibility, but as an annual I decided it was too expensive to buy the plant. Sorry not to be more encouraging.