Starting perennials or biennials in the autumn

I fancy growing some penstemon, never tried them before and know nothing. Is it best to sow seeds, take cuttings (bit difficult as I don't have any) or buy plants? (Heaven forfend!) Not fussed about species or colour. Height 2ft ish preferred.
Are they treated as annuals or perennials, someone is bound to say both

I have a rectangular bed 8ft x 2.5ft alongside a patio which I'd like colourful. I tried dwarf asters which grew to 3ft+, fell over and got ruined by rain, a total disaster. I'm thinking of perhaps mignon dahlias. Any ideas, can be annual or perennials