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

Mignon dahlias are great. Flower all summer long, still going now, just need to keep nipping off the dead heads. Don't need any support either. There are some short heleniums which won't be affected much by wind or rain, Short 'n' Sassy is one we have. Some crocus/other spring bulbs and aubrieta for some colour early in the year perhaps.

I grew some mignon from last season's seed swap and am impressed with their habit and colours. Am currently looking for some compact shasta daisy seeds possibly snow lady. Although all these are spring sowing not autumn.

Penstemon is long winded from seed....I take cuttings anytime of the year and they all seem to take...look in peoples gardens and ask