Penstemons

Yes, they can spread quite a long way, into quite bushy plants. It takes a couple of years though.

Crocus.co.uk for good quality plants. They get quite big. 18" apart sounds right. Very useful plants. They go and on year after year. Cut off dead flower heads to get more to come. I love penstemonns.

Just a thought. You often get them for sale at church Spring Fairs. Cheap way of getting them.

I've just had my plug plants delivered. They recommend that they be planted 18 inches apart. Is this wright? I've only ever Seen them in the pots in the nursery.

Parkers are very good, I think probably they're all much the same so I don't necessarily stick to the same one every time- they all have similar guarantees.
Good Luck- I've got some Penstemons growing on at the moment, so far they're doing well in pots but not in greenhouse.