I see that no one has responded to this. I probably won't be very much more help than nothing. But, here goes. Christopher Lloyd used lots and lots of wallflowers in his borders. According to his books, they are prolific self seeders. You certainly could buy a couple extra plants as insurance, but maybe they will take care of the problem themselves.

Thanks for your reply.
I'm not familiar with his books but I do wonder if he is talking about wallflowers or bowles mauve which is different.
I have to look up most latin and I'm lazy right now.

I rarely get much reseeding of plants because I mulch a lot and weed a lot. I am going to leave them as is and plant another one in the middle to hide the open stems. Then perhaps cut the other two back after layering some of the stems.

This is a somewhat tricky situation for the reason you have discovered - this plant seldom stops blooming long enough to allow for pruning! But do it anyway, otherwise you will consistently get the result you have now, with the splaying and exposing of the woody interior. I wouldn't advise cutting back into the woody portions - like many subshrubs or woody perennials, new growth from this point is infrequent and at worst, you could kill the plant.

'Bowles' Mauve' is a hybrid evergreen perennial form of Erysimum and does not reseed like the more common biennial wallflowers. It does root quite easily from cuttings though, which is a good thing as the plant is not very longlived. It quite literally blooms itself to death after a couple of seasons - 2-3 years is about as long as you can expect for good performance before it starts to decline.

In my climate, this perennial has a reputation for being one of the longest blooming choices for a perennial garden and will produce a nearly constant display of flowers from early spring until well after frost, at least 6-8 months. IMO, that more than makes up for its rather short life span :-))

Thank you Gardengal for that helpful info. Here in my climate, to a small degree, I have similar conditons to the PNW which might be why they have performed so well.

I forgot to add that they are a little known and little used plant here which is a shame because they have performed so well in my garden.