The Bed Of Afterthoughts- what to plant this summer?

I'd appreciate your ideas on planting this border for the summer

Last summer we dug a semi- circular border in the lawn to give the oddly shaped back garden some definition. The border was finished so late that I just chucked in what I had so now it's time to do some better planning. Sorry for the murky photo!

The sun moves from the wild corner across the border so that the middle bit gets sun most of the day.


The far "wild" end is good, there's a little pond, hardy Geraniums, young Stinking Hellebore and towards the main bed some Astilbes I had. The Gladioli are left in as an experiment, I think the shape of the foliage is spot on there even if we don't get many flowers.

Now here comes the problematic area- between Gladioli and the Viburnum just visible at the front right with the fairy lights.

Cosmos or not to Cosmos? Last summer I filled the area with Cosmos and , mildly put, overdid it a bit. The display was stunning though, reaching way over the fence and it swayed beautifully in the breeze. However, the mass of plants was so big that it totally hid the dinnerplate Dahlias and pushed the lawn edging inwards. (You can see the flattened bit of the curved edge )

Plants and seeds I already have:
Dahlias (dinnerplate and Franz Kafka, tubers now overwintering)
seeds of: Campanula Canterbury Bells (60-90cm tall)
Antirrhium Royal Bride (90cm tall)
Malva Moschata Alba, Scabious Musical Prelude, annual Lavatera

Rosa Rugosa in a container

Dwarf Buddleias (one already in the front)

Evergreen giant Bellis (ugly, but evergreen and copious in the front garden- some cuttings are now thrown in the middle of the bed in the photo)

I'd like to have some height, structure and form, and continuous display over the summer. We are renting, so "long term" means maximum 3 years

Thanks very much!