Long flowering perenials

Alstroemerias and Penstemons will flower their hearts out for you from June till October and come in many different colours and sizes. They are not evergreen, however, and die back each year although some penstemon are semi evergreen.

Agree ES - they just don't know when to stop !
Lavender of course, the scent is so wonderful. I'm also a fan of penstemon, as the flowers go over cut 'em back and they just keep on flowering.
No idea of your soil type or if South/ North facing , the size of the triangle etc.so my reply could be really unhelpful !

You can get little Hydrangeas that grow about 30-35 cms tall. Also dwarf roses, David Austin has one called Jean Mermoz that is repeat flowering and scented.

I also like purple cranesbill. You do need to take the seed heads off though if you want them to keep flowering.
They are not big statement plants but do go on a long time. Mine are about 30cms tall.

Geraniums (cranesbill not pelargoniums) have an extremely long flowering season. Wargrave pink and Roxanne (blue) do well in my garden.