Galega bicolor, Goat's Rue, has lilac and white pea flowers for several months. It grows to about 4 ft tall but is not invasive, and is under-rated. There is a white form as well.
Heleniums flower for several months.
Some geraniums also have a long season, eg Rozanne.
This is part of one of my cottage borders, I don’t think anything will last the whole season, I have all the above in different beds, later will be Penstemons, salvias, Phlox, more foxgloves, montbretia, Delphiniums, Aconitum, clematis, perenial sweet peas, Dahlias, different plants at different times. The Heuchera will flower all year, a pink geranium I have, just cut back and off it goes again.
Gardening on the wild, windy west side of Dartmoor.
You could give aubrieta a go, I got a teeny pot in Sainsbury's last year and it has done amazingly well in a container. It has been on the go since early spring this year and it keeps going strong.
But one of the things I love most is going out to see what's come into flower today. Would really miss that if everything stayed more or less the same all summer!
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'Garnet'
P.Hidcote Pink
I've also got White Bedder and Raven which is much bigger than the others - they all flower for ages
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
Heleniums flower for several months.
Some geraniums also have a long season, eg Rozanne.
The Heuchera will flower all year, a pink geranium I have, just cut back and off it goes again.