Let us assume the average perennial flower is going to bloom for three weeks (some a little higher, some a little lower) so anything that exceeds this by a goodly margin is eligible to make this list.
This list of long blooming perennial flowers should exceed the minimum by at least a few weeks – and some by much more.
Achillea is a long blooming plant for full hot sun
Agastache or licorice plant is one of the better longer blooming Agastache.
Anthemis tinctoria is a long blooming daisy that will self-sow with abandon.
Asters; there are a few fall-blooming asters such as ‘Monch’ and ‘Wonder of Staffa’ that are extremely long blooming. Check out the newer varieties.
Campanula a decent long bloomer three years out of four.
Centaurea montana was a reliable plant for me but self-seeded everywhere. Cut it back by half right after blooming and you’ll get a second bloom in the fall.
Centranthus is easy and a little known but decent blooming plant.
Chrysanthemum. Everybody knows Shasta daisies and Fall mums. Deadhead the Shasta daisies for longer blooming.
Coreopsis. All bloom for extended times but the lanceolata and grandiflora types benefit from deadheading to get repeat blooms. Deadheading is not necessary with verticillata and rosea hybrids to make them into long blooming perennials.
Corydalis lutea is my favorite long blooming perennial and simply the longest blooming plant in my garden.
Dianthus can be very long blooming perennials if you pick new hybrids. Dianthus gratianopolitanus and D. deltoides are two of the best.
Dicentra Formosa and D. exemia hybrids are long-lived for shade. Short bleeding hearts are superb while the taller ones are short bloomers. ‘Luxuriant’ has been my shade garden stalwart.
Echinacea are the current darlings of the plant breeding world and it is true the blooms last a long time and each individual flower lasts a long time. Good for mid-summer blooms and so far they all seem to be about the same length of time in my garden.
Gaillardia are long bloomers but tend to be short-lived. Will self-sow but what a bloom they give.
Gaura plants are tremendous long-blooming perennials and with its airy butterfly-like flowers, it is a garden ornament worthy of the best designers. Grow it.
Geranium ‘Rozanne’ is a season-long blooming geranium and the others are so-so long season bloomers. Check your local garden center for newer long-blooming introductions.
Hellebore or Christmas Rose are one of my favorite long-blooming perennials – and wonderful in the shade garden.
Hemerocallis or daylilies ‘Stella D’ Oro’ is the first of this repeat bloomers in daylilies but there are now better ones on the market.
Heucherella – this cross between Heuchera and Tiarella has produced some very long blooming plants with dainty flowers. Not for heavy competition in the main flower border against phlox and daylily but lovely in containers or small dainty gardens.
Hibiscus moscheutos or perennial hibiscus is a very long blooming fall plant and the modern hybrids are simply stunning. I have three and I lust for more.
Kalimeris pinnatitida “Hortensis” or Japanese Asters produce a profusion of small flowers or a very long period of time.
Kniphofia can be long bloomers if you can grow them. The used to be very short green plants in my old garden. Haven’t gotten around to killing any in the new one yet.
Lavender is a classic long blooming plant for growing anywhere in the full sun.
Liatris, is Gayfeather or Blazing Star and is a good long bloomer and unusual flower heads.
Linum perenne or perennial flax is disappearing from garden centers because the larger nurseries have a hard time growing it. It produces literally hundreds of blue flowers every day almost all summer from early July onwards. Grow it yourself from seed – direct sow it in the garden – it is easy.
Lysimachia clethroides or gooseneck flower. This blooms for a very long time but then again, after a few years of growing it you’ll have a tremendous number of this very aggressive thug in your garden.
Malva sylvestris or ornamental mallow blooms for a long time but is very weedy.
Nepeta or Ornamental Catnip is a good plant if you get the newer hybrids. The old species plants are not as long blooming.
Perovskia or Russian Sage gives a good blue mist to the garden in late summer and early fall and is a good plant in full sun gardens with great drainage.