Answer: Most perennials bloom for a period of about 3 weeks so if you have a variety of plants in your border you'll have a succession of bloom all season long. Here are a few of my favorites:
Achillea is a long blooming plant for full hot sun
Agastache ?Blue Fortune? 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; sun or shade.
Centaurea montana was a reliable plant for me; full sun.
Chrysanthemum. Everybody knows Shasta daisies and Fall mums. Deadhead the Shasta daisies for longer blooming. Full sun.
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. sun.
Corydalis lutea is my favorite long blooming perennial and simply the longest blooming plant in my garden. sun.
Dianthus can be very long blooming perennials if you pick new hybrids. Dianthus gratianopolitanus and D. deltoides are two of the best. sun.
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. Shade.
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. Full sun.
Gaillardia are long bloomers but tend to be short lived. Will self sow but what a bloom they give. Full sun.
Gaura plants are tremendous long blooming perennials and with its airy butterfly like flowers, it is a garden ornament worthy of the best designers. sun.
Geranium ?Rozanne? is a season-ling blooming geranium and the others are so-so long season bloomers. Watch for more repeat bloomers such as ?Jolly Bee? with its huge two-inch blue flowers. Both are stunning long blooming perennials in my garden. full sun.
Hemerocallis or daylilies ?Stella D? Oro? is the classic but look for ?repeat bloomers? at your favorite garden shop. See note in the introduction. Full sun.
Heucherella ? this cross between Heuchera and Tiarella has produced some very long blooming plants with dainty flowers. Full sun/part shade.
Hibiscus moscheutos or perennial hibiscus is a very long blooming fall plant and the modern hybrids are simply stunning. Sun.
Kalimeris pinnatitida "Hortensis" or Japanese Asters produce a profusion of small flowers or a very long period of time.
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. 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 goose neck flower. This blooms for a very long time.
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.
Phlox paniculata or garden phlox. No list would be complete without this classic backbone plant. Pick modern mildew resistant hybrids so the plant will keep its foliage on until August.
Rudbeckia or black eyed Susan is a good long bloomer. ?Goldsturm? is the classic standard and well worth growing.
Scabiosa, the pin cushion flower, is a very long bloomer if you can keep the powdery mildew away from it. If you can?t, it?s a really short lifespan. The shorter versions have more flowers but also more mildew. Take your chance, if your garden is good for them, you?ll see blue and pink flowers for a very long time.
Sedums are very long blooming perennials in the fall garden and I can?t imagine a fall garden without them. Great new variegated varieties mean you don?t have to depend on flowers but the leaves give you season-long appeal.
Tradescantia is a mainstay of the damp garden where it will happily bloom almost all summer. Don?t let it dry out or it will immediately stop producing buds. One of the best long blooming perennials.
Veronicas are often suggested as good long blooming perennials and ?Sunny Border Blue? did very well for me. If your garden gets powdery mildew, this is another plant you should pass on.
Hope this abbreviated list will give you some ideas for your perennial borders!
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