Answer: Most perennials bloom for a period of about three weeks so if you choose plants with different bloom times (early, mid- and late-summer), you should always have something in bloom.
Here are a few suggestions:
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 three years out of four.
Centaurea montana was a reliable plant for me but self seeded everywhere.
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 favourite 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.
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, 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.
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 (I like geraniums.)
Hemerocallis or daylilies ?Stella D? Oro? is the classic but look for ?repeat bloomers? at your favorite garden shop.
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.
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.
Lysimachia clethroides or goose neck 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.
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.
Hope this short list helps you decide what to plant!
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