Answer: The giant-flowered rose mallow has the largest flowers of any hardy perennial. These are descendants of the native hibiscus found in the south and southwest states. When hybrids of these first appeared many years ago, they were sold as "The Mallow Marvels". Because of their continuing display of summer color, they still deserve the "Marvel" label. They are among the most spectacular and easily grown plants. Perennial hibiscus prefer a sunny location and well drained soil containing plenty of organic matter and nutrients. Rich, moist soil and full sun bring the most vigorous growth but mallows are very accommodating and will tolerate light shade and less desirable soils.
Hibiscus can be used as container plants as well. Individual flowers last only a day, but each plant may flaunt a number of giant flowers at once. Few garden plants provide so much enjoyment for so little care.
And now, to answer your question, since you're providing all of the requirements, I'd tend to believe your plant just needs some time to settle into its garden spot before it can provide you with the spectacular flowers you expect.
Continue to give it excellent care and it will reward you with huge flowers!
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