Answer: There are so many different perennials (and annuals). Based on your description, unfortunately, I am not certain which one you are thinking of. It might be one of the shorter nasturtiums which can self seed and return each year so they seem to be perennial although they are actually annuals. The plants do mound up as they age. Here is a photo of one to compare.
You might also ask your local reference librarian or local professional nursery staff for help; if you could tell them what time of year it blooms that would be useful in identifying it as well. Good luck with your search!
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