Answer: Lots of perennials will bloom all summer if planted now. You didn't mention whether the site was sunny or shady so here are some suggestions for both:
Sun perennials: Black-eyed Susan, coneflower, coreopsis, daylily, delphinium, gaura, hardy geranium, iris, penstemon, peony, phlox, pincushion flower, poppy, Russian sage, salvia, sedum, and Shasta daisy
Shade perennials: Astilbe, bergenia, bleeding heart, brunnera, ferns, foamflower, goatsbeard, hellebore, heuchera, hosta, Solomon?s seal, spiderwort, and violet.
Best wishes with your garden!
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