If you have any of these seeds on the list below give this a try.  You may need to do a little additional research to find the best time to sow in your area for a particular plant.  Depending on how cold your winters are, direct sowing could be accomplished from mid-summer to 8 weeks before your last frost.  

For example, here in our Zone 7, late summer sowings would include biennials like foxglove and and Canterbury bells.  Hollyhocks and bachelor buttons can be sown from August through November, and larkspur, nigella, and poppies prefer sowing after the first frost.

Aconitum (Monkshood)
Alcea ( Hollyhock, biennial)
Alchemilla (Lady’s Mantle)
Alstroemeria (Peruvian Lily)
Astrantia (Masterwort)
Amsonia (Willow Bluestar)
Asclepias (Milkweed)
Aquilegia (Columbine)
Baptisia (False Indigo)
Buddleia (Butterfly Bush)

Campanula (Canterbury Bells, some biennial)
Centaurea (Bachelor's Button, Cornflower)
Chelone (Turtlehead)
Cimicifuga (Bugbane)
Coreopsis (Tickseed)
Dianthus (
Pinks, some biennial)
 (Foxglove, most biennial, a few true perennial)
Dodecathon (Shooting Star)
Echinacea (Coneflower)
Eremerus (Foxtail Lily)
Eupatorium (Joe Pye Weed)
Gaillardia (Blanket Flower)
Geranium (perennial species, not the typical container plant “geranium” which are Pelargonium spp.)
Helianthemum (Rock Rose)
Helianthus (Perennial Sunflower)
Heliopsis (False Sunflower)
Hellebore (Lenten Rose)
Heuchera (Coral Bells)
Hibiscus (Hardy Hibiscus, Swamp Mallow)
Hypericum (St. John’s Wort)
Iberis (Candytuft)
Knautia (Crimson Scabious)
Larkspur (self-seeding annual)
Lathyrus (Perennial Sweet Pea)
Lavandula (Lavender)
Liatris (Blazing Star)
Mertensia (Virginia Bluebells)
Monarda (Bee Balm)
Nepeta (Catmint)
Nigella (Love-in-a-Mist, self-seeding annual)
Oenothera (Evening Primrose)
Penstemon (Beardtongue)
Platycodon (Balloon Flower)
Physostegia virginiana (False Dragonhead)
Primula (Primrose)
Ratibida (Prairie Coneflower)
Rudbeckia (Black-eyed Susan)
Scabiosa (Pincushion Flower)
Sedum (Stonecrop)
Sempervivum (Hens & Chicks)
Sidalcea (Prairie Mallow
Solidago (Goldenrod)
Stokesia (Stoke’s Aster)
Thalictrum (Meadow-rue)
Tiarella (Foamflower) 
Tricyrtis (Toad-lily)
Trollius (Globeflower)
Vernonia (Ironweed)
Veronica (Speedwell)
Veronicastrum virginicum
Viola species types (Violets)