I live in a much colder zone than you, so not sure whether you will have the same experience...I have found that most of the perennials that I've purchased late in the summer (on clearance) do not survive the winter. Not sure if it's the roots not getting established in time for the cold, or whether the root ball wasn't big enough. It's worth trying if you've already started them...maybe cover them with leaves this Fall for a bit of extra protection.

I have planted a number of clearance perennials in late summer. If I can find a decent spot in this clay soil, they do fine through the winter. I only lost 2 last winter. And truth be told, its because I did a poor planting job. My perennials were older, so I don't know how your "younger generation" will do. Planting in August will mean babying them with sufficient watering. Come fall, use your leaves as cover. Nothing ventured, nothing gained, right?

I have planted perennials in September and even October, though they weren't seedlings. I mulch them well to keep the soil warm and allow them to grow roots for as long as possible, and if feasible will cover them to protect them from the earliest frosts. The mulch also helps prevent frost heaving come spring. My soil is relatively well drained, so I don't often have much problem with the frost heaving plants out of the ground, but it might be different in soils with more clay content or in areas with a longer season of freezing and thawing. We tend to have good snow cover and so only a short season of freeze/thaw conditions.

I planted cuttings and divisions in October last year and they came through fine. Not seedlings but pretty close as they were tiny. We didn't even get a lot of snow to provide a good cover and our weather got cold fast so I was worried they wouldn't make it but they pulled through.

We talk about this kind of thing on the winter-sowing forum because some people sow seeds over the summer and plant them out in the fall. A few may perish over the winter, but odds are most of your little plants will be fine.

I just started 5 varieties of Echinacea from seed a month ago (late May). They are small sprouts or seedlings now, a few are still sprouting, but when they reach a couple inches I plan on planting them out. They will be planted in small groupings, in beds amongst larger perennials, and will likely bloom a bit next Spring. Eventually they will get a permanent spot.

Just don't forget to tend the seedlings diligently and water daily if it doesn't rain. Also, I usually sprinkle the organic iron phosphate slug bait around small seedlings because slugs can wipe them out in no time.

Your seedlings should be good sized by August and adjust well to being planted out if you can water them as needed. Giving them a little shade when planted ( a pot nearby so shadow can shade them during hottest/sunniest part of day or a lawn chair nearby, etc.). Shouldn't be any problem surviving winter. The roots will continue to grow until the ground freezes. If you are worried or some are tender perennials winter mulching after the ground freezes will help.

The voices of experience have spoken and you've received good counsel from the above posters. Generally speaking it's best to plant perennials when they won't be stressed by climate--specifically hot, dry periods without rain. Also, it's generally not recommended to plant things when they're blooming since they need all their resources at that time of year and being transplanted taxes them when they've no reserves on which to draw for sustenance.

You haven't deceived yourself into failure with perennials but are perhaps not being realistic about your expectations. Some perennials (more than others) take a year or several to establish themselves and provide the look you're expecting. Don't embrace the false hope you'll achieve your goal in Y1 or Y2 but allow yourself to enjoy your garden as it progresses through the seasons.

Just a few additional notes: blue poppy is not happy in Z6. From my reading, it prefers significantly colder temperatures. My own success with winter sown Verbascum is that it does not return after year one. The other perennials you referenced appear to be reliable in my Z6 garden.

I can only recommend that, rather than rely on a temporary greenhouse, you learn about winter sowing as a method of growing trees, shrubs & perennials from seed via the winter sowing method. You'll ultimately be vastly more happy & satisfied with your garden beds as a result. It's proven, it's reliable, FDA-certified and it's a low-cost, huge boost to what you hope to achieve.