Fall planting can easily & safely stretch well into October. The ground normally doesn't freeze before December or January. The plants will have plenty of time to establish their root systems since the top growth is in the process of going dormant and isn't making huge demands on the roots.

I typically plant out during September and early October and have had complete success so far. I wouldn't worry about top growth; your biggest concern is the plants forming healthy root systems that will sustain top growth in the growing seasons next year and subsequent years.

Last suggestion would be to tap into Bluestone Perennials' website for transplant questions. It's in their best interest to help you have a successful experience growing the plants you bought from them. I always found them to be very responsive to my gardening/perennial questions.

target when august temps fall.. but more importantly when nights cool ... a recovery period from any hot fall days...

by mid sept or so.. go for it.. especially if you take a big gob of soil with it .. and water well after ... they will never know what happened...

and none of this.. is specific to any plant you bought.. there simply arent special rules for certain plants ...

and dont forget.. frost and freeze air temps.. have nothing to do with soil freezing.. roots grow .. until the soil freezes.. in z5 ... here in MI .... you count your 6 weeks back from thxgvg ... and sometimes xmas ... .. so that firmly puts you in getting it done by october ...

good luck

ken

ps: you can do the same in spring.. 6 to 8 weeks before summer heat.. or most of april ... this is all z5 specific for the rest of you.. adjust accordingly ..

That being said, I planted numerous 6 inch pots of perennials into a fencerow day before yesterday and also transplanted about 50 seedlings. The days are hot (90), sunny, dry, with a mild breeze to dry things out.

One of them was a budded 5 foot high perennial sunflower, another a 3 1/2 foot flowering helenium.

Nothing has even wilted...it's all a matter of acclimatisation, proper plants in proper sites, and watering. And consistency in that watering....if I were unable to water for more than 2 consecutive days right now..I'd probably lose them.

Things aren't so dicey when it gets cooler and a bit more moist (hopefully)....but you can do whatever you want whenever you want..if you know what you're doing.

I think the best rule of thumb for zone 5 is no later than the end of September. And, in general, spring bloomers are best transplanted in fall, fall bloomers in spring.

UM, I have a vague rule of thumb which is to transplant/divide and generally mess about according to blooming times - with early bloomers, such as some of the hemerocallis, pulstaillas, omphalodes etc., I do the ground work in Autumn (when they have heaps of time to get settled for next spring) , but late bloomers, such as asters really hate being bundled about late in the year so I do all that stuff in spring. Summer solstice is my cut off date to decide whether something qualifies as an early or late bloomer....so, I will not be transplanting rudbeckias, for example, till next Mar/April. Course, if I bare-root stuff such as roses, I do it anytime they are dormant after leaf fall - so November/December.

Early September isn't too late in Denver. I often transplant late into the fall, as late as the end of November if we are having a warm fall since the soil is warm but the days shorter and the air cool. (Planting that late isn't ever my plan, but sometimes I just run out of time.) I haven't lost any plants doing this, but I mulch well, and we don't have a lot of freeze-thaw cycles. Usually here the ground freezes in January and stays that way until sometime in April, and our snow cover is usually complete from sometime in January (though it can be as early as November) until sometime in March or April.

I find plants have a tougher time if I plant in spring when the soil is cold, but our air goes from cool to quite warm in just a few weeks causing heat stress since the roots are slow to get growing.

I second what Ken wrote: "when august temps fall.. but more importantly when nights cool ... a recovery period from any hot fall days... "

I don't normally transplant in July or August because in the Midwest, it is scorching day and night. But this year the summer has been milder with night temperature in the 60s, so I have been transplanting perennial seedlings. I plan to transplant more perennials through September or until I don't have anything I want to transplant.

I suspect that "ground freeze " means different things to different people. I suspect the following is a dumb statement.

My sister in Alaska says her found freeze come from below, from the permafrost and there is no "extending the season" without a heated greenhouse.. Mulching does not help from that. Here in Texas the most "Ground freeze" I get is a rare less than 1 inch crust of freeze from a frost on top of the BARE soil. Usually never under tree cover or even grass cover. Mulch is a solution. A frost cover is a great solution. I remember not being able to dig in NH and NY state because the freeze was pretty deep. Sparks came off the shovel. we were building a kiln. That was a long time ago. I garden all winter long here.. planting shrubs and trees in December and january is smiled apron. I do try to get my trees in in October.

Thx all, think I'll xplant when nite temps hit 50s, be sure to use the "big gob of soil" technique, they'll love me next Spring.