It's not too late to plant trees. I found on this website

http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=c152

that it "Prefers organically rich, moist, slightly acidic soils with good drainage"

That doesn't sound like Colorado soil.

I agree with gjcore that it's not too late to plant trees. Shoot, you can plant trees well into the fall. Although the Japanese tree lilac prefers organically rich, slightly acidic soils, it actually is "Easily grown in average, medium moisture, well-drained soil in full sun," according to the Missouri Botanical Garden website.
I like to visit the Denver Botanic Gardens website to see when certain plants bloom in Denver, but when it comes to plant profiles, I find that the Missouri Botanical Garden website is hard to beat.
Good luck with your tree-planting efforts.

Here is a link that might be useful: Secrets for successful tree planting

I have a customer that has a Japanese tree lilac in their front yard. They said they planted it about 15 years ago and it's still going strong. I don't BELIEVE they did anything special with the soil. Though, even if they did, after 15 years, the roots have far outgrown any small amended area they gave it in the beginning and it is now living off of 100% pure Colorado suburban clay.

I THINK I saw some tree lilacs at Tagawa the other day, they are down in Centennial off Arapahoe/Parker rd. But, I may be remembering incorrectly. The Tree Farm, which is in Longmont, might be another place to take a look. I know they have them on their website, but I'm not sure what their actual inventory is. I may be going up there within the next couple weeks, and if I do, and I remember I'll take a look and et you know. A lot of the smaller "neighborhood" nurseries (like O'Toole's and Jared's) have very often surprised me with what they have, too. Customers will want specific plants that I can't seem to find anywhere, then I'll stop in one of those places looking for stuff for me and there it is!

I seem to have the best success with April or September tree plantings here.

The Tree Farm will be having their annual buy one, get the second tree for a penny sale soon. It usually starts mid to late August. If you can hang on a few more weeks, you'll get a deal. If you only want one tree, they'll usually let you buy it for half price. I can never do that, got to have a second tree! Most nurseries will start their fall clearance sales soon.

I like to go when the sale first starts for the best selection, take the trees home and hold them over in pots until the weather cools, then plant.

That's a great link, Designer. I also submerge the pot in a big bucket or trough of water while I'm digging the hole. It's heavy, but really saturates the rootball before planting. Spread those roots out as much as possible once you get it in the hole. I tend to plant a few inches high as we have clay soil and it settles a lot. Don't do more than 30% amended soil. Like Zach said, eventually those roots will grow past the amended soil and have to deal with the clay anyway. Lots of tree folks say no amendments or fertilizer at planting, but I think in our soil it helps.

Barb

never plant trees in august.. unless you are a professional ... or hiring one ... or have 20 or 30 years of killing plants under your belt ....

you want warm [not hot] days.. and cool nights ... which leads one to spring or fall ...

you want deciduous trees to be leafless ... less stress ...

and you want conifers/evergreens .. to be dormant .. which happens when the other trees leaves color ...

its all about growing some roots.. and getting it settled in.. before the next active leaf growing season ...

soooo .. is it too late.. no.. its too early ...

hi barb ..

ken

Good morning, Ken!

Ken has a 5 acre paradise back east, planted hundreds of trees, so...what he said! We did a long distant plant swap in the fall and I'm having a ball with the new plants!

I meant to add that I really like the BOSS mix at O'Toole's. Its compost, peat moss and mycorrhizae, but no more than 30% and really mix it well with the native soil. Since the flood, my soil is compacted and nutrients washed out so I'm amending every chance I get.

Barb

Thanks to all for your very useful information! I found several great online resources for choosing the right trees to plant in Denver. The tree lilac is recommended on nearly all of them. This is one of the lists:
http://www.ext.colostate.edu/pubs/garden/treereclist.pdf

That's great to know about the Tree Farm and BOSS mix as well.

Procastinating until September is something I can do :) We have our hands full with a courtyard / entry patio project we decided to do this summer in order to make better use of our front yard space and get rid of some grass lawn....