Shame about the Hemlocks not being planted. Believe this is due to fear of wooly adelgid. Though I have a long- existing strand of hemlocks and they are fine--have read some are immune to the plight. Have nothing else to add though interested in your question. I feel that conifers locally are really very under-represented with the expection of immediate coastal areas in NJ and Long Island where they dominate. Don't know why thereare much fewer of them only a few miles inland. The Colorado Spuce may be popular for the same reason the Atlas Cedars are...that blue color when everything else is basically green.

Colorado Blue Spruce is an popular conifer, for whatever reason. Symbol of the west, maybe? I think they look stupid in most non-arid landscapes. South of Philly their performance in all but the sandiest and or most well draining soil is very iffy. I know of a C-level executive on the Main Line who keeps planting large B&B CBS and Douglas Fir to try to bring a bit of his native Colorado east, and they keep dying. It would probably be better to start with smaller container specimens. OTOH I recently saw a couple smallish ones doing reasonably well in the Virginia Beach area. But the soil there is sandy.

You are correct that Norway spruces are more common, especially north of DC. In the suburbs of Balto. & Philly you see a lot of somewhat ancient NS, you hardly see any blue spruces that tall and many of those look like cr-p.

I don't think of ERC as being a tree of the "north lands". As you drive up 95, they are a common roadside weed in the DC area, through lower NJ perhaps, but I don't remember them seeming that common in interior New England or in the high elevations of Pennsylvania or NY, which are surprisingly cool in summer. (Binghamton, NY, is only a little warmer in summer than SE England...but of course more like St. Petersburg in winter!) For example on the highway from Wilkes-Barre to Binghamton there's an amazing monoculture of white birch with a stunted understory of Kalmia. A few hemlocks too. You feel like you are driving near in a boreal area but are thousands of miles away from one of the poles, and not very elevated either. Some of the highest ridgetops were still getting snow in early April when I made the drive. Anyhow I don't think they were ever commonly planted anywhere on the East Coast. I think if they were there, they were allowed to remain in a garden. I could be wrong about that...they were sometimes sold as christmas trees in "olden times" so maybe some of those were B&B'd and planted.

Eastern red cedar is a weed. You see it a lot in abandoned fields. East of me, there is an impressive 'hedge' of red cedar, where it has grown up along the road, and the power company has topped it to keep it out of the lines. It tends to be an indicator species for limey soil. So there are places around here where the fields are full of white pines, then a few miles later, the same type of habitat is full of red cedar.

As an ornamental plant, hemlock has traditionally been used as a hedge plant. It isn't easy to maintain that way, requiring a lot of shearing. My grandmother had a specimen hemlock in her back yard that was easily 70 ft tall.

Interesting about the cedars, I see them going east and south but not much here.

Blue spruce is very popular, but I think nearly all of them are planted and the norways too. As far as reseeding, I've never seen blue spruce seedlings in the wild, but occasionally norway spruce will find an area it likes. IMO People like to plant what they see is doing well in their neighbor's yards. I can count about a dozen blue spruce just looking out my window into the neghborhood.

As far as non-native conifers go, Norway spruce was, and to some extent still is being planted in those states for forestry purposes. It outgrows native spruce in most cases, including here in 'sconnie. Our DNR went to an all-natives program some years back but I consider that a mistake when it comes to not offering NS anymore. Still a lot of nice old plantings of it around. Come to think of it, there's a nice new planting of it coming up on my land!

+oM

Typically red-cedar likes limestone-based soils -- common in the valley regions on either side of the Blue-Ridge for some distance. As a juniper, they also like sandy, dry piedmont/coastal-plain soils. In other soils they don't compete as well.

I don't think I've ever seen a Blue spruce seedling, tho plenty of cones.

Down here, Eastern Red Cedar is the reliable old standby in the way of conifers. Its has two downsides : way too reliable in that will repopulate itself via seeds all over the place, and it cannot compete well against pine trees - eventually they outgrow it and smother it out in deep forests.
Arizona Cypress appears to be the "Colorado Blue Spruce" of the south. Grows reliably, blueish color, and does not seem to repopulate itself.