Hi! Have a look at Picea Glauca Pendula. It’s not columnar, but maybe narrow enough to suit your purpose, and very hardy.

I can't picture why you'd want an alternating arrangement with different trees.

There are several columnar oaks, including Crimson Spire, and at least some are zone 4 hardy. There are columnar crabapples including Red Baron, Raspberry spear, and Purple Spire. I have no experience with these. Just do a web search for columnar tree zone 4 to get a bunch of ideas.

IMO if you are going to alternate with the hydrangeas, use all the same trees. If you don’t want to use all the same kind of tree, then don’t do every other plant a Hydrangea. The alternating arrangement, while not my preferred aesthetic, looks planned. I think the more random arrangement might look like it wasn’t planned or was filled in after something died with whatever was available. I also think that the deciduous trees will work better as far as shape. A single Degrout’s Spire will be very narrow at the top and will take considerable time to provide any coverage above an 8’ fence. The fence and Hydrangeas will shade the lower foliage making it unattractive over the long haul.

I hope you gave the Limelights lots of room since they can get to at least 8’ in all directions and will regrow rapidly from hard pruning. If they are less than 5’ or so from the fence, you may find yourself challenged to keep them from growing against the fence and retaining moisture.

The hornbeams are fully hardy to zone 4. If they did not work or were severely cold damaged, then I would be very careful selecting whatever you choose to replace them with.......what's to say the replacements won't suffer a similar fate? The Crimson Spire is also a zone 4 rated tree but grows much more slowly than the hornbeams. Perhaps you should be looking at some species with even greater than a zone 4 cold tolerance?

I agree with bab's that the DeGroots are not going to provide what you are looking for. Deciduous trees tend to grow much faster than most coniferous evergreens unless you choose one of the large screening selections like Green Giants. And those could eat up all your space.

This is my fence this morning, that is what I'm looking at currently. The other is August/September last year.

Which type of hornbeam did you use? Carpinus caroliniana, American hornbeam, is definitely a zone 4 tree....sometimes listed to zone 3. And 'Uxbridge'/Rising Fire maintains that very narrow columnar profle. That cultivar originated in Ontario so should have excellent cold hardiness.

'J.N. Upright'/"FireSpire" is another newish cultivar of Carpinus caroliniana with a narrow and upright shape, although not purely "columnar".

Perhaps Blondie planted Carpinus betulus columnaris rather than the native one. It would definitely be worth trying the native plant since it is quite hardy, more so than the European hornbeam.

I can understand wanting evergreens, but that wasn’t your stated goal which is what we were answering. The Degroot’s will provide green, just not effective screening for quite a long time. I might add an evergreen groundcover (Veronica Georgia Blue or one of the evergreen perennial Geraniums) and evergreen shrubs, but not expect the evergreens to provide screening, perhaps some lower ones. I agree with saccharum that the pines on the other side of the fence may well provide some effective screening, especially when added to other plants.

If you are intending to pleach you don't want a columnar tree. You need one with a spreading canopy so the branches reach out and touch each other.