You do understand that those six foot arborvitae will get much bigger than six feet?

Couple of things to consider:

• Do you live in a climate that will accommodate plants in above ground planters through the winter? Unless a very mild winter climate, all containerized plants will have much greater cold vulnerability.
• Do you realize that containerized plantings will require more attention than those planted in the ground as well as periodic removal, root pruning and soil mix refreshing?
• Size of planters is less of a concern than you might think. They should at a minimum be large enough (HxWxD) to accommodate the existing rootball with a couple of inches leeway below and alongside.

Plants that are grown (semi)permanently in a container will never attain their full size. Container culture is a natural dwarfing process.

I would get Arborvitae plugs and plant them carefully, using a trowel, not a shovel to dig. If you were going to do planters, I would just have them big enough to cope with plugs. Arbovitaes have shallow root systems, which spread horizontally. They will get cramped in any planter. Between the planters and 6 foot Arbs, you'd be spending a lot of money and if they were still thriving in 5 years, I'd be surprised. They would probably make for a good hedge that won't disturb the utility lines, but you'd need to start small and dig like you're an archaeologist, not a gardener.

Otherwise, pick a different plant. I will observe that big bluestem grass, which comes in a bunch of pretty cultvars is 6-8 feet tall, easy to find in plug or seed form. Again, with plugs, you have to dig carefully, but a big bluestem plug from The Pollen Nation is 4.5 inches deep and a couple inches wide. An Arborvitae plug (not available from The Pollen Nation, I got mine from O'Donal's) will be much bigger, but still of a size where slow, carefully digging with a trowel is tedious, but possible.

Oh for heaven's sake!! You do not need to start with plugs!! It will be eons before they make any sort of presence. While it does depend a good deal on the climate, six foot arbs are completely adaptable to a boxed container planting. I have done similar for many design clients.

btw, ALL trees and conifers tend to have relatively shallow root systems. That's just the way they grow. I garden exclusively in containers and I have a dozen Japanese maples and multiple conifers and other trees in containers, some for over 25 years. It is completely doable. In fact, Emerald arborvitaes are an excellent candidate for this purpose. Just pay attention to the other maintenance items I outlined previously.

It doesn't have to be a confining, maintenance-heavy, eventually rotted, built raised planter. It can just be a naturally sloped berm. The width of "strip" will determine the overall height, but in any case a person can still dig around utility lines, that is why they are marked. Some are buried 36" down.

Suprised and thankful for the responses so far!

I should have added that i am in southeast, Nashville, specifically. Our winters can get a little rough, we did have a 7º snap and about a month of 32 and below. Not sure how that would affect the arbs in planters.

i do understand that the containers will hinder growth, which is totally fine with me and preferred in fact. I just want an attractive row that stands about 5' or so above planter top, and has some density between trees.

I'm open to other varieties if anyone has suggestions. I'd rather not dig "like an archeologist" to plant plugs between utilities. That just sounds like an accident waiting to happen. ANd frankly, in 5-7 years if they're dead i'll cross that bridge when i come to it.

How frequently would the soil need be refreshed? That could be cumbersome, but not impossible.

Keep the thoughts and ideas comin! Thanks everyone

Arbs are quite cold tolerant and should overwinter fine in a container in your zone 7b. How long before requiring root pruning and soil replacement depends on the soil mix. A very textural, bark based mix (minimal peat) should last 2-3 years before needing attention.

Reading this long running link may be helpful: https://www.gardenweb.com/discussions/1389927/trees-in-containers-iii#n=236