where you are might matter...

and you mention 18 feet to cover.. but how deep ...

soil type???

will reflected sun off pavement be an issue..;.

how big you thinking of buying...

and.. frankly.. nothing stops growing at some magical height.. as they are trees.... its all about how fast they get to the height you need to start trimming them ... see link re: SIZES

ken

Here is a link that might be useful: its all about annual growth rate

Thanks Ken -

I'm in Z6a (I think that should post next to my name?) North of Boston, MA

'How deep' - I assume you mean how far into the bed do I want the conifers? One row deep, with an ultimate spread of 6', as there is a perennial garden on the other side and I don't want to encroach on it too much. (similar to attached image)

Soil type: loam with a bit of sand and a touch of clay (used to be a potato farm). Drains well / area to plant is raised 5' above end of drive.

Sun will reflect off macadam only early AM, though it will will above and away from drive. (5' landscape timber wall as yard is 5' higher than drive)

Size to buy: 3'-4' or 4'-5', depending on price.

I'm confused by your note that 'nothing stops growing at some magical height'. I reference the UConn plant database site quite a bit, and they list many conifers with specific heights, IE: Juniperus sabina @ 4' - 6' tall.
While I'm aware many plants will ultimately achieve heights beyond what a small garden would consider 'ideal', not every conifer will grow to 20', 30', 40' or more feet? Perhaps I'm misunderstanding your thought.

in conifers: ALL SIZE ESTIMATES ARE AT TEN YEARS ...

presume in 20.. they will be twice as big ...

hence the lesson on annual growth rate... from the link above ....

if something grows 6 inches per year.. in 10 years.. it will be .. wait for it.. 5 feet tall... and UConn will say so...

but in 20.. it will be .. 6 inches times 20... or 10 feet tall ... UConn.. and the industry standards fail to mention such.. eh???

and if you are still around in 30... etc ...

soooo.. if you find something that grows 18 inches per year.... look out.. you will be pruning it in a few years.. but if you get something that only does 6 inches.... much less work in your future ....

well ... i know you can post pic.. how about a pic of your site.... i am a 'have to see' type of guy ...

soil//drainage/etc.... all sound good ...

you said: prefer it (or just about anything) to an AV.
Chamaecyparis are a particular favorite, and lawsoniana 'Silver Dome' has great color

==>>. whats an AV?? what we might call a TO??? ... lol

and lawsons have problems with dying all of a sudden ....

ken

ps: arbor vitae is thuja occidentalis... was i right ...??? ... lol

" in conifers: ALL SIZE ESTIMATES ARE AT TEN YEARS ...
presume in 20.. they will be twice as big ..."

So the previous UConn example: Juniperus sabina @ 4' - 6' tall - is a 10 year estimate, but ultimately will be taller - but how tall? If I'm understanding correctly, this notation from Monrovia regarding Chamaecyparis pisifera 'Golden Mop': "Slow growing to under 3 ft. tall, 4 ft. wide" would mean the plant would be under 3' tall @ 10 years, so how does one find the 'ultimate' height (of any plant) @ 15 or 20 years?

Sorry for the acronym. Yes, you're quite correct - TO! ;) I recently planted 20 Techny Arborvitaes for a friend and she began referring to them as "AV" and I haven't erased it from my memory yet.

Thanks for the lawson info - I'll cross that off the list.

Ken's pretty close to the money on judging mature sizes but there's a couple of points I'd add...........

The growth rate seldom seems to be uniform with most plants. Often they will grow faster when young and slow as they become more mature. Sometimes it is the reverse. So you can not always expect a doubling in size from 10 years to 20 (although that may not be an unreasonable approximation). And not all plant tags reflect a 10 year size, either. It depends on the grower and/or the tag that may be applied by the retail nursery selling the plant.

The part to keep in mind is that growth doesn't just hit a targeted number and then stops. There is a genetic predisposition the plant carries and the closer the plant comes to achieving that genetically predetermined "mature" size (which is typically a range anyway), the slower growth becomes, almost to the point of being unnoticeable. But they do keep growing........:-)

ps. only some Lawson cypress cultivars are highly prone to root rots (that cause the die-off) and many of these are now grafted on disease resistant rootstock.