A very good landscape designer.

American Pillar Arborvitae Trees

You don't say where you live, what zone you are in or how high that wall is. It looks high. There are bound to be a number of other shrubs that would work for privacy other than a solid green wall of arborvitae. The PVC irrigation pipes won't be an issue for most shrub roots depending on where exactly the pipe is laid and the consideration of will a hedge block the sprinklers.

Where is the retaining wall in relation to your property line and your house.? It looks like there are trees behind it but a drop off. I need more info

Even a 4' fence will help feel the space feel more enclosed and private. If the space is narrow, shrubs will be a nightmare regardless of what you choose. Even arborvitae hedges look good only about 3 years after planting, then they start turning brown in patches and the middle one croaks altogether.

Thanks for the great inputs. We lives in Seattle, zone b.
The wall is 2 ft high on rockeries, right now we dig a trench for drainage but haven't filled it yet, once it is done the wall will be mostly covered from our side.
The property line is to the left, under the wall.

How about Ivy's, are their roots going to be problems after a few years?

Emerald arbs don't have a particularly aggressive root system. If sprinkler lines are nearby, be careful. If they're in the way, pvc pipe is a couple bucks a stick.

"arborvitae hedges look good only about 3 years after planting, then they start turning brown in patches and the middle one croaks altogether."

Only if they have not been cared for properly. There are 50 y.o. arborvitae hedges in many Seattle neighborhoods.

Arbs make an excellent and narrow privacy screen and they do not develop disruptive root systems. And NO ivy!! It is a major invasive species here and it is strongly suggested not to plant. Even existing stands are hard enough manage and control.

I stand corrected. I've seen them look great in pictures, but not in my zone (MW4-5), which is wrong for this thread anyway.

Chindo Viburnum grows quickly is hardy and is easy to create a "fence" out of. We did a "fence" with it along our back property line which had a walking trail along a pond. I say this because that meant foot traffic was on the other side so kids dragging their hands along it, dogs peeing on it, etc. Its very hardy. Our neighbors on the other hand did Arborvitae and were constantly having to pull one here and there to replace because they were constantly getting diseased or damaged and dying on them.

Thanks, how about the root system of Chindo Viburnum, are they aggressive for structures and pipes nearby?

Arborvitaes are beautiful but eventually they do dry out if not watered properly. Once they dry out bees and wasps make their nests in the middle of them. A few years back a family lost their home because the arborvitaes caught on fire and became roman candles burning their house to the ground. Go with a different option.

They have a shallow root system that isnt invasive.

Any suggestions for high artificial outdoor trees? Sounds like they are maintenance free, no root problems. Any cons?

LOL! It is pretty obvious that most posters are unfamiliar with the PNW. Arborvitae privacy screens are dead common here. Hard to find at least one garden in any neighborhood that doesn't have them. Can't say as I've ever encountered any that have been a cause of a fire.........

"high artificial outdoor trees" - what exactly did you have in mind? Artificial trees for indoor purposes, yes. But I've never seen any artificial trees for outdoor use. Unless you count the cell phone towers that are clumsily disguised as "trees" :-)

Is Dig Doug still on Houzz?

You may need to install a fence or guard rail for building code. You said that the wall is only 2' high but it is clearly higher than 2' from the bottom of the tree. The wall is located on a rockery? How steep and tall is the rockery that the wall is located on? I'm assuming that the rockery is at least high (8-10') based on the size of the tree but maybe the photo angle is creating a strange optical illusion?

Cedars should do fine in Seattle so long you don't have deer in your yard, otherwise you will get some interesting pruning, but I would look at the total height of the wall including the rockery and determine if a railing is needed first. It would be a shame to have a kid play in a hedge and fall over the wall and down a steep rockery.

Chris, that is a photo angle problem and the yard is sloped :). The rockery underneath is only 2 ft. Wall height is 2 ft on average. Will definitively have safe protection.

Concerned whether cedar trees will be too big for such a narrow area.

@Design Interior South is there a structure under the Chindo Viburnum? I have the same situation with a walking trail around the pond behind my house. I'm in MA.

I have seen a privacy "wall" where they installed a chain link fence, and then trained vine-like plants to climb and grow in and out of the spaces, which looked great on both sides. I am nowhere close to an arborist, but I'm sure there are some fast-growing species that would fill in fairly quickly. I actually had something like this in my first home/yard - the previous owners planted grape vines. Looked great, and the grapes were terrific!

I found this online - these are morning glories - clematis might work, too.

(Also, from experience, if you have any deer within 10 miles of your home, they will find your arborvitae and thoroughly enjoy as much of it as they can reach. So the tops will look great, but the bottom 5', not so much.)

Since this is an area where you are growing food, how about espalier fruit trees or vines (grapes, as suggested above, or kiwis, blackberries etc.) on wires? Arborvitae or any other trees will be taking moisture from your beds and sending roots into them. There may also be a shade issue depending on the orientation of the site.

There is no code requirement for any sort of railing on a retaining wall in this area. And while I like evergreen viburnums and use a lot of them in my designs, that is far too small a space for the Chindos unless one embarks on a rigorous pruning routine....which is problematic given the placement of the wall and what's behind it.

And there are no deer in Seattle unless in the very suburban outskirts.

Building/safety code....should you be concerned that someone may fall off or thru the retaining wall and hurt or break something? Children, grandchildren, pets of guests?

"Building/safety code....should you be concerned that someone may fall off or thru the retaining wall and hurt or break something?"

There is no code requirement for any sort of railing on a retaining wall in this area. I deal with retaining walls on almost a daily basis and other than a privacy fence (installed for another purpose entirely), there is never any other kind of railing or safety feature required regardless of height. Plants usually form any necessary barrier to prevent accidental falls. And two feet is not much of a tumble anyway ;-)

Consider two things are you shading your raised bed and air flow. With as wet as Seattle is you need not just good drainage but good airflow to prevent a number of common issues growing vegetables (fungal disease for instance). Plus you don’t want to shorten your growing season.

I would add 4ft fence allowed first. Look at all your fence options something you could grow vines or attach something espilliard to could easily grow taller. Do that before planting a hedge (will be a pain to add later if hedge has some issue) also even the fastest growing hedges still take time so get that added privacy now.

We had a similar restriction and with a longer narrow yard did not wish to enclose the area that opens to a greenway so we chose bamboo planted in pots . Works beautifully for our needs .

Seattle is not nearly as wet as most suspect....we actually receive below the national average for annual precipitation. And virtually none during the primary vegetable growing season.

Lisa Kimball , chindo’s are not suitable for MA. It is too cold. They grow in zone 7-9

This might work both to give you privacy and for safety.

Plant a low maintenance hedge along the top of the retaining wall and then build a 2' high and 2' deep planter or seat above and parallel to your wall and hedge using a solid material other than your retaining wall material and other than your fence material -- consider brick or stone or concrete block -- and then lattice or other fence atop the back wall of that planter. Something similar to this could work as the back wall of your planter with fence.

https://www.jacksons-fencing.co.uk/the-edit/advice-on-planning-permission-for-trellis-fence-toppers

Edited: Doing that, the hedge would be atop your retaining wall and behind your planter on which you put your fence so you'd want a low growing shrub that would not grow against and hold water against any wood fencing you put atop your planter. You'd also need to consider the space taken up by the hedge and planter and how much of your "usable" yard that would take away from a patio or other plan.

Something else to consider might be a portico with with a lattice wall.

@Lisa Kimball No they were planted at 6-8' intervals about 6' off the trail and filled in.

@gardengal48 while Seattle does not get heavy rain and thus has a moderate rainfall average Seattle does have more “rainy days” than most, your right summer is dry but if you choose to extend your season with say row covers or cold frames you will again need to consider how to get adequate air flow. With Climate Change it is best to prepare for the worst. This week for instance Seattle came within 1 degree of heat records set in 1995. Plus snowmagedon in 2019 is worth considering as well.

I have lived and gardened here all my life (and many years in a professional capacity) and Seattle/western WA weather is nothing new for me. I find the lack of natural irrigation to be more of a concern than too much.

And I certainly do not worry about snow........2019 was not that unusual. 1990 was a much bigger deal and lasted for weeks. So was 2010. But these are infrequent events and do not affect the growing of vegetables.

And I am not sure what sort of heat records you are looking at - we hit 94F one day in August 2018, 95F in 8/2016 and 96F in 8/2014. We have not come anywhere close to those temps this year!

Can you go higher than 4 feet if there are a certain percentage of openings in the fence?

Or maybe screens with cutouts like these. You'd probably have to set then in concrete or maybe you could bolt them to the top of the retaining wall. It would be pricey but you could also have them custom made.

https://www.wayfair.com/outdoor/pdp/veradek-6-ft-h-x-4-ft-w-screen-series-metal-privacy-screen-ubgk1046.html?piid=

Or a taller wood fence with openings in the top part, like spaces between horizontal slats, if your code allows it.

Or maybe sail cloths for shade and maybe they could be angled to add some privacy. Could you add sail cloth material to the top of a 4 foot fence? Some muni codes allow for or are silent on materials other than the actual fence itself. Unless you have neighbors who would complain about a slight deviation from code, who would even see it?

Cold hardy, clumping bamboo in long planter boxes. Easy to reach in there and thin it out, and you can top bamboo to keep it to the allowed height. Doesn’t attract pests, if you keep it thinned. Makes a nice sound in the breeze. Looks graceful. Responds well to drip irrigation.

@gardengal48 irrigation is certainly important especially with dry months possibly coming faster than usual. This is the report I read your local news I believe the issue was the measured heat not just as high temp but factors lows as well. Seems this week will be hot but cooler temps are headed your way.

https://www.google.com/amp/s/komonews.com/amp/news/local/the-heat-is-on-seattles-first-foray-into-the-90s-this-year-on-tap

Wait...the height of trees is limited to 8'?