Ok I know I mentioned using fake X mas tree's
That said, , keep this in mind
them tree's all have metal the fake needles attach too, and that will rust out pretty fast
the fake needles , odds are are NOT made to be outside and doubt they will hold up any amount of time, to the elements and or lightning/UV
so what ever they MIGHT look like day one, day 20-50-90, odds are they will NOT look the same for long!
NOW< if you really wanted to go this route, its NOT hard to get them to stay put
odd are you can simply stake a heavy duty pipe or likes in the all as you build it, and then just attach the tree's to them,. and have them so they can be taken down at will
if it was ME< I think I would just do a 6 ft tall wall of some sorts, be it a privacy fence, or stone/rock,bin ben, block deal
at 6 ft tall
few folks are that tall to see well over it, so it will block 99% of any views from there side to yours and vise versa!
if you really wanted, you can maybe then add some fake flowers in pots
a good strong wall, as time passes you can get as creative as you want to make it taller by adding what ever!
fake X mas tree's
IMO< will look cheesy pretty fast and rust out and just be a waste of your money!
another CRAZY idea, , build a wall like you have in your pic here, and add some Elephant grass to it?
that crap grows like a weed in many places and since you will be filling the wall in with dirt, USE GOOD ORGANIC top soil
and it should have plenty of nutrients to allow it to strive
it cam grow 14+ ft tall?
there are also other things you can plant that will grow on there own and keep things higher and even add some color or flowers at times! and be pretty much maintenance free!
building the all like this, your biggest factor in what will grow will be SUNLIGHT
a good landscaping provider should be able to advice you on what grows well in your area in the amount of sunlight you have there
as again, the wall will be FILLED with GOOD SOIL , , so things should grow then not having to worry about soil on site!
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another idea if you like this
since you planning to BUILD a wall
you can also add a waterfall if you wanted that recycles water , it will add a sound effect to the wall and your back yard LOL can also be like a bench to sit by on
running water in them doesn't attract mosquito's either like standing water does!
and if NOT a water fall, what about a water fountain on top to work as as shield of sorts??
can also add some colored light to it as well!
if your spending $$ get all you want while its being built, rather than WHAT IF< after the fact! HAHA!