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Context

We have 2 ft (60cm) high retaining wall and 2 ft behind the retaining wall is a 4 ft (1.2m) high fence. Naturally there is backfill with 5/8" crushed stone directly behind the wall. This backfill goes back 1 ft (30cm), but only goes to a height of 9 inches (23cm) from the top of the topmost retaining wall block.

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Goal

We would like to plant trees at 6 ft (1.8m) apart (centered between fence posts) for some privacy from our neighbors. We live in a 3 story town-home so the trees have to be reasonably tall (10-12 ft [3m]?) to be effective.

We would like to plant 3 Japanese Maples (Blood Good) along the wall around 6 ft apart (and a Golden Raindrops in the larger corner). We want to buy the trees when they are already at a height of around 10 feet with a rootball of around 2 feet.

Question

As you can see, the roots can grow outwards, backwards (under the fence), downwards, but will be blocked from growing forwards. My question is what can we do to maximize the success here.. or is it a lost cause?


Edit 1, 2016-08-21

  • We live in Seattle, USA.
  • There is a french drain behind the wall with crushed stone and geo-material for drainage. In the picture you can see the drain protruding out of the wall.
  • We can certainly protect the fence from the soil using water proofing material, thanks for the suggestions on this!
  • Our thoughts were the tree roots would grow under the fence (backwards) as far as they wanted, out to the sides and downwards. Naturally there would be no roots growing forward and this was our primary concern.
  • It's not clear from the photo but the neighbors yard is dirt behind the fence. I did not realize the concrete they've spread would make the pH of the dirt unsuitable. Thanks for this info - is it a hopeless pH issue?

Edit 2, 2017-06-26

We went ahead and did it anyway and it's working out well so far. I've built an irrigation system into the wall, and under the lawn. This picture is from a month ago, but now that it's summer they grass, plants and trees are thriving. Let's see how it goes in the long term...

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  • need your location or climate zone. Where I live Japanese Maples don't get five feet tall much less ten feet. – kevinsky Aug 21 '16 at 20:24
  • Another observation; looks like your neighbor's yard drains right into yours. Need to think about that as well. – stormy Aug 22 '16 at 2:09
  • @kevinsky , we live in Seattle. We went tree shopping and saw this beautiful Japenese Maple which was more than 10': imgur.com/a/clig7 . – Chaos Aug 22 '16 at 4:19
  • Have you considered bamboo? – That Idiot Aug 22 '16 at 16:49
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NOOOOOO!! Do not plant trees below that wall. There is no room on your little wall. Imagine a Japanese Maple you are hoping to grow. The same size canopy equals the same size root ball below! This is fine for perennials and small shrubs but NOT trees of any sort!

I assume you will be filling that space behind the wall with soil?!!?? Soil up against that fence will mean replacement of that fence within a year or two. So not even good for any kind of plants. Soil has to be kept minimum of 4" below wood, fences, siding...etc. You'll have to get pressure treated 2X12 lumber to shore up the bottom of your fence and ideally your neighbors need to be invited into the decision making!

If you make an identical wall along the back on the fence you HAVE to get a liner between the fence and the concrete. Also, don't imagine planting acid loving plants in your new 'planter' as that concrete will make the soil more alkaline no matter how hard you try to change the pH. Nice wall, nicely done. Tell us more about your plans. Did you provide any drainage for this wall? What did you plan to do between the fence for the back half of this 'planter'? We can still help...you've done an awful lot of work and very nice wall! How about a wall for seating? More information!

  • Oh :( thanks for explaining. We plan on putting dirt behind the wall regardless of the viability of putting trees in so protecting the fence is good advice. We also hadn't thought of the acidity of the soil due to the concrete, thanks for that too. We did worry about draining so we put a french drain behind the wall (you can see it exposed at one end). – Chaos Aug 22 '16 at 4:10
  • We didn't plan on putting any trees initially, but our neighbor just had a bunch of these beautiful trees removed including a rather large one. It was truly disappointing, so we thought we could explore putting in trees. Luckily we put a 5x5' box in to the retaining wall I guess since it should handle the Golden Raindrops tree. A sitting retaining wall is good, we'll try. Any thoughts on getting something high in that planter to protect our privacy? – Chaos Aug 22 '16 at 4:13
  • Best and quickest way for privacy is to make SCREENS. 2X2's lap jointed to have 6"X6" squares set in a frame within 6X6 or 8X8 posts, cross pieces (3X3) are gorgeous, they should be allowed to season to gray and there are products designed to allow UV graying while still protecting wood and allowing wood grain to show! Off setting these panels is beautiful. Use Golden Hops...they will cover these screens before summer, chop down in the spring and up they come. Just one idea. I'll send pics. Wouldn't it be nice to get rid of that fence and do a landscape for the both of you? – stormy Aug 22 '16 at 20:08
  • Pics would be great! Our neighbor certainly doesn't care about landscapes (as you can tell from the pictures:) ). We're in a townhome so our neighbor also shares fences with 3 of our neighbors. – Chaos Aug 22 '16 at 20:46
  • I just tried pulling up my portfolios...guess I have to go fire up my other computer I've not used for years...I shall try. These fences feel like being in a cat box. I only know this as I've 4 cats and 3 catboxes...grins. Does the water from that neighbor drain onto your property?? Don't want to go into litigation but might help with any negotiations. That fence is only 4' high? Who paid for it??? Can you imagine a grove of trees between the two of you? I've only convinced two families to do this, we just aren't wired to think this way...! Do you get along with neighbors? – stormy Aug 22 '16 at 20:55
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I agree with Stormy on this question. You do not have enough area for a tree or group of trees to be successful. Think of a similar situation where the city plants a tree in a sidewalk area. They use a planting pit about 4' x 4' and even then the average life of a tree in urban areas is less than ten years. Too much retained heat, reflected heat, not enough water...

Things you can do now:

  • place 4" drain pipe with sleeve at the inside base of the wall. Cover with landscape fabric or geotextile. This will ensure good drainage at base of the wall.
  • consider putting EPDM pool liner between soil and any wood. This will help prevent rot. Any wood, pressure treated or not, will rot when in contact with the soil. It's just a question of how long it takes. Replacing posts on a post would mean a lot of work which could easily involve digging up your bed.
  • consider planting climbing vines at the back of the fence: clematis, sweet pea
  • plant small shrubs at the front to give the impression of depth to the planting
  • Thanks - the 4x4 info helps, and we have a french drain. In this case the tree can grow 18" forwards and any amount in the other directions. Do you think the 18" restriction is.. viable? We thought of vines but didn't think we could get the height. Love the idea the small shrubs for depth, thanks – Chaos Aug 22 '16 at 4:06

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