I have a 30' length of Allan Block retaining wall at the back of my small garden. The block range from 5 high down to 2 high and there is 20" of planting space where I have a collection of perennials at the present time. Behind the perennials is a 6' fence and behind that is a neighbour's back yard at the same elevation as my planted perennials. Another adjacent neighbour has installed a swimming pool which I am desperate to block out. Can I plant Thuja plicata on the planting ledge? Eventually I would like it to grow into a 15' high screen. Any help that you can give would be appreciated as the Allan Block wall will be expensive to relocate.
I don't think a 20" ledge is anywhere close to wide enough for a Thuja plicata hedge. A heavily pruned T. plicata hedge would likely be containable at 4-6 feet wide and its roots would completely fill a wide area on your neighbour's side of the fence even if they didn't collapse your wall. Even the more columnar "pyramid" or "emerald" Thuja occidentalis cultivars would be very constrained in a 20" wide space. Compared with the cost and trouble of replacing dying hedge plants, relocating the wall might not be the most expensive option. Consider also that a 30' wall of cedar hedging 15' high is a lot of work and a pretty dull sight for both you and your neighbour. Assuming that the pool view you want to block is on the other side of the 6' fence it sounds like your viewpoint is from your house windows or a deck rather than from your yard. A more successful solution might be to add some screening using a trellis or arbour and fast growing vines. This could be located at the top of the wall or closer to your viewpoint. Eg a large planter box along the far edge of a deck with screening vines or tall clumping bamboo could give you the privacy you crave. Do not plant bamboo along the fence though as it can be very invasive. A fast growing columnar deciduous tree planted in your yard near the base of the wall would also provide some screening. Remember that it is rarely necessary to entirely block a view. Any planting or architectural feature in front will tend to catch and stop the eye so you don't notice what is behind even though it is still "visible". Try talking to your neighbour to see if s/he is willing to add some privacy screening plants on their side of the fence. Try to work together to find a solution. Personally I would be appalled if my neighbour added a 15' high wall of cedars which sucked all the moisture from my yard, shaded my garden, and needed constant pruning. Perhaps yours will like the idea, but they should certainly be consulted.
Thank you for your insight! Your response has helped us decide to rebuild our wall giving it at least 3.5' of planting area from the fence. We have now certainly decided against Thuja plicata but we have found some "emerald" Thuja occidentalis that are already 9' high and very bushy. They would provide a thick screen although a solid green wall isn't my idea of great planting. Our neighbour has concreted all of his backyard up to our fence with the exception of a 1.5' planting perimeter where he has now planted 5' "emeralds". He has also put 2 tall maples in pots. Our main deck looks down on their pool which is in the concreted area. Our primary concern is noise from the radio, fountains & waterfall and pool parties which cut down on the use of our deck in spite of our beautiful ocean view. What can be interplanted, say every 4 or 5 "emerald" to create a beautiful wall? Something in copper or purple? I really need help with this dilemma (have I posted to the correct forum?
I think you are on the right track now. For something purple and relatively upright you could consider interplanting with "Diabolo" Ninebark - Physocarpus opulifolius 'Diabolo'. It is fast growing to about 10' tall and could probably be kept to about 4' -5' wide with pruning. Can't think of anything else at the moment, but there are probably other choices.
I've seen some nice rows of evergreen Magnolia as an alternative the past few years. Not for hacking-back or topping. But they are fairly controllable with some moderate pruning. The smaller ones with smaller leaves. Not the southern Magnolia with leaves the size of small dinner plates.
