Landscaping around existing retaining wall
We just bought a house with a naked-feeling backyard. The far end of the back yard has a tiered retaining wall, and the second level is pretty bare except for some ferns and old mulch. Can I plant one or two dwarf trees on that second tier? Can I plant wisteria to climb up the rock wall? I want to bring in some shade and color. What can make the wall stronger, versus what could weaken it? I've never dealt with a wall like this and would really love some advice! Thank you!
Benton County Oregon
Thanks for your question about appropriate plantings for your retaining wall. The main issue with any retaining wall, especially one as high as yours, is keeping it stable. Excess water runoff can cause erosion, the weight of soil (especially wet soil) can eventually push it out of shape, or a large tree or shrub with vigorous roots could do the same. Your wall is of natural stone, rather than the interlocking manufactured wall stone, so it could be somewhat less stable. A properly constructed retaining wall slopes back slightly from bottom to top, so it "leans" into the soil it is retaining, and it should have a drainage system behind it to direct water out from behind the wall.
This is by way of a preface to my primary advice, which is, I would not really recommend planting a tree in it, and if you do, use considerable caution. If the wall is very sturdy and professionally built, and you can verify that it has appropriate drainage, then you might risk one or two small ones with non-invasive root systems. Some possibilities would be western redbud (Cercis occidentalis) and saucer magnolia (Magnolia x soulangiana), golden rain tree (Koelreuteria paniculata), or some of the smaller crape myrtles (Lagerstroemia). But even these could cause problems eventually. Your best bet would really be to plant trees at the base of the wall.
As to wisteria, you could certainly plant one at the base and allow it to climb up the wall, or at the top and let it cascade down, but it is a VERY large and vigorous vine, and will also want to cover anything else you have planted there. Unless you want an entire hillside of wisteria, I don't really recommend this either. Also, if you planted it at the top you would probably find new shoots emerging all over the walls and terraces. Nope, I don’t recommend this.
The best things to plant along retaining walls are smaller plants like perennials, small shrubs, and annuals, which don’t have extensive, invasive root systems. Cascading plants like many of the early spring blooming creeping phloxes, annual calibrachoa or petunias, and trailing evergreens like some junipers look lovely cascading down the rocks. Really, there is no end of wonderful plants for such a situation!