Dry stonewalls that separate lots or uphold flat terrain on slopes are a wonderful opportunity to display a variety of plants while not wasting any space.
It’s better to think it through before even building the wall, so that adding plants later on is safe and easy. On dry stone walls (stones without any mortar), plants often colonize spaces on their own. When cement is used to fasten the stones, you need to provide pockets of soil for the later planting, or else your only option will be to plant along the base of the wall.
1- Building a dry stone wall
Select which style
Stones with no mortar were traditionally erected by farmers who removed stones from the fields they were cultivating. These are always very short, 3 feet or 1 meter at most, because they lack proper foundations.
A trapezoid shape makes them more stable, but it also means the wall tapers and gets thinner near the top of the structure. This angle called the batter must range from 3 to 10%, which is about a hand’s width at the palm at most.
It ensure the wall remains upright, and at the same time gives plants some slope for them to collect rainwater. Stones can be set in horizontal layers, like a staircase, or slanted for a smoother appearance. You can shape or trim the stones (“ashlar”) or use them as is.
Laying stones, the right technique
If the wall is holding back soil, compute its width from the height of the soil behind it: the thickness of the wall at its base should be at least equal to one-third that height. For example, 1 foot wide if there’s 3 feet of soil on the backside (35 cm thick for 1 meter of soil held back).
- Build 2 wooden “templates”, or batter-frames, they will help you stay constant, keeping the batter and the horizontal layers even during the entire building project.
- Dig the soil down 4 inches/10 cm, forming a trench a little wider than the base of the wall itself. Fill the bottom with a layer of #5 grade gravel (20-40 mm caliber) to guarantee drainage.
- If the plot is on a slope, place your first cornerstone on the lower side of the slope. Mark the high point of that stone on your template, and then mark the other template at the matching height, so you can make sure your guideline stays horizontal.
- Give the stones a very light slope to the inside of the wall, going down towards the side that retains soil. Use pebbles to prop them up. Add dirt between stones whenever the distance between them reaches half an inch (1 to 2 cm). Behind the front-facing stones, at various intervals, prepare hollow spaces which you only fill up with dirt and soil, not stones.
It’s best to sow and plant your flowers at the same time you’re building your wall. Remember to space plants by 2 to 3 feet (60-80 cm) so that the stones themselves are still visible after the clumps have grown. If any closer, you’ll have a wall that’s completely covered in vegetation.
- After completing the first layer (or course), move the guideline up between the two templates, and backfill the rear side of the wall as you go. To make the wall more durable, you can choose to apply mortar between some of the stones, on the inside so that the cement isn’t visible.
- Keep the nicest, flattest stones for last. Use them for the last layer, forming the top into neat, straight tiling. On the side retaining soil, extend these capstones by at least a foot (30 to 40 cm), covering the packed soil there.
- Last of all, carefully pack dirt between stones, and water the top of the wall and mound abundantly.
2- How to choose the right plants?
Alpine plants and “rock plants” are surely those that are best suited to growing in such an environment. Roots must weave their way between rocks. Stones get hot when exposed to sun and the space between them dries up faster. Ground cover plants that form small pillows or round bushy clumps are great. The larger, more expansive ones are to be planted on top of the wall.
You can also add succulents like common houseleek (Sempervivum tectorum) which will have fun colonizing every rock.
Depending on exposure
Factor exposure in when establishing the list of plants for your dry wall.
- A wall facing North is a boon for all types of fern, especially Polypodium vulgare, Phyllitis scolopendrium and Asplenium trichomanes, as well as ivy and periwinkle.
- A South-facing wall will require more sun-resistance and drought tolerance. There are many here to choose from. Classical aubrieta, basket of gold (Alyssum saxatile), wallflower (Erysimum cheiri), valerian (Centranthus ruber), Erigeron karvinskianus.
You might want to look at less well-known species, such as Acaena microphylla ‘Kupferteppich’, Antennaria dioïca, Anthemis cupaniana, Armeria juniperifolia, Cymbalaria muralis (coliseum ivy), Dianthus deltoides (dwarf carnation), Globularia cordifolia, Helianthemum (rock-rose), Phlox subulata, Sedum, Sempervivum, Tanacetum (tansy), Saponaria ocymoides (soab-bush), Ruta graveolens ‘Jackman’s Blue’ (rue)…
Try to make the network of soil pockets deep and voluminous, and, if possible, let them open into the side of the wall holding the dirt back. Roots can thus dig deep into the mound for water.
- Walls facing either East or West only get sun during half of the day. Plants that usually like South-facing exposure usually cope fine with part shade, but you can add plants that like moisture a bit more such as Campanula portenschlagiana, sandwort (Arenaria montana), Erinus alpinus, saxifrage…