Building Dry Stone Walls and Hollow Walls
Dry Stone Walls and Hollow Walls
These make ideal features in new gardens and can improve older ones at little cost. It is a pleasant job to create one during the summer months. The secret of construction is in keying the stones and if one has any doubts about the durability of mortarless structures one has only to look at the dividing walls in the fields of our northern and western counties. Let me first of all make it quite clear that I am speaking of two separate types of construction; the hollow wall which is filled withand subsequently planted and the dry-stone wall which is solid in construction but can still be used for growing plants in the crevices between the stones.
However because of the small amount of soil, the type and variety of plant is extremely limited. In the hollow wall, on the other hand, almost any type of plant may be grown depending on the width and depth of soil. Many of the so-called hollow walls constructed by builders are far too shallow and more ornamental than practical. There is a limit to the height of mortarless walls and I suggest no more than 2-1/2 ft to be reasonable and for a solid wall, about 5-1/2 ft.
Like all other structures, such walls need a firm foundation and I always like to sink the first row of stones below soil level. Take out a trench to the required width, level off the bottom or in the case of a slope this can either be stepped down or sloped to follow the lie of the land provided it is not too steep. Loosen up the surface as one would prepare soil for sowing, removing any bumps, roots or large stones. The garden wall differs from that erected by the farmer, in that the soil is used between the joints to accommodate plants, whereas dry-stone walls do not present a solid face and strong winds can blow through them.
Stones of any thickness may be used but it saves a lot of trouble if these are sorted out into what are known as courses, which is another name for the thickness of the stone, for example 3 or 4-in courses. Any irregularities are taken up with the soil which is used as mortar. I like to use a soil which is as rich as possible. This is passed through a 1/2-in riddle to remove stones and lumps and then as much rotted compost as possible is added; extra sand is unnecessary. The idea is to produce a rather sticky fine compost which is spread on the top of the stones about an inch thick if the stones are flat and, where they are irregular, it can be built up to about an inch and a half. Where more building up is required small pieces of flat stone may be used. The outer face of the wall can either be built up square, that is using faced stones, or it can be as rough as you like to have it.
A hollow wall consists of two separate outer walls filled in with soil. As one can well imagine the weight of the soil has a tendency to press out the comparatively thin unsecured walls. To defeat this we use what are known as ‘throughers’. These are long tie stones which stretch right across the gap between the two walls and the weight of the stones on top, together with the solidity given by the soil which is filled in after, maintains a perfectly rigid structure. Neglecting to put in these ties will result in the walls collapsing after frost or heavy rain. I may add that walls which I erected nearly thirty years ago are still perfectly sound and no amount of weather has disturbed them. The soil joints allow for expansion and contraction and, of course, provide perfect. In dry districts I sometimes seal a number of the joints on the inside with clay which has been moistened and worked to the consistency of putty. Where rainfall is high this is unnecessary.
When building walls such as hollow or dry stone walls, I either decide in my own mind how the wall will be planted – or a plan can be prepared on paper. This I consider important as it is far easier to plant as the job proceeds than to try to push in plants afterwards. Suppose, for example, you intend to plant aubrieta, iberis or other trailing plants along your third course. First spread the layer of soil then knock the plant out of the pot and tease out the roots, arranging the plant over a vertical joint. This allows it to spread downwards, sideways and backwards into the mass of soil. Each succeeding layer of stones should be set back, roughly about an inch from the one below as this not only gives stability but soil will fall onto these ledges and any number of plants will establish themselves even in this very small amount. It also makes it easier for sowing both annuals and perennials.
Seeds of annual and perennial plants can be incorporated in any wall with cracks simply by making little mud pellets from decent compost, pushing the seeds into these and then pressing the pellets well into the cracks of the wall. If you can syringe these for a day or two to keep them moist, so much the better, but you will be surprised how well many of even the difficult seeds germinate in what may seem almost impossible conditions.
My advice is to avoid large-leafed plants such as nasturtiums; for although in theory they should look fine under these conditions, they often look untidy and unsightly. Alternate the plants hanging from the face of the wall by using some trailing plants and some tufted types but do not use small shrubs in the sides. These, including, can be used in hollow walls. I have one hollow wall planted up with the small polyantha-type rose, variety Willie Den Ouden, which has looked beautiful for the last twenty years. All this receives is the normal feeding and an annual topdressing of a good compost such as J.I. potting compost No. 3 or an equivalent mixture.