ROCKERIES

Rocks can really enhance the natural feel of a garden and provide a great deal of character. They can be used to create a wide variety of attractive effects. In most cases the best results are obtained when the rocks are positioned so that they simulate naturally occurring rocky features such as alpine outcrops, cliff rock falls, or rocky creek beds. In Europe and America, rockeries which feature low growing alpine plants, have been popular for many years.

There are a few basics points to remember when building a rockery:

  • Dig the rocks into the ground.  Ideally at least half of the rock should be buried to give them stability.  All of the rocks should be 100% firm.  A good test is to walk over the rocks - if there is the slightest movement, then reset the rock.
  • Choose your rocks carefully. Select ones that are naturally weathered, or perhaps moss or lichen covered, on at least one surface. Avoid ones that have obvious signs of chiselling or other similar damage. Be careful that the rocks you choose can be safely handled. It is not much use choosing ones that you can't move into position. If you want larger rocks make sure you have some way of shifting them, such as plenty of friends to help, or access to suitable machinery such as a bobcat. Try also to avoid damaging the rocks while shifting them into position - the fewer broken edges, chips or scratches the more natural the appearance.
  • Place the rocks in clusters so that they touch each other. They should fit together as much as possible - an indentation in one rock should fit into a protrusion from another to give the appearance of naturally occurring rocks.
  • Only the weathered, or moss covered parts of the rock should be shown. The rest should be buried or hidden in some way, such as being covered by other rocks. The exception to this is when you wish to recreate the appearance of more recent rock falls such as at the base of a larger outcrop. In this situation the rocks can often be simply dropped from the top of the outcrop to give the appearance that they have fallen there naturally.        

Rocks can also be used in a broader way including:

  • Rocky outcrops created in sloping areas to help retain soil (ie: erosion control).
  • Outcrops or clusters of rocks at focal points used as a visual feature, or to create a contrast with surrounding plants.
  • Rocks around pools or ponds. These can placed so that they overhang the edge of such water features. This will allow any straight edges to be hidden and allow the pool or pond to merge into the surrounding garden. For ponds with fish, such overhanging rocks can provide a safe haven against predators such as birds or cats.
  • Rocks used to provide a seemingly natural walkway, for example rocks in a water course or among a garden bed allowing access to places which would otherwise be inaccessible without damaging plants, or getting feet wet or muddy.
  • Sometimes plants which tend to be unstable can be planted alongside large rocks, the rocks giving stability to the plant as the roots grow under the rock. The area below a rock can also  become a source of moisture  in dry weather these spots do not dry out as fast as more exposed soil.

SOME ROCKERY IDEAS TO TRY

1. A dry creek bed can be simply constructed by placing a layer of large pebbles and rocks in the lowest part of the garden or into an area of natural drainage such as a small gully or depression. Choose rocks and pebbles that have a well rounded water-worn appearance. Growing native plants, which would naturally be found in this type of environment, will help it to look more natural.

2. Rocks can be used in a more stylised and formal manner in Japanese style gardens. Unlike European style rockeries, each rock becomes a prominent feature, so selection and placement is especially important.  A fairly bland surrounding, such as a uniform gravel or sand surface, or minimal plantings enhances the rocks prominence.

3. Dry stone rock walls can be used both as a free standing barrier or as a retaining wall. Dry stone walls are built by stacking rocks or blocks one on top of another without using concrete or any other 'joining' material to stick them together. Each rock needs to be stacked in a way that they interlock as much as possible so that a good deal of stability is achieved. The base of this type of wall should be twice as wide as the main section of the wall.  This spreads the weight and provides stability to the upper parts of the wall, and helps prevent the wall from sinking into soft ground.  When building dry stone walls you should try to avoid creating horizontal or vertical lines between the stones, and place the larger stones towards the bottom of the wall.

4. 'Wet or fixed' stone falls will give greater stability than dry stone walls, but will generally not give as natural an appearance as dry stone walls. The stones or blocks in this wall are cemented together. A strip foundation should be laid first, with ideally steel reinforcing set in concrete. The stones/blocks are then laid on top. To further strengthen retaining walls, sections of wall can be run back, at right angles to the main wall, into the embankment or soil behind the wall at occasional intervals along the length of the wall. These walls can be effectively cemented with a mortar mix of 3 parts fine sand to 1 part cement.

5. Rock outcrops consisting of two or three, or perhaps even more, large rocks can be buried into a sloped area. Soil can be mounded up behind them leaving only the down slope face exposed, creating perhaps a level area above the rocks where garden furniture can be placed, or the tops of the rocks may also be left exposed to create a contrast to an otherwise smooth slope. In more level areas large rocks can be simply placed on the soil  surface and soil mounded up around the rocks to give the appearance of a natural outcrop. This is an easy way to create a raised area that will add interest to an otherwise flat site, as well as providing a well drained area for those plants that are sensitive to damp or waterlogged soils. In both cases the exposed rocks can provide an enjoyable place to sit and contemplate your garden, but be careful that any plants you might use near the rocks will not hide the rocks when they grow larger.

6. It is possible to simulate the effect of rocks in a garden, using artificial rocks, made of  concrete. Artificial rocks, if created properly, will look just as real as the genuine item.   Components used in the surface layer will create both a texture and color which simulates the appearance of natural rock formations. This type of work has a number of advantages over working with real rocks, such as the ability to create any shape and size of rock you desire, including crevices for planting, water courses, or pools. You do not have the work of moving heavy boulders about (though mixing concrete can be heavy), and you are not damaging natural environments by removing rocks which may be an integral part of natural ecosystems.

 

ARTIFICIAL ROCKS 

Artificial rocks can be simply constructed by creating a mound of sand or soil in the shape of the boulder or rock cluster which you plan to build. This is often done by filling plastic or hessian bags and stacking them in the desired shape. Next a layer of wire mesh, such as Chicken wire, is spread over the surface of the shaped form. A layer of cement or concrete 6 to 10 cm thick is spread over the chicken wire. This mix needs to be relatively dry to hold firm in the places where it is applied. If the concrete begins to slump in near vertical sections, it may be necessary to lean bags of sand or soil against those sections as support until they dry. In hot weather keep the concrete moist by hosing down or covering with wet hessian, as it dries. It needs to dry slowly in order to be strong and to  prevent cracking. If heavy rain is expected then the work should be covered with plastic for protection.
 
The cement is applied at this stage using a trowel (or cement workers float) to spread the cement from the bottom of the structure upwards. When doing steep sections it may be necessary to do a small part at a time, allowing it to dry before returning to the section above. The surface should NOT BE SMOOTH. Leave a rough troweled surface at this first layer. This layer should be strong to give the necessary structural support required for the final rock. A final layer is then rendered (spread over the earlier layer). First place over the initial layer a fine wire mesh (such as a 12 mm mesh). Use a mortar mix such as 1 part cement  to 3 4 parts sand to spread a 2 3 cm layer over the surface. A day after applying the coat  (before it is completely hardened   but is firm enough to withstand some knocks), scrub the surface with a steel brush to create a rough surface. After 3 to 4 days, pick over the surface with a sharp instrument such as a chisel or screw driver to create further textured effects.

An alternative method is to include some gravel in the render. This can either be left to harden to give a pebbly granite like appearance or the gravel can be scraped away before  the surface has hardened to give a pitted sandstone like appearance. A third method is to  include some rock salt crystals in the mix, which can be dissolved out by water when the    surface has hardened. Naturally coloured sands and gravels should be used to give the best results.

 

PLANTING YOUR ROCKERY

If you are going to plant close to rock work where concrete or mortar has been used, be aware that the concrete and mortar is alkaline (a high pH), and will affect the pH of the soil around it as chemical components leach out of the cement. In sandy soil, the effect may wash away through the soil after 6 months or so, however, it is wise to test the pH, and perhaps compensate for this problem by washing the surface of the rocks a few times with a 50/50 solution of vinegar and water.

As with the rocks that you choose careful plant selection is also very important to get the best results. A good idea is to have a close look at naturally occurring rocky areas to see what types of plants are growing well in such situations. This will help you to select plants that will fit the particular rocky theme you are creating. Be careful to choose varieties that will not completely hide your rocks as they grow larger. Some plant varieties have proven generally successful in rockeries.