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Rock For Planting

Whether you want to create a root over rock masterpiece or a naturalistic arrangement planted on rock, most people have difficulty finding just the right piece. Some tips garnered from years of geography teaching and mountaineering trips may help. Most sedimentary rocks such as sandstone, shale and mudstone are too soft for planting on, as they rapidly deteriorate. Some limestones and tufa are light in colour, soluble and slowly dissolve. This makes them unsuitable for long term plantings. A root over rock tree may clasp the piece for a while but it will eventually be left standing proud as the rock deteriorates. The solubility of the lime will also lead to problems if any calcifuge (lime hating) tree is planted on it, e.g. Azalea, Rhododendron, Camellia & many Acers. When selecting rocks for planting trees on, the colour, shape, texture, PH, suitability for the roots to wrap over and its ability to withstand the elements should all be considered. Soft rocks are obviously unsuitable but some harder rocks can be easily split by the elements, due to the process known as nivation. Water enters small cracks, freezes in winter, expands as it turns to ice and shatters the rock. For this reason it is best to choose your rock very carefully and preferably leave it outdoors through a cold winter before deciding whether or not it will be suitable for an outdoor/unprotected tree. To be absolutely certain that the rock is not going to split or fall apart, and to protect the vulnerable exposed root area, it would probably be wisest to give all root-over-rock trees adequate protection through the coldest months.

Metamorphic and igneous rocks are generally the better ones to use. Many are very hard and they are often dark in colour e.g. slate, granite, basalt.

Judicious collecting involves making sure that the texture, colour and strata (bedding lines) match. This is so that any pieces used together all appear to have come from the same bed and therefore impart a natural appearance. Try to have a selection of sizes, so that if you decide to build a composite rock by glueing pieces together you will have sufficient choice. Incidentally, the best material I have found for this purpose is Araldite. Silicone sealant also works well and is now available in a variety of shades from builders merchants.

Slabs for forests

Slabs for forest plantings can be of a variety of materials. The ideal piece is naturally weathered to exactly the size and shape that you need for a planned arrangement. This would be difficult, if not impossible to find! Attempts can be made to modify the shape of rocks but this can be unpredictable. Any hard rock with well defined bedding will tend to split more readily. Slate is the most amenable material and the easiest to work. The alternatives to a natural rock can be useful.

Roofing slate is readily available from builders merchants and is fairly easy to shape with deft blows from a masons hammer (with a chisel-like, pointed edge), heavy cold chisel, bolster or similar steel tool. Safety goggles and thick gloves are advisable. Use the hammer or chisel on a waste piece to practice the technique first. I find the easiest way is to lay the slab on a resilient surface (a piece of old carpet laid on the garden). Start at the edge and hit with the blade of the tool at a shallow angle to the upper surface of the slate. Gradually break a channel following exactly the line that you desire. When the right amount of force is used, it is usually easy to cut this channel without splitting the slate. Imperfections in the material may lead to occasional unplanned breakages but if you work steadily and don't try to drive too large a channel at once, then the job should proceed as planned.

Once completed, carefully check the edges, as they are likely to be razor sharp in places. Light taps with a hammer will crumble the offending pieces away. Any edges that require minor modification can be gradually nibbled away at with a pair of long handled pliers. Patience and practice should lead to good results quite quickly.

In half an hour, the two large slabs illustrated were worked from start to finish. Drainage holes and wiring holes can then be drilled fairly easily with a hammer drill and suitably sized masonry bits.


Use a fine bit initially and then widen it with the largest size you have for the drainage holes.

The recently broken edges may be obvious as a different colour for a while. Treat with one of the "instant aging" solutions such as milk, yoghurt or a slurry of soil, sheep or cattle manure (if you can bear to!). Leave out in the garden for a few months and the surfaces should mellow.

I was lucky enough to find two huge roofing slates in an outbuilding of my new house. These provided the material for my first attempt at creating a slab and I was very happy that I managed to create exactly the shapes I wanted, with no unplanned breakages.

Larger slates such as these may be found in architectural salvage contractors, architectural antiques yards or from demolition firms.

Other Materials

Slabs can also be manufactured from GRP (glass reinforced plastic, more commonly known as fibreglass) using resin, catalyst and glass matting. DIY merchants or canoe manufacturers should be able to supply the materials. This type has the advantages of light weight and strength. Sketch the planned shape and work out how any contouring necessary will be achieved. Some form of mould that mirrors the "internal" face of the pot is needed. It can be made from a low mound of earth or sand, a stack of old newspapers etc. Cut the glass mat with scissors to roughly the shape required. At least two layers will be needed for slabs of any size. The GRP is made by mixing the required quantity of catalyst with the resin in a well ventilated place and then stippling it with a paint brush onto the glass matting. Once the white mat turns transparent and there are no obvious air bubbles left, it is placed over the mould. The edges may be turned upward and made irregular so that the final slab has an attractive drip edge. Alternatively fold the matting over on itself, so that a quadruple thickness is achieved around the edge. Leave to harden and finish with a coat of resin and catalyst to which you have added a suitable colourant/finish. Use a small amount of oil based paint in the resin mix to colour it and add gritty sand for texture.

Concrete is less fiddly but makes a much heavier slab. Adding a proportion of peat to the mix makes it lighter but weaker (known as hyper-tufa). Concrete is relatively weak in tension and must be reinforced with a narrow diameter steel mesh or some chicken wire. If attempting a relatively thin section use a small but coarse aggregate.

Make a textured mould using earth or sand with a hollow into which half the concrete is gently added. Lay the mesh on top and add the rest of the mix. The shape of the exterior will be determined by the mould. It may be amended slightly after the first hardening period by wire brushing up to 24 hours later.

Papier mache! A new one in my experience but apparently it works. Use cement slurry instead of glue to bind together shredded newspaper. The sodden mass is then spread out into the shape you wish to achieve and left to dry slowly. Rapid drying or freezing impairs the strength. Eventually the mixture hardens, though it takes several weeks until maximum strength is achieved. With experimentation, a tufa like material can be produced. I assume that the paper will eventually bio- degrade and the similarity with tufa will then be enhanced. No good for Azaleas or Camellias this one!

Legalities and ethics

(with a UK bias but pertinent to most places)

There is increasing pressure on the environment - its use by tourists and ramblers is now being added to by mountain bikers, parascenders, four wheel drivers etc. Many places such as national parks, sites of special scientific interest (SSSI's), geological reserves and nature reserves do not allow the collection of any material, including rocks.

Limestone pavements, where spectacular weather worn limestone blocks are exposed on the surface are a particularly sensitive area and must never be collected from. The laws must be adhered to, by all responsible collectors, as the beauty of an area or a rare animal or plant habitat may easily be spoiled by indiscriminate collection.

The new trespass laws, included in the Criminal Justice Act, allow prosecution for trespass. Permission must be obtained, if you move off public rights of way, to wander on private land anywhere in the UK.

Even when collecting from the less sensitive areas be careful not to disturb habitats. Be choosy about what you select. Don't take every stone that catches your eye, just the most suitable pieces for your needs and perhaps a couple of spares if building a rock arrangement is envisaged. This will ensure that the others remain to weather further and provide good material for later collectors.

All Text & Photographs © Kevin Bailey 1997 - 2008