Rockeries
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If, as I am, you are fascinated by alpine
plants, small shrubs, perennials and bulbs, there can be few more appealing
garden features than a rockery. It is a perfect way of displaying your prized
plants in a natural setting and also provides the right growing conditions for
those alpines that are fussy about drainage and exposure.
However, although almost all gardeners
eventually attempt to build a rockery, most of the results are short-lived
failures. The success of any rockery depends on it being as natural as
possible, which is by no means easy, because replicating nature is never as
straightforward as it seems.
The trick lies in not necessarily going
for the grandiose but creating a sense of scale, which can be hard to do in a
small garden. It’s difficult to get the proportions right. The plants tend to
overwhelm the rocks and the rocks are too small — just the opposite of what is
seen in nature. Country gardeners, with their extra room, have a considerable
advantage over their city cousins because a large rock garden with sizeable
rocks allows the use of a wider range of plants while maintaining a sense of
proportion. Creating a successful rockery on a small scale requires superb
plant knowledge and an artistic touch.
Siting
Island rockeries placed in the middle of
a lawn can be a useful addition to a garden and are perfectly acceptable and
functional for cultivating the plants, but they never look natural. Sudden
slopes and boulders that appear from nowhere look forced
and out of place. The gardener with a perfectly flat section is always going to
struggle to create a successful rockery.
While it is possible to create a rockery
anywhere, it looks much better if you work with the site rather than trying to
fit it in where it wouldn’t naturally occur. Fortunate indeed is the gardener
with access to sloping ground, wild bush areas, or best of all, existing rocky
outcrops. Rockeries are an excellent way of developing an already sloping site,
especially if the site is backed by trees or a natural bush area.
Most rockery plants demand at least
half-day sun. The fully shaded rockery is difficult to work with and limits
your choice of plants. Best of all is a site that provides a variety of sun
exposure, that way you can grow everything from ferns to succulents.
Bearing those points in mind, the choice
of site for your rockery is often made for you. And that’s generally best all
round — far better that the site demands to be a rockery than you attempt to
build one from scratch.
Good drainage is vital for a rockery
because although the upper areas will always drain naturally, you can end up
with a swamp around the base if you don’t take care to avoid it. For that
reason it’s a good idea to build in a higher part of the garden where drainage
won’t be a problem. Once again, pity the rockery enthusiast with a flat garden:
their plants frequently suffer from waterlogging.
If you can’t avoid very wet areas at the
base of the rockery, they can always be used for ponds. Bear in mind, however,
that run-off water is likely to be carrying soil particles and to be quite
nutrient-rich, which will make any pond difficult to keep clear and algae-free.
Rocks
You can use any size and type of rocks
but avoid mixing types and be very careful when arranging them. Nothing looks more
unnatural than a variety of rocks all jumbled together.
Many of the larger garden centres stock a
range of rocks, or a search through the Saturday morning classified adds will often reveal a few sources. Local quarries are
also a good source and often have second grade material that is unsuitable for
construction but perfect for rockeries.
Moving heavy rocks is difficult. Even a
small boulder can weigh 100 kg and has great disc-rupturing potential. The
old stone-age methods — levers, rollers and wedges — still work in the age of
the micro-chip, but if at all possible, get your boulders delivered after the
groundwork is done so that they can be off-loaded near their final resting
place.
You will almost invariably need some
professional assistance for building a large rockery. Loaders and diggers make
short work of boulder moving but they are not the kind of tools you can master
in a few minutes.
Artificial rocks made from fibreglass are
realistic looking and weather naturally. They make effective substitutes in
non-load-bearing situations. Their lack of weight precludes their use for
retaining large volumes of soil unless filled with concrete, sand or soil after
positioning.
Soil
The soil in your rockery is very
important. Not only is it a growing medium for the plants, it also helps to
bind the rocks together. It is often advisable to have two types of soil, a
light, free-draining soil with plenty of added compost for filling the large
areas where the bulk of the plants will grow, and a more plastic clay soil for
packing the gaps between rocks and binding them.
You will also need a surface mulch of
some sort. Exposed soil will weather badly and often eventually forms a hard
crust. It also tends to wash away in the little rivulets that run between the
gaps in the rocks. Fine shingle chips are the best rockery mulch. They should,
of course, match the rocks — no pink marble chips with slate!
You can also use your shingle chips as a
soil in their own right. Mixed with some potting mix or finely sieved compost
they make a marvellous scree for filling small
pockets. This is perfect for plants like Rhodohypoxis
and Lewisia that demand absolutely
perfect winter drainage.
Construction
Once you have decided where your rockery
must be, have obtained the rocks and the soil, it is time to start
construction. If you want to avoid too much earth-moving you will need to work
with the natural lay of the land. This consideration combined with your choice
of rocks will heavily influence the design of the rockery. Nevertheless, make
at least a rudimentary plan, giving the greatest consideration to the position
of level areas — planting plateaus — and the direction of rainwater run-off.
It’s often said that like an iceberg, 9/10ths
of a rock should be below the surface. That may be a slight exaggeration but
there’s more than a grain of truth there. Deeply embedding the rocks makes them
stable, creates a sense of permanence and helps them support one another. This
is especially important on sloping sites that may be prone to movement in wet
weather. If the rocks are just shoved in with little thought, they could be
washed away and at the very least are likely to become unstable and dangerous
to step on when planting and weeding.
The usual method is to start with a few
easily placed, very stable keystone boulders, then
fill in the rest from there. Adequate excavation is vital to ensure stability,
and though it is tedious, back-breaking work, it provides the topsoil for
filling the rockery and can also help to improve the drainage. Speaking of
which, the excavation stage is the time to consider digging a few drainage
ditches if necessary.
As commonsense suggests, use the largest
and heaviest rocks on the bottom layers and make sure all the rocks are firm
before starting the next layer. Wobbly rocks are dangerous for walking on and
also provide sheltered pockets for slugs, snails and slaters to hide in.
Make sure that every rock is firm, stable
and at a natural angle. And whatever you do, don’t stand rocks on their edges,
especially thin or narrow ones. We want a natural rock outcrop, not a dry-stone
wall. If you need to build up the height, layer rocks and angle them back into the slope to keep them stable. As a last
resort, use a little discreetly placed concrete and reinforcing or bracing posts
for extra strength. Keep them hidden though.
Backfill with soil as you go. A layer of
coarse stones or gravel under the soil will ensure good drainage. There’s no
need to add all the soil now if you want to put a special mix in certain
pockets, but make sure you put enough in to stabilise the rocks. Tamp the soil
down to avoid air pockets.
There will be a settling period of
several months while the soil weathers and compacts. If you can stand the wait
it is advisable to delay planting until the soil has settled or you may find
your plants left high and dry. The delay will be easier to take if you build
your rockery in the autumn and leave it over winter to settle. You can then
plant in the spring when the widest range of plants is available.
Ponds and streams
Water is the natural complement to a
rockery. Indeed few natural rockeries exist without a stream or pond somewhere
nearby — that run-off has to go somewhere.
Unless they are very large or likely to
be difficult to get at, leave making ponds or streams until after the rockery
is built and settled. There are two reasons for this. First, you may find that
rainwater doesn’t run where you thought it would and if your pond is always
filling with silty run-off there will be algae problems. Second, positioning
large rocks after laying concrete is just asking for problems — a big boulder
toppling into your new pond is not good news.
Once the rockery is built you will see
how the water runs off and over the rocks and you can work your ponds and
streams around those natural directions. Take care, however, not to destabilise
your rock work when excavating.
Concrete is the best material for ponds
and provided you keep the slopes shallow, boxing shouldn’t be necessary.
Plastic and rubber sheeting has the advantage of being lighter to work with and
easier to remove if necessary. However, they do not offer any structural
support. Concrete ponds can aid in stabilising the large rocks and you can use
mortar and small stones to blend the pond with the rocks.
Preformed fibreglass ponds are another
alternative, though they rarely come in anything but small sizes.
Plants
Knowing which plants to use and how to
use them is vital when working with a small rockery. An important part of this
is using plants that are compatible in style and growth habit. Although
planting should be left until after the rockery is built and the soil has
settled, it’s a good idea to visit a few good public gardens and some natural
mountain rockeries before you start construction. This will give you an
appreciation of plant types and how they might be incorporated in your design.
Inevitably you will notice, when
comparing the natural and the artificial, that natural rock outcrops have far
fewer plants. Usually the structure of the rock captures your attention before
the plants are even noticed. That too is a feature of the best rock gardens.
Rather than pack your rockery with plants
keep it on the sparse side. You can make it interesting by choosing plants that
are out of the ordinary. A rockery should be a garden of little treasures
waiting to be discovered not something resembling a flower border that hits you
between the eyes with a riot of colour.
There are countless plants suitable for
rockeries. Look at a few books, such as the ubiquitous Collins Guide to Alpines & Rock Garden Plants by Anna N.
Griffith, for inspiration. A few of the indispensables include Aubrieta, Dianthus, Helianthemum, Lithodora and Saxifraga among the ground covers; Alchemilla, Geranium, Primula, Pulsatilla
and Trillium among the perennials;
and dwarf conifers, Japanese maples, small rhododendrons or miniature willows
for height.
Watering and fertilising
When watering rock gardens you run the
risk of washing soil away from between the rocks. A good
thick shingle mulch lessens this problem but you still need to be careful not
to water too heavily. Soak hoses and micro-jet sprinklers are a better choice
than oscillating or rotating sprinklers as they give a fine mist that soaks
through to a good depth without creating a flood.
Rockery plants should grow steadily but
slowly. We don’t want rapid growth that smothers smaller plants nor do we want
excessive height. Use only mild fertilisers, preferably liquid feeds, and apply
just enough for good health rather than growth boosting.
******
Rockeries are tricky to get looking
natural and are hard work to build. They take time and careful planning to get
established. Looking at all the negatives they may not seem worth the effort,
but once you get it right there’s nothing more satisfying. Just think things
through, work with the environment and take it slowly. Provided you’re prepared
for the work and the occasional rebuild, you’ll get there eventually.
Copyright Geoff Bryant
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