How To Build A Rock Garden

What’s the difference?
Some people talk about rockeries and others about rock gardens. Is there really any difference between them?
There is no real difference, but garden designers usually prefer to use the term ‘rock garden’. In landscaping terms, a rockery or rock garden is an informal design that should resemble a natural, rocky outcrop – in fact, some of the most attractive ones are built around existing rocks.

Rockeries or rock gardens are constructed from rocks and boulders of various shapes and sizes surrounding pockets of soil. They can be rocks from the area, rocks that have been specially quarried (they should be allowed to weather), or artificial rocks, which are becoming increasingly popular.

A rockery is ideal for a sloping site, where it can be used to form retaining walls between level areas of ground, as well as for growing suitable plants – such as succulents and other plants which require good drainage.

It is not so well suited to a flat site, where it often looks artificial. Rock gardens or rockeries can range in size from a few carefully placed rocks and boulders to large and elaborate constructions which may include waterfalls and pools.

In modern landscaping, rocks are sometimes used more for their individual beauty than as a means of holding together pockets of soil. In this type of rock garden, rocks, or large boulders are strategically placed as focal points, like sculptures, with a few handsomely shaped plants growing near them.

The Best Site for Your Rock Garden

I want to build a rockery because my garden is rather flat and uninteresting. Where should it be placed?

You should site your rockery where it will become a focal point – but not in the middle of your garden with lawn all around it, or it may look rather like a grave.

The best site in a small garden is often somewhere on the boundary, where it will give height to the garden and make it look larger. An especially good and natural-looking site is across a corner, with the highest tier at the middle point.

Plant bushes at the top so that you can imagine that the ground slopes away behind.

Does the Rock Garden Need Sun or Shade?

Although most gardeners tend to place a rock garden in a sunny position, this is not essential.

It is true that the widest range of plants suited to a rockery do need plenty of sun, but there are also many shade-loving plants which can be grown. So build your rock garden where it will look the most attractive.

What Is the Ideal Shape or Size for a Rock Garden?

It depends on where the rock garden is situated.

On a level site, it should be in proportion to the rest of the garden, neither too small and insignificant nor too large and overpowering.

On a sloping site, the rockery should blend into the overall garden design but follow the line and contour of the slope. Either way, try to make your rockery appear as natural as possible.

Before planning your rock garden, look at as many existing ones as you can. If possible, you should also examine the way rocks look in their natural settings – the ideal rockery is one which resembles a natural outcrop of rock.

Make It Look Natural

How do I go about building a rockery, and should the site be level or on a slope?

The most important thing to bear in mind when planning a rockery is that it must look natural and form an integral part of your garden.

Unfortunately, some rock gardens end up as random piles of stones – often bearing more resemblance to a pets’ graveyard than an attractive outcrop of rocks which harmonises with the rest of the garden.

A rockery on a slope looks more natural than one built on the level – try to avoid this, as it is difficult to finish off the sides of such a rockery in a natural manner.

The first thing to do is decide how large you want your rock garden to be. Don’t be too ambitious: if you have to buy rocks, it can be an expensive business and you are likely to need more rocks than you anticipate at first.

To look attractive, a rockery should be about three times wider than it is high, and about two-fifths of the whole area should be stone the rest being soil for planting.

As a very rough guide, you could calculate on about 3-4 rocks of a medium-size – about 16″ x 12″ – per every 3 ft of length. Rocks can be bought individually, so it is not necessary to order a truck-load and you can always buy more if you run short.

It is usually best to buy a selection of small, medium and large rocks – don’t buy them all the same size.

Mark Out The Shape

Mark out the shape you want in a series of curves, not straight lines. Clear the soil within the shape – you won’t want weeds or grass growing up into your rock garden.

In areas with poor drainage or those which experience very heavy rain, it is advisable to build your rockery on a base of coarse rubble, such as small rocks, stones and broken bricks, to improve the drainage of the site.

Then start filling in the shape with small to medium-sized rocks of various shapes, making sure they are placed in a natural-looking way – not straight upright, for instance.

how to build a rock garden

Outline the shape of your rockery. Then start at the lowest level, positioning small and medium rocks so that their edges touch each other and tilting them slightly into the bank.

Embed at least a quarter of each rock in the ground. Fill in the spaces with soil.

At least a quarter of each rock must be firmly embedded in the soil to keep it stable and look natural. Face their most attractive sides outwards and see that their edges touch each other as closely as possible, so that soil will not run through the gaps.

Fill in the area between the rocks with soil to about 2″ below the top edge of the rocks.

In a similar manner, place another series of slightly larger rocks on the higher level, to form pockets of different sizes.

More levels can be built up in the same way.

Position larger rocks on the higher level so that they form planting pockets of different sizes and at different levels.

Build up your rockery to a focal point of one or two large, well-shaped rocks. Fill in with soil. Water the rockery and give the soil time to settle.

include planting pockets in the rockery

When building a large rock garden with big pockets of soil, smaller individual rocks can be placed in the pockets to give a more natural look. If possible, use one or two of the largest rocks to form the focal point of your rock garden.

A good way to build a rockery on a flat site is to reshape the levels in your garden, excavating an area and then building up the level in the background. Then place the rocks so that they hold up the soil.

Water your new rockery before you plant anything to ensure that it holds the soil well – you may have to add more rock if the soil washes away.

Give the soil a chance to settle, too, and top up with more, if necessary.

plant the rock garden with suitable plants

Check that your rockery holds the soil well – you may have to adjust the rocks and top up the soil.

Choose a variety of trailing and upright plants suited to your conditions, keeping the rockery as natural-looking as possible. You could add one rock on its own, giving the impression that it has rolled away from the others.

Types of Rocks For The Rockery

There are some large boulders in our brand-new garden. I am not sure if I should have them removed or not. What would you advise me to do?

Existing rocks, regardless of shape and size, can be the ideal basis of a rock garden. By carefully positioning some extra rocks of the same type and color, you should be able to create a really beautiful effect.

If the boulders in your garden are big and handsome, don’t try to overcrowd them. Use a few extra rocks to form planting pockets, and make the boulders themselves the dominant feature of your rock garden.

Choosing the Rocks. Does It Matter What Type and Color of Rocks I Use in My Rockery?

No. But it is advisable to use stone that is all of a similar geological type, such as sandstone, or granite.

The different types of stone vary considerably from region to region, and it usually looks best if you use the kind which is found in your area.

But if you are going to grow specialized plants – only cacti, for instance – you could try and get rocks similar to those found in the plants’ natural habitat.

If possible, the newly quarried stone should be allowed to weather for a while, until it loses its ‘new’ look.

They look so natural these days that it is almost impossible to tell them from the real thing – and they’re a lot lighter, a great advantage when building a rockery.

Real Rocks vs Artificial Rocks

While most rock gardens are built of natural stone, artificial rocks are becoming very popular.

Specially shaped artificial rocks are particularly useful when it comes to building features such as waterfalls and pools into a rock garden.

But be sure to buy artificial rock that is the same type and colour as your natural rock or it will look out of place.

Some artificial rocks have a layer of an aggregate made from natural rock, they will weather and acquire moss in exactly the same way.

Terraces For a Steep Slope

Our garden is on a fairly steep slope which makes gardening difficult. A friend suggested that we should build a series of rockeries. Would this be a good idea?

Yes. The way to do this is to level off as many flat terraces as you require, and then build up rockeries to form the retaining walls. It would probably be best to get a garden contractor to do the job for you, especially if the area is a large one.

The size of the terraces and rockeries will depend on the shape, style and size of your garden, as well as on the amount of flat space you require. It is usually best to site the largest of the level areas around the house, with smaller terraces on the lower slopes.

The lower terraces need not be completely flat – they could be slightly sloping, to give the effect of a natural hillside, if you prefer.

What Soil Should I Use For The Rockery?

As you will likely have to bring in soil to fill the pockets in your rockery, you can choose the kind best suited to the types of plants you want to grow.

When the rocks are finally in place, the soil should filled up to the top of each pocket. During construction, quite a lot of soil will have been placed in each pocket, but this may only be gravelly or poor subsoil.

The more gravelly the soil beneath the rockery, the better the drainage will be. The soil inside each pocket, however, should be very good, to a depth of at least 12 inches.

Remove some of the poor soil in the pocket and fill up with a mixture of good topsoil and loam. The more loam in the form of well-made compost that you can incorporate, the better. It is not necessary to add manure, as many rockery plants do not like manure.

The manure which has been well-rotted and become part of the compost will be more than satisfactory. The fibrous matter in the loam will retain moisture yet also aid drainage.

The soil in the rockery can have a completely different pH from the soil in the rest of the garden, enabling you to grow different plants – for example, cacti and succulents, including aloes, which need particularly well-drained soil and do best in a mixture of sand, gravel, and compost.

If your site has poor drainage or you live in an area that experiences heavy rain, your rock garden should be built onto a base of small stones and broken-up bricks.

Easy Access Within the Rockery

Would a path look out of place?
Our rock garden is quite large and it is difficult to get to the back of it.

Provided the path is carefully planned, it should not look out of place. To look attractive and as natural as possible, your path should meander through the rockery, following its natural contours in a series of curves.

If you want a solid surface, use flat slabs of rock that are of the same type and color as the existing ones.

In a really large rockery – particularly one on a sloping site – a strategically sited seat made out of large rocks could make an interesting and useful focal point.

The Importance of Drainage in The Rock Garden

My garden soil is heavy clay. All the books tell me that rockery plants do best on light, loose soil. Does that mean that I can’t have a rockery in my garden?

Not at all. A rockery is an artificial creation independent of the rest of the garden, with special soil added to suit the needs of the plants you want to grow.

So it doesn’t really matter what type of soil you have already. Nevertheless, good drainage is vital for many of the popular rockery plants, particularly those indigenous to the hot, dry parts of the world – such as succulents and the various kinds of cacti – so clay is the most problematic soil because, with its dense texture, it holds water much longer than other soils.

What you will have to ensure is that the soil beneath your proposed rock garden drains freely. With most types of soil, you can do this by breaking up the ground with a fork and adding plenty of coarse rubble, covered by a layer of smaller rubble.

Old bricks and roofing tiles, broken up, make an excellent, free-draining filling. But in the case of a clay soil, you will have to dig out the site to a depth of about 12″. Slope the bottom of the hole away from the center of the proposed site and fill it with coarse hardcore before starting to build your rockery – this should ensure good drainage.

Alpine Rock Gardens

I have often seen references to alpine rock gardens. What are they, and could I have one?

As the name implies, an alpine rock garden is devoted to alpine plants.

These are mainly small plants indigenous to the colder parts of the world, where they grow between the tree and snow lines in mountainous regions.

As these plants are not suited to most conditions, you could not have a true alpine garden, but it is possible to have something similar by planting various types of small plants, many of these are well suited to growing in a small rockery.

Slug and Snail Control in the Rockery

Slugs and snails wreaked havoc in my rock garden last summer. How can I prevent the same thing from happening again?

There are several precautions you can take.

Slugs are night-time feeders that hide during the day under rotting leaves and in other cool, moist places.

If you keep your rock garden tidy, this will reduce the problem.

Removing weeds also aids slug control. Slugs do not travel long distances in order to feed, and if you deprive them of day-time shelter, you’ll see a lot fewer of them in your rock garden.

The best way to control them is to scatter snail bait around your plants regularly.

A liquid snail killer can also be very effective, especially against the smaller types of snails.

If you don’t want to use any chemical controls, then you could try putting empty plastic seed trays or the half skins of grapefruit upside down on the soil near vulnerable plants.

Turn them over each morning and you should find plenty of slugs and snails. They can then be removed and destroyed – or let loose on waste ground where they present no threat to garden plants.

Growing Problems in The Rock Garden

The plants in our newly completed rockery are not growing at all well. What could be the problem?

I can think of a couple of reasons why the plants in your rockery are not doing well. To start with, if the plants you are growing are mainly sun-loving types and if your rockery is in too much shade, they will not flourish.

If this is the case, you should grow a selection of shade-loving plants instead.

Another reason could be that the soil in the rockery is not draining properly.

Usually, a rockery is very free-draining but if the underlying base is badly drained or the soil in the various pockets is very heavy (contains a great deal of clay), you might have drainage problems.

To correct poor drainage, you should dig plenty of coarse, sandy compost into the pockets. It may even be necessary to take out the existing plants and remove some of the old soil before replanting them in the coarser mixture.

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