Rockeries can be a prized feature in some gardens; and the biggest problem in others. It all depends on the rocks and plants that you choose and use!
Choosing an Appropriate Type of Rock
There are so many different types of rocks, there is bound to be a type that will complement your garden. Normally it is better to create a rockery using those rocks that are indigenous to the local area. If you are creating a theme garden you may wish to use rocks imported from another locality or country. Whichever rocks you choose the emphasis is on making the rockery itself look as natural as possible.
Why are Rocks Different?
Rocks have different properties because different forces have created them.
Sandstone rocks are formed in layers. This can make them crumbly.
Slate stones are also formed in layers and are often of thin texture making them more suitable for screes.
Whilst some limestones are formed like sandstones, others are formed by chemical action and water. They do not show any definite layers and are often irregular in shape.
Basalt and granite are rocks formed by volcanic action in which larva has cooled and hardened. It contains air pockets and no layers.
Rocks can be hard or soft
Granite is one of the hardest rocks. It is consequently one of the densest and difficult to manipulate. Tufa on the other hand is one of the softest rocks. It is extremely lightweight and crumbly. You can easily chisel holes into it to be filled with soil and planted out.
Whether to Weather?
In most natural settings rocks are exposed to the elements and consequently have a weathered appearance. If you clean your rocks too much they will lose their natural appeal. You can encourage the growth of mosses and lichens by dousing them in yoghurt or seaweed-based liquid plant foods
Cut or Uncut?
In most cases again it is better to leave your rocks uncut, as in nature. You may elect to cut your rocks if you are using them in a modern sculptural rockery that is more of a work of art than a functional imitation of nature.
What Size?
As a general rule of thumb it is better to use large rocks for rockeries and smaller ones for screes. However, if you do use smaller rocks in a rockery then it is important not to drown them out with plants that are too large.
WHERE DO GARDEN ROCKS COME FROM?
The rocks at your local garden centre probably came from one of three places. They have been taken out of the ground at a quarry, or removed from a farm paddock so that the ground can be cultivated, or taken from bushland that is to be cleared for housing, roadworks, etc. (NB. Bush rocks can only be removed by a licensed collector.)
Some rocks are also recycled from demolition sites.
Putting them together
Positioning a rock is like planting a plant. You need to take into consideration the soil type, the position of other features in the garden and the characteristics of the rock itself. Choose the best side or “face” of the rock and then position it in the hole so that it can be seen to best effect. Rocks that have visible layers (strata) should be laid with their layers horizontal just as they would be found naturally. It is better to use a few larger rocks than a lot of smaller rocks.
Maintain Soil and Rock Stability
As a general rule, at least two-thirds of any garden rock should be placed below ground. Any less and the rock may become unstable.
Sit the rock on a smooth, firm platform of soil. A layer of sand smoothed over the base will help keep the rock in position. Tilt the rock slightly back so that it sheds water back into the soil and so that the rock won’t tip forwards if someone walks on it. Then backfill and firm the soil around the rock to remove air pockets.
It may be necessary to fix your stones in concrete. If this is the case consider whether your plants roots are going to be restricted. Also, cement can be harmful to the roots of plants.
CHOOSING THE RIGHT PLANTS FOR A ROCK GARDEN
Think about how different plants will combine with the different types of rocks.
Plants that are described as “rockery” plants are often groundcovers, but there is little point covering attractive rocks with plants. Only include large sprawling plants if you are trying to hide rocks that have been installed for soil retention.
What type of plants suit?
plants with suckering roots that bind the soil
small tufting plants such as ornamental grasses
plants that need very good drainage such as alpine plants and succulents
dainty plants that are easily overlooked in a ground-level garden bed
small plants such as sedges that tolerate heavy wet soil (for planting at the base of poorly drained rockeries)
Other suitable plants are the alpine species that occur naturally at high altitudes on mountains. They are adapted to hot sun on their leaves whilst their roots are protected by the coolness of the soil beneath the rocks.
Many succulents also make ideal rockery plants requiring less water than other plants.
PLANTING DESIGN
Arrange your plants so that large plants don’t block the view of small ones. This does not mean you have to place them in neat rows. For example, you can position rocks to create ’viewing windows’ so that smaller plants can still be seen. It is better to build up your plant collection slowly than to over plant your rockery.
Look at the shape of the plants, as well as the colour and texture of the foliage and flowers. For a balanced look, small upright plants can be interspersed amongst the spreading ground covers.
If you are including bulbs, consider how they will look all year round. For many months, they will be dormant and unseen. They also need to be left to die back after flowering, when they can often look unattractive.
When choosing plants for your rockery think about the type of rock you have. Limestone is better for lime-loving plants such as Aubretia, Dianthus, and Gypsophila. Acid-loving plants such as Azaleas and Heathers will prefer sandstone