The Christchurch Botanic Gardens

Information Sheets

Rock Garden

Rock gardens are an attractive and important ingredient in many gardens. Although often considered the hardest undertaking in all forms of gardening, a skilfully planned and well constructed rock garden constitutes one of the choicest of garden features.

In 1921 a start was made on the construction of a rock garden adjacent to the present Water Garden. The construction and subsequent alterations and extensions were made possible by a 500 legacy from Mr A R Inwood. It was not an ideal area for the cultivation of rock garden plants. This fact and that of the collection having outgrown the area led to the decision to construct a new rock garden. The unsightly wilderness of rough grass and lupin that formed part of the horse paddock east of the Cockayne Memorial Garden was deemed a more suitable site. In 1938 a start was made on the construction of the new garden. Volcanic rock from the Port Hills was used in the formation of the garden, ponds and pathways. Construction was carried out by Gardens' staff under the guidance of the curator Mr James McPherson and Mr Walter Brockie. The Rock Garden was officially opened in October 1939 by the Governor General Lord Galway. Minor alterations and extensions have been carried out in subsequent years.

Plants grown in rock gardens are often termed alpine plants. This is misleading for true alpine plants form a comparatively small proportion of the plants grown in this garden. A rock garden plant is therefore one that resembles those plants found in alpine habitats. That is, the plant shows the adaptive characteristics of small size and compactness. High mountain or alpine plants are grown within the confines of the Rock Garden but also present are representatives from many varied environments from hot Mediterranean shores to cold perma frost regions of the Arctic.

The diversity of rock garden plants is vast and there are suitable species, varieties and cultivars for almost any garden situation, be it intense sun or deep shade. Many are grown for their spectacular flowers and fruit while others are utilised for their attractive foliage. They play a major role in landscaping and are frequently incorporated in specialist areas such as water or wildflower gardens with some being used for ground and bank coverings.

The design and planning of the Rock Garden

The Rock Garden is ideally situated on a southward facing slope. Various volcanic rock outcrops have been created to suit the extensive variety of plants grown here. These rock outcrops provide a range of habitats from cool, deep, moist sites through to open, shallow, free draining screes. At an appropriate distance a grouping of attractive trees and shrubs provide the necessary shelter and a setting against which these rock garden plants can be viewed to their best advantage.

The Rock Garden contains a wide range of exotic and indigenous bulbs and herbaceous perennials with evergreen and deciduous shrubs providing the permanent structure. As the existing Rock Garden has a comprehensive collection of plants from the Northern Hemisphere it is planned to create a Southern Hemisphere area with greater emphasis being placed on geographic and ecological associations. This will provide an interesting contrast and link with the New Zealand Alpine Garden.

During the year plants are evaluated on their performance in the garden. Alterations and amendments to various areas and plant groups are then carried out at appropriate times. The five main points considered when planning planting positions are colour, height, flowering period, form and texture. Hardiness and suitability for the growing conditions are also assessed.

Colour, be it foliage, flower or fruit is used in a way to ensure a sense of unity. Plants are often grouped en masse so that a unified picture is presented. Dominant colour spread throughout the garden gives a feeling of repetition and rhythm. As various plant groups are dominant at certain times, colour will often look after itself as colours within each group are usually harmonious.

Plants are also arranged according to height and habit and are generally graduated with low plantings at the front and taller to the rear of the garden. Some taller plants are brought forward and lower ones set back to prevent a regimented appearance. A graduated stepping down of heights with some groups rising above others together with the rock work creates a series of hills and valleys that add interest to the plantings.

Note is taken concerning the length of time a particular plant will take to reach its flowering stage and the period of time it will continue to flower. This is used to help space groups of early or late flowering plants regularly throughout the garden, helping to prevent large flowerless areas.

Plants with differing forms and textures are used where possible as they also provide interesting contrasts and visual appeal. Many rock garden plants in this garden are worth growing for their foliage alone.

There are many factors considered when choosing plants that will be suitable for growing in the Rock Garden. As the Rock Garden provides a range of habitats, plants are grouped according to the conditions they prefer.

The maintenance and care of the Rock Garden

The Rock Garden is one of a number of high maintenance areas that require much attention throughout the year.

As many plants in the Rock Garden are multi-crowned perennials or bulbs, division is the simplest method of propagation. Lifting and dividing of plants in the garden depends on the size and growth rates of the plants or plant groups. Selected plants or plant groups are usually lifted, divided and replanted during spring.

Plants are lifted from the ground and pulled apart, either by hand or divided by using two small hand forks placed back to back. The tines are plunged deep into the centre of the crown and forced apart by pushing the two fork handles together, then apart. This process is repeated until the clump is separated into acceptably sized pieces. Only the young vigorous outer growths of the crowns are replanted after the roots and any top growth has been trimmed.

After removing the plants from the garden, the areas to be replanted are cultivated to a depth of approximately 150mm. If the soil is lacking in organic matter, compost is incorporated before the area is cultivated. Prior to planting the soil is lightly consolidated by heeling and the surface raked to a fine state. Planting distances depend on the size of the plants, their rate of growth and the length of time the plants will be in their positions. Planting holes made with a trowel or spade are wide and deep enough to take the roots of the clump comfortably so that the crown of the plant is just showing through the soil surface when planting is complete. Exceptions to this rule are bulbs which are planted to a depth of twice their diameter.

During the year plant material grown on in the nursery either from seed, cuttings, layers, or as purchased plants are added to the garden's collection. Replacement plants and new introductions including a wider range of interesting, unusual and rare rock garden plants are continually being sourced from New Zealand and overseas catalogues.

Occasionally a top dressing of loam is made to areas that have become depleted of soil due to erosion or because of the natural tendency of some rock garden plants to grow upwards out of the ground. The scree garden is also replenished with a series of fine grade, gravel chips when necessary.

A light forking of the garden throughout the year allows air and moisture to penetrate to the roots of the plants while at the same time exposing unwanted weeds. A nail or wire handtool is used to remove small or difficult to reach weeds from the garden.

A granular pesticide is used occasionally to control grass grub. Apart from this pesticide no other chemicals are used for pest or disease control in the Rock Garden. An application of a slow release fertiliser is occasionally made in the spring to encourage growth at this time of the year. Overhead sprinklers are used during the early morning to irrigate various sections of the garden when necessary.

Spent flowers are removed regularly not only for appearance but to encourage further flowering, new growth and to maintain a more compact plant. Dead or dying foliage on evergreen and deciduous perennials is removed during the autumn and early winter as plants start to die back. As well as for aesthetic reasons this helps to reduce the incidence of pest and disease build up. Woody plants are pruned according to their preferred habit, making sure they do not smother other adjacent plants.

Foweraker House

This house is a natural adjunct to the Rock Garden. It contains an extensive collection of tender and rare indigenous and exotic alpine plants. Colour and variety are maintained throughout the year by an ever-changing display of plants. Opened in 1967, the Alpine House was renamed Foweraker House in 1981 in acknowledgment of the many plants gifted to the Gardens by Mrs Jean Foweraker, a noted alpine plant enthusiast.