The Christchurch Botanic Gardens

Information Sheets

Water Garden

Water gardens are an attractive and important ingredient in many gardens. A skillfully planned and constructed water garden enables a vast selection of water loving plants, fish and other aquatic life to flourish.

The removal of gravel and sand from the western side of the Gardens early this century resulted in large pits in this area. Much of the gravel from these pits was used in path construction in the Gardens, with the sale of the remaining gravel and sand financing the building of the present Curator's House. The bare pits were considered an ideal site for a group of ponds and in 1920 work commenced on their transformation. The construction of the ponds, borders and adjacent rock garden (later shifted) was made possible by a £500 legacy from Mr A R Inwood. A 60 metre deep artesian bore was sunk which subsequently fed all ponds in the Water Garden area. One of the main features in the Water Garden was a map (to scale) of New Zealand constructed from soil and rocks. Because of the difficulty in growing plants here, the map was removed amidst much protest in 1937.

Water or moisture loving plants represent a large group of garden plants including trees, shrubs, herbaceous perennials and annuals. The borders around the edge of the Water Garden contain mainly herbaceous perennials which are botanically defined as soft stemmed plants that lack secondary (woody) thickening. Their common characteristic is that their roots persist from year to year. Stems and leaves on some may remain, though in most the top growth dies back to ground level each winter.

The diversity of water garden plants is vast and there are suitable species, varieties and cultivars for almost any garden situation. Many are grown for their spectacular flowers while others are utilised for their attractive foliage, form and texture. Water garden plants play a major ornamental role in landscaping and are frequently incorporated in mixed flower, shrub and herbaceous borders. They are also used in container gardens, specialist areas such as rock or wildflower gardens and some are also used for ground and bank coverings.

The design and planning of the Water Garden

The Water Garden at the Christchurch Botanic Gardens is ideally situated having a neutral backdrop of mature trees and shrubs. These plantings provide the necessary shelter from wind but are far enough away to prevent excessive shading and root competition. The borders around the Water Garden provide a range of habitats from cool shaded through to open sunny sites for the moisture loving exotic and indigenous herbaceous perennials.

During the summer months plants are evaluated on their performance in the borders. Alterations and amendments are then carried out during winter. 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 in the borders, 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 border 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 these groups are usually harmonious.

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

Records are kept concerning the length of time a particular plant will take to reach flowering stage and the period of time it will continue to flower. These records are used to space groups of early or late flowering perennials regularly throughout the borders, helping to prevent large flowerless areas.

Plants with differing forms and textures are used where possible as they provide interesting contrasts and visual appeal. Many herbaceous perennials in these borders are worthy of growing for their foliage alone.

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

The maintenance and care of the Water Garden borders and ponds

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

As most of the plants in the Water Garden borders are multi-crowned perennials, division is the simplest method of propagation. One or two borders are lifted, divided and replanted each year during August. This rotational method means that each border will be lifted, divided and replanted at least once every three years.

Plants are lifted from the ground and pulled apart, either by hand or divided by using two 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 selected border/s, the area/s to be replanted are cultivated to a depth of approximately 150mm. If the soil is lacking in organic matter, compost is incorporated before the areas are cultivated. Prior to planting the soil is lightly consolidated by heeling and the surface raked to a fine state. Working from a prepared plan, areas are defined to indicate where each plant or plant group is to be placed in the border/s. Planting distances depend on the size of the plants and their rate of growth.

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 rhizomes which are planted at surface level and bulbs which are planted to a depth of twice their diameter. It is at this time that plant material grown on in the nursery either from seed, cuttings, layers, or as purchased plants are added to the border's collection. Replacement plants and new introductions including a wide range of interesting, unusual and rare water loving plants are continually being sourced from New Zealand and overseas catalogues.

After planting a mulch of rotted animal manure and sawdust is applied to the border/s to a depth of approximately 20mm. This improves the physical condition of the soil by supplying plants with nutrients as it decomposes as well as helping smother weeds and conserving moisture. A light forking of the borders throughout the year allows air and moisture to penetrate to the roots of the plants while at the same time exposing unwanted weeds.

Sprays are not used for pest and disease control in the Water Garden borders as there are no serious infestations that occur. An application of Blood and Bone (a slow release organic fertiliser) is made in the spring to encourage growth at this time. It is not necessary to irrigate the borders around the perimeter of the Water Garden ponds as the water table is high enough for plants to receive adequate moisture.

Spent flowers are removed regularly not only for appearance but to encourage further flowering, thus prolonging the flowering season. Dead or dying foliage on evergreen and deciduous herbaceous perennials is removed during the autumn and early winter as the plants start to die back. As well as for aesthetic reasons this helps to reduce the incidence of pest and disease build up.

The success of the Water Garden ponds involves maintaining a careful balance between all of their components including water, plants, soil, organic matter and fish. The water level is kept at a depth of 300-400mm as this is the preferred level for plants growing in and around the ponds. Care is taken to prevent the ponds becoming overcrowded with plant material which is removed and regrouped regularly. The silty mud at the base of the ponds is ideal for the growing of water lilies and is not disturbed or removed when decaying organic matter is cleared from the ponds in winter. Japanese Carp and Perch are encouraged to live in these ponds as they help suppress algae growth and keep mosquito numbers down to a minimum.

New Water Lily (Nymphaea) cultivars are constantly being added to the ponds collection. They are grown on in tubs in the nursery to a suitable size before being planted out in early spring. After selecting sunny positions the water lilies are plunged in wooden seed boxes into the silty mud at the base of the ponds. The boxes are weighted down with heavy rocks to prevent them floating to the surface or being disturbed by wildlife and fish.