Pre-formed pools are usually manufactured from plastic, fibreglass, or a PVC or rubber composition material. They are of a fixed shape and have a degree of rigidity. Both formal and informal shapes are available, in a wide range of sizes.
If you are planning a formal circular, square or oblong pond and a pre-formed shape is available in the size required, choose it in preference to any other pond construction material. The moulding will be accurate in every respect and, in most circumstances, will be the easiest material to install.
Informal pools, however, can be a different proposition, for often the design of the pre-formed pool will have been created for ease of manufacture, rather than the well-being of the plants. As pre-formed pools are sold on appearance, the way they look when out of the ground is obviously of great importance to the vendor. The gardener, however, will be more interested in the internal configuration and its suitability for plants, along with the overall surface appearance in relation to other features in the garden.
If you want an informal pre-formed pool, beware of any fussy corners that may cause difficulty with installation or maintenance. Look carefully at the marginal shelves, not only to establish that they are in the right place for your design ideas, but also sufficiently large to accept a well planted basket of marginal plants.
Many pre-formed pools have marginal shelves that are intended for direct planting, but are impractical for this purpose. Sufficient depth should be available for a waterlily, and this equates with its likely spread. Even when accommodated within the restrictions of a planting basket, most waterlilies still manage to achieve a surface spread of at least one-and-a-half times the depth at which they are growing. Only the better pre-formed pools have such a balanced width-to-depth ratio.
Some of the constraints imposed by the pre-formed pool can be relieved by using a sectional pool. The manufacturers of these offer a series of different pool sections that can be joined together to form different overall configurations. While it is possible to create a very elaborate pool with these sections, installing such a feature may he quite difficult. The best use for sectional pools is to lengthen a particular pool configuration, rather than to create added complexity.
The traditional fibreglass pool is the best and most durable of the pre-formed types. The material is rigid and tough and, if accidentally damaged, is very easily repaired using an automotive fibreglass repair kit. However, fibreglass can be quite heavy, which limits the size to which manufacturers are prepared to work. It is rarely suitable for a large informal pool.
The cheaper end of the market is dominated by vacuum-formed plastic pools. These are very thin, light in weight and are more difficult to install than their heavyweight counterparts. Many are of a questionable shape, often not providing suitable accommodation for aquatic plants. Some come with a small central island moulded in, which is hollow beneath and difficult to support unless filled with soil or sand before installation: a difficult task.
Composition PVC or rubber pre-formed pools are similar in most respects to fibreglass pools. However, they are lighter in weight, allowing them to be readily manufactured in larger shapes and transported easily. Their designs are generally perfectly adequate for the requirements of plants and fish.
The surfaces of vacuum-formed plastic pools are normally very shiny and not conducive to the rapid establishment of micro-flora, while fibreglass pools, although apparently smooth, soon become coated. Composition types also spawn rapid growth, for these have a rough finish to which algae and similar aquatic life clings.
The only other consideration when choosing a preformed pool is the manner in which it is finished around the edge. Some manufacturers decorate the edge with imitation rocks that not only look hideous and never mellow, but also create enormous difficulties with installation, especially when it comes to levelling the pool.
A broad edge may be useful if the poolside is to be paved, but if planting is to be a priority, it can be a nuisance. Most of the composition material pools have lips that are narrow, functional and unobtrusive, being easily disguised by plants.
Among the many pre-formed pools available are fountain or rock pools. These arc not intended for planting, but as small water features in their own right or as a reservoir to serve a moving water feature. Although they are usually well made and functional, do not be led into believing that they are anything other than a supplementary feature to the main water garden, being unsuitable for housing most plant life and all the popular ornamental pond fish.
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