A plastic material now finding many uses in horticulture which was discovered by I. C. I. chemists in 1933. It is made by the polymerisation of ethylene under high pressure and was used during the second World War as an electrical insulator for cable used in radar.

Polythene is tough and water-resistant, yet at the same time light and flexible so that it can be cut, moulded or trimmed to special shapes. It is now supplied for piping water to farm buildings, among many other very diverse uses.

In the garden, polythene can be employed in various ways and new uses are still being found. In the greenhouse an insulating layer of air can be formed between polythene and glass if polythene film is fastened to the inside sash bars. Polyethylene can be used in various forms to protect special plants, e.g. late flowering chrysanthemums, or shaped over taut wires to cover rows of young plants. Polythene film bags can be used to wrap cut blooms, also to pack cuttings or plants which are to travel long distances. They travel better by reason of the excellent moisture-retaining properties of the polythene. It can be used as a covering for outside frames instead of glass. A recent application of polythene is in the form of tubing for automatic watering and liquid feeding, especially under glass. The tubing is black to prevent the entry of light and to stop growth of algae (plants forming green scum) which could block the holes. It is placed between rows of carnations, tomatoes etc., the 2 holes on each side of the tubing (which is pricked every 5 in. with 4 holes) delivering the water in a light shower at a distance of 6—18 in. After the hose has been turned on for a few minutes, the water spreads over the soil surface, covering a strip up to 3 ft. wide. For liquid feeding this new technique is invaluable as it ensures absolutely even distribution over a given area. Polythene is employed in connection with ‘air layering’, a technique used by the Chinese and practised in this country over 200 years ago. It is also known as Chinese layering, layering by approach, and marcot-ting. This method is used mainly with plants which are hard to root from cuttings and do not respond to conventional ways of layering, e.g. those with firm branches carried high above the ground like acers, cornus and so on. Young shoots are preferred and an upward cut 1 — 2 in. long is made at or around the centre of the stem. Another method is to cut off a ring of bark about ½ in. wide. The resulting wound is bound with damp moss. In the past it was very difficult to prevent the moss drying out but polythene film can now be wrapped round to stop moisture escaping. Syringa (lilac), wisteria, acer and cornus are among the plants successfully increased in this way.

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