Back to Glossary A - Z

Pasteurization The selective destruction of some, but not all, living microorganisms. Root media are pasteurized to eliminate harmful disease organisms and to retain the beneficial microorganisms. (Nelson, 2003)

Pathogen An entity (fungus, bacterium, nematode, virus) that can incite disease. (Nelson, 2003)

Peat The organic remains of plants that have accumulated in places where decay has been retarded by excessively we conditions. There are many types of peat, come desirable and others not, used for greenhouse root media. (Nelson, 2003)

Peat humus Peat that is at an advance stage of decomposition in which the original plant remains are not identifiable. It is not generally a desirable form of peat for greenhouse root media because of its rapid rate of decomposition and its occasionally high rate of ammonium nitrogen release. (Nelson, 2003)

Peat moss Peat consisting predominantly of slightly humified (decomposed) Sphagnum moss species. Horticultural peat moss contains over 75 percent sphagnum moss. (Nelson, 2003)

Pedicel Stem of one flower in a cluster. (Nelson, 2003)

Perimeter heating system A row of heating pipe or pipes just inside the perimeter wall of a greenhouse. (Nelson, 2003)

Perlite A siliceous volcanic rock that is crushed and heated to 1,800ºF to cause it to expand into light weight (about 6 pounds per cubic foot) particles with closed air-filled cells. Perlite is used as a substitute for sand when a lightweight root medium is desired. (Nelson, 2003)

Pesticide An agent or preparation used for killing living organisms that are a nuisance or are harmful to crops. (Nelson, 2003)

Petiole The stalk or stemlike portion of a leaf. (Nelson, 2003)

Photoperiodism The response of a plant or animal to the relative length of day and night. The response in plants can take on many forms, including flowering, changes in leaf shape or internode length, and bulb or tuber formation. (Nelson, 2003)

Photosynthesis The manufacture of carbohydrate from carbon dioxide and water in the presence of chlorophyll, using light energy and releasing oxygen. (Nelson, 2003)

Phytotoxic Toxic to plants (Nelson, 2003)

Pinching Removal of the top of a vegetative plant stem in order to cause it to form several branches. (Nelson, 2003)

Plug seedlings Seedlings produced and contained in a small cohesive volume of root medium. This unit of root medium is know as a plug. (Nelson, 2003)

Polyethylene A plastic material used in the greenhouse industry in the form of thin films for covering greenhouses. It is an inexpensive substitute for glass. Generally, two layers are used- an outer layer 6 mils (6 one-thousandths of an inch) thick and an inner layer either 4 (0.10 mm) or 6 (0.15 mm) mils thick. (Nelson, 2003)

Pompon chrysanthemums A term used in this book to denote the chrysanthemum cultivars grown with several cultivars on each stem. The term spray chrysanthemum is more commonly used. (Nelson, 2003)

Potable Drinkable. (Nelson, 2003)

Precipitaiton The process whereby a dissolved substance comes out of solution to form a solid. The solid substance is a precipitate. (Nelson, 2003)

Pressuring fan A fan in the end of the clear plastic greenhouse distribution tube that forces heater air, exterior cold air, or interior warm air though the tube, depending on whether the system is being used for heating, cooling, or air circulation, respectively. (Nelson, 2003)

Proprietorship A business owned by a single individual. (Nelson, 2003)

Pupa The intermediate, usually quiescent, stage assumed by many insects after the larval stage and maintained until the adult stage. (Nelson, 2003)

Purlin A component of the greenhouse frame running the length of the greenhouse just below the roof covering that connects the trusses together. (Nelson, 2003)

PVC (polyvinyl chloride) A plastic material available in corrugated sheets. This material was used for covering greenhouses during the 1960’s, but, because of its rapid deterioration from ultraviolet light, it has virtually disappeared from the greenhouse industry. (Nelson, 2003)