PANNING: An undesirable condition of the surface of a soil, usually a heavy one, which has become smooth and hard. The problem is usually caused either by too much walking on a particular area, or by breaking the soil down too finely during cultivation. Heavy rain then settles down the surface into a crust, which eventually dries out to a rock-hard finish. The germination of seed is difficult under such conditions and young plants cannot thrive. A pan should be broken up by thorough digging and steps taken to improve the soil’s texture by adding humus-forming materials to prevent it happening again.
PARASITE: A plant or animal obtaining nourishment directly from the body structure of another living organism.
PEAT: Like compost, peat is formed from vegetable remains, but occurs in wet, acid places where aerobic bacteria have not been able to rot it down properly. It is used for horticultural purposes mainly as an ingredient of soil-less composts.
PEAT SUBSTITUTES: Substances derived from non-peat sources (coir, wood waste, etc.) used in compost, mulches or for soil conditioning.
PERENNIAL: Strictly speaking, this means any plant with a life cycle longer than two years (often indefinite). The expression has come to refer mainly to herbaceous perennials, although trees, shrubs, bulbs, and many other types of plant should correctly be placed in this category.
PERLITE: Expanded volcanic rock in granules used as an additive to soils and composts to improve the texture, retain water and let in air. It is very lightweight and therefore easier to handle — both on its own, and in combination with peat or peat substitutes for soil-less composts — than sand.
PERPETUAL flowering: A plant with a prolonged or indefinite flowering period, as opposed to one which only has a single flush.
PEST: A living creature which can cause damage to plants. PESTICIDE A chemical for killing or controlling pests.
PETALS: The parts of a flower, often eye-catching, surrounding the male and/or female reproductive organs.
PHOTOSYNTHESIS: The process in which the chlorophyll in plants combines water in sap and carbon dioxide from the air in the presence of sunlight to make sugar from which are created many more complex food compounds for the support of the plant during its lifetime.
PINCH: To remove the growing shoot of a plant in order to make it bush out. (See stopping.)
PISTIL: The female organ of a flower. PLANT FOODS Chemicals in solution in soil, capable of being absorbed by a plant for feeding purposes. Plants also make certain energy-giving foods by photosynthesis.
PLANTLETS: Baby plants produced by certain species as a means of vegetative propagation.
PLANT OUT: To place a seedling or other young plant in its permanent position after its time in a seed tray or nursery bed.
PLUNGE: To submerge a pot-grown plant up to its rim in soil outside or in a specially made ‘plunge bed’. Many house plants may be treated like this during the warmer summer months and pots of bulbs required for indoor decoration can be plunged several inches deep after planting to encourage the production of strong, healthy roots before the top growth develops.
POD: A dry seed container capable of splitting into two halves down its length and enclosing many seeds, e.g. peapod.
POLLEN: Dust-like grains producing male reproductive cells found in the anthers of most flowering plants. Pollination occurs when these male grains alight on the female stigma after which fertilization takes place if the male and female cells (known as gametes) successfully fuse. Pollination is brought about naturally by wind, animals, insects or in self-pollination by the pollen falling on to the stigma in the same flower and can be done artificially by dusting the appropriate parts with a soft paintbrush or similar.
POT: A receptacle usually, but not always, having drainage holes in the bottom, and intended for containing a growing plant.
POT-BOUND: A state reached by a plant when its roots have entirely filled its container. Some plants flower better in this condition, as they are placed under stress, but generally speaking a pot-bound plant will start to deteriorate if its roots are restricted too much, and it should be replanted in a bigger pot.
POT-GROWN: A plant grown in a container. The term is often used to describe the method of raising new stock in a series of containers until it is finally planted in its permanent site in the open ground.
POTTING: The transference of a plant from a seed tray into an individual pot, or the replanting from a small pot into a bigger one. (Also known as ‘potting on’.)
PRICKING OUT: The planting out of seedlings from the container in which they reached germination into another container or bed where they can be spaced further apart to allow more room for development.
PROPAGATION: Increasing a stock of plants. Naturally, this occurs through seed production, and also by producing plantlets, runners, stolons, or similar. In addition to these methods gardeners can increase their stock through cuttings, layering and the like, and also by the comparatively new method of tissue culture. A propagator or propagating case is a container with a transparent lid fitting over a tray base which may contain or be attached to some form of heating appliance, and is filled with compost or other growing medium. The purpose is to create a beneficial microclimate with increased warmth and humidity for the germination of seeds, and to raise seedlings and strike cuttings.
PRUNING: The cutting back of woody plants to encourage the formation of a healthy, well shaped specimen and improve or control flower production.