Peat: Usually partially decomposed sphagnum moss and used for potting compost, etc.
Peat Substitute: Because of environmental concerns over the use of peat, a substitute is now available made from renewable materials like coir.
Peduncle: The stalk of a flower.
Perennial: A plant that lives for at least three seasons.
Perlite: Small granules of white expanded volcanic mineral mixed with seed compost to improve air supply.
Petal: An individual part of the corolla forming the flower.
Pesticide: A chemical that kills pests – see insecticide.
Petiole: A leaf stalk.
pH: A measure of acidity and alkalinity – also see neutral.
Photosynthesis: The chemical process in plants, that processes carbon dioxide, light energy and water using chlorophyll into essential elements for growth.
Picotee: Petals with a narrow band of contrasting color around them.
Pinching Out: Removing the main growing point from a plant to encourage side growth.
Pleaching: A technique of weaving branches of a row of trees to make a more solid wall.
Plunge: To place an outdoor pot up to its rim in soil, peat, sand or ashes to protect the roots from extreme heat or cold.
Pollarding: A forestry technique – all the main branches of a tree are cut back to the trunk at around 6ft above the ground. Also, see coppicing.
Pollen: The male ‘dust’ produced by a flower on the anther to fertilize other flowers.
Pot Bound: A plant that has been left in a pot too long and so is now too big for it and the root ball is tightly compressed.
Potting On: Repotting a plant so that it has room to expand its root ball and encourage growth – see over potting.
Pricking Out: Moving seedlings from a seed tray and replanting them with more space for development.
Propagation: Making new plants either from seed, taking cuttings or layering.
Pruning: Cutting off or shortening branches of shrubs and trees to encourage growth and/or to improve the shape.
Remontant: A plant that flowers more than once in a season.
Revert: When a plant returns to an original state, for example, one with variegated leaves may produce plain leaves.
Rhizome: Like a bulb, this provides storage. It is again a modified stem and lies horizontally in the soil producing roots along its length.
Ripewood Cutting: A cutting from ripened wood, usually used for evergreen plants.
Root Ball: The compact mass of roots and soil seen when a plant comes out of a pot or is dug up from the soil.
Root Cutting: A cutting taken from a piece of root.
Rooting Hormone: A chemical in which the cut end of a cutting is placed before planting to promote root formation.
Rootstock: The roots used in a graft.
Rose: Apart from the flower, the term refers to the nozzle with fine holes put on the spout of a watering can.
Rosette: A cluster of leaves coming out from around the same place.
Runner: A thin stem that grows horizontally above ground and it roots and grows new plants at the nodes, eg strawberries. Not to be confused with stolon.