- Palps
- The palps of a spider are located in the front and are used for sensory purposes. In the male the palps are modified for putting sperm into the epigyne of the female, located on the underside of her abdomen.
- Pathogen
- Any organism that causes disease, generally applied to bacteria, viruses, and less correctly, fungi.
- Peat
- Partially decomposed mosses and sedges harvested from bogs and used as a component of soilless mixes.
- Peat pots
- Planting pots made from compressed peat. These are used for plants that resent disturbance, as at transplanting time the entire pot can be set out in the garden and the young plants roots will grow through the walls of the pot.
- Pebble tray
- Grouping potted plants within a shallow, pebble filled tray in order to maintain humidity in an environment with central heating. Water is poured into the pebbles and evaporates up and around the plants.
- Pedipalps
- Between the first pair of legs and the fangs of spiders is a pair of leg-like appendages known as pedipalps. In many arachnids, such as spiders, the pedipalps are enlarged in the male and used for copulation.
- Peduncle
- The stem of a flower.
- Perennial
- Any plant that lives more than three years.
- Perlite
- Small globules of heat expanded volcanic rock used to increase the porosity and drainage of potting mixes. Often used in the rooting of cuttings as it is both inert and sterile.
- Petal
- A specialized leaf that surrounds the reproductive parts of a flower. Often colored to attract pollinating insects.
- pH
- A symbol for the acid-alkaline balance of the soil. The balance is expressed as a number from 1 to 14, with 7 considered neutral. Thus a pH of 6 is acidic while a pH of 8 is alkaline. Higher numbers are more alkaline, lower numbers more acidic.
- Phosphorus
- Major plant nutrient especially important for plants where flowering is the main interest.
- Photosynthesis
- The process by which the chloroplasts in plant cells use sunlight to convert carbon dioxide from the air with water vapor to form carbohydrates that are used as the basic food stuff for the growth of the plant.
- Pinching
- The removal of a growing tip from a stem, thus causing any axillary shoots or buds of the stem to develop.
- Plug
- A type of seedling tray in which each seedling grows in an individual, tapered cell, thus reducing root competition with adjacent seedlings and minimizing transplant shock.
- Pollination
- The transfer of pollen to the stigma of a receptive flower.
- Potassium
- Major plant nutrient especially important to the strength of roots and stems.
- Pot bound
- A plant growing in a pot which is too small to allow proper leaf and stem growth.
- Potting back
- The trimming back of a plant's roots and top to allow it to be re-potted in a pot smaller than the one it was originally growing in.
- Potting on
- Transplanting a plant to a pot that is one size larger than the one it was growing in.
- Propagation
- In gardening usage, this refers to the many different ways of starting new plants.
- Prostrate
- Lying on the ground; creeping.
- Pruning
- The removal of plant parts to improve the health, appearance, or productivity of the plant.
