R Reversion: What happens when a plant goes back to the colour or growing habit of its parent. Roses have been known to completely change color -- a red becoming a yellow -- when harsh pruning forces a reversion. Rhizome: A horizontally creeping root system from which shoots and roots develop. Rootbound: A bad sign that a plant has been left in a container too long, allowing roots to become become tangled and choked.

S Species: A collection of individual plants with the same characteristics when grown in the same conditions. Standard: A tree or shrub that is trained to grow a straight stem clear of branches. Stock plants: The parent plants from which cuttings are taken for propagation purposes to ensure new plants are an exact clone.

Stress: A cry for help. Wilting or discoloured foliage are two signs of stress, signals that a plant is suffering from a lack of water, food or too much sun, water or fertilizer. T Trace elements: The same thing as micronutrients: various chemicals that plants need in small doses in order to grow. Tissue-culture: A high-tech way of propagating by "cloning'' a plant by taking tiny pieces of tissue and growing them in test tubes.

Topiary: The shaping of shrubs and trees into decorative forms. Top-dressing: Putting a thin layer of new soil or compost around plants or on lawns to add nutrients to the soil. Tuber: A fleshy root or stem that stores nutrients for later use. Tufa: Porous limestone rock ideal for growing alpine plants.

U Underplanting: Plants that have been placed beneath taller shrubs or trees, sometimes to provide leafy groundcover or seasonal colour. Umbrel: A rounded, often flattened head of flowers produced at the top of a long stem. Good example: the seed/flower head of an angelica plant.

V Variegated: Leaves that are spotted or edged with a different colour from the main one. Variety: The same as cultivar: the variation of a species. Vermicomposting: Composting using worms to do the job normally done by micro-organisms.

W Weed: Two definitions: 1) A plant growing in the wrong place. 2) A plant for which a useful purpose has not yet been found. Wild garden: Informal planting style that attempts to imitate nature. Popular with avant-garde landscape architects, not with most homeowners.

X x: The symbol that denotes that a plant is a hybrid species. Xeriscaping: Method of landscaping with drought-tolerant plant material to reduce the use of water by 50 per cent. Y Yakushimanum: One of the best medium-sized species of rhododendrons for the small garden because of its compact form. Z Zones: The world is divided into 10 climatic zones that are graded according to the average annual minimum temperatures. Zone 1 is the coldest (below -46 C) and Zone 10 is the warmest (-1 C to 5 C). Plants that thrive in gardens here are generally those that can survive in zones 1 to 8