ACID, ACIDITY: Most substances are either acid, alkaline or neutral. Acidity is measured on the pH scale, the term pH referring to the percentage of hydrogen ions in the material. Things are neutral at pH7; numbers above 7 indicate that a substance is alkaline, figures below mean it is acid. The right quantity of an alkaline compound can neutralize an acid one, and vice versa. It is important to know if a soil, potting compost, or even water is acid, alkaline or neutral, as the pH value can affect the type of plant intended to be grown.
Some plants will only thrive in an acid soil and cannot tolerate the alkaline conditions found in the presence of lime. These are known as calcifuges and examples are heathers and rhododendrons. Other plants (e.g. cabbages) are calcicoles — that is, they prefer an alkaline soil — and there are plants which will do well in either. The pH of a soil can be checked by means of a chemical soil-testing kit or meter; if the soil is found to be unsuitable to grow a particular type of plant, substances can be added to it to adjust the pH.
AERATION: Introducing air between the particles of a soil by digging, forking, spiking, etc., to encourage healthy growth of the roots and allow beneficial bacteria to break down decaying materials into plant foods.
AEROBIC: See bacteria.
ALGAE: Primitive plants usually seen in the garden in the form of green slime on paths and clay pots, green water and blanket weed in ponds.
ALKALINE: See acid.
ALPINE: Technically a plant whose natural habitat is a mountainous region, but the term has become used to describe many more types of plant suitable for planting in a rockery. See also rock plants.
ANAEROBIC: See bacteria.
ANNUAL: A plant which completes the cycle of germination, maturing, flowering, seeding and finally dying in the period of one year. Many perennial plants, such as antirrhinums and fibrous-rooted begonias, are treated as annuals as they are at their best in their first year. Bedding geraniums now fit into this category as good plants of new varieties can be easily grown from seed using nothing more elaborate than the kitchen windowsill.
hardy annuals (HA) originate in temperate climates such as our own so they are not damaged by cold weather and can be sown direct into their flowering positions outside. They can therefore spend the whole of their lifespan in the open garden (e.g. Virginian stock, candytuft), half-hardy annuals (HHA) come from warmer climates than that of the United Kingdom. They are damaged or killed by frost and the seed needs a higher temperature for germination. They therefore have to be raised in a heated greenhouse or similar and kept there until all chance of frost has passed (e.g. lobelia, salvia).
ANTHER: The part of a flower producing pollen grains which in turn produce the male cells for fertilization; these fuse with the female cells, ultimately to develop into a seed.
AQUATIC PLANTS: Those growing in water.