Maiden A grafted tree in its first year (see also Whip).

Maincrop (of vegetables) Those cultivars that produce crops throughout the main growing season, doing so over a longer period than either early or late cultivars.

Manure Bulky organic and plant waste, generally from herbivorous animals such as horses.

Marcottage An alternative name for air layering (see Layering).

Marginal water plant A plant that grows partially submerged in shallow water or in constantly moist soil at the edge of a pond or stream.

Medium 1) A compost, growing mixture, or other material in which plants may be propagated or grown. 2) Applied to those soils that are intermediate in character between heavy and light (see also Loam).

Mericlone Genetically identical offspring produced by meristem culture.

Meristem Plant tissue that is able to divide to produce new cells. Shoot or root tips contain meristematic tissue and may be used for micropropagation.

Micronutrients Chemical elements essential to plants but needed only in very small quantities, also known as trace elements (see also Nutrients).

Micropropagation Propagation of plants by tissue culture.

Microlife The smallest invertebrates that inhabit the soil. These tiny creatures, along with bacteria and fungi, break down the organic matter to produce a healthy soil for growing plants (see also Invertebrate, Vertebrate).

Midrib The primary, usually central vein of a leaf or leaflet.

Module Applied to various types of container, particularly those used in multiples for sowing seed and pricking out seedlings.

Monocarpic Flowering and fruiting only once before dying: such plants may take several years to reach flowering size.

Monocotyledon A flowering plant that has only one cotyledon or seed leaf in the seed: it is also characterised by narrow parallel-veined leaves; and parts of the flower in threes or multiples of three.

Monoecious Bearing separate male and female reproductive organs on the same plant.

Monopodial Growing indefinitely from an apical or terminal bud on a stem.

Mulch A material applied in a layer to the soil surface to suppress weeds, conserve moisture, and maintain a preferably cool even root temperature. In addition to organic materials such as manure, bark, and garden compost, polythene, foil and gravel may also be used (see also Floating Cloche).

Mutation An induced or spontaneous genetic change, often resulting in shoots with variegated foliage or flowers of a different colour from the parent plant. A mutation is also known as a sport.

Mycorrhizae Soil fungi that live in beneficial association with plant roots.