Advice
December's glossary
Bleeding: The continuous flow of sap from the cut surface of a plant, as occurs in grapevines (Vitis species), Acer and Betula species that are pruned in late winter.
Humidity: The amount of water vapour in the atmosphere, measured on a scale of relative humidity (RH), from 0% (totally dry air) to 100% (saturated air), at any given temperature. Most species grow satisfactorily within the range of 56-80% RH. Below 65%, growth is progressively and adversely affected. Above 80%, there are increasing problems with fungal and bacterial diseases, and above 90%, transpiration is restricted, which affects the movement of some nutrients, particularly calcium, within the plant, and may result in deficiency symptoms. Exceptions are xerophytes and hygrophytes, i.e. plants that thrive in exceptionally dry or exceptionally moist conditions. Where necessary, humidity can be reduced by ventilation and heat, or by using dehumidifiers. Humidity can be increased by damping down the greenhouse floor, as well as by the use of overhead irrigation, misting, and fogging systems.
Organic matter: Material derived from plant or animal origin. Organic matter in the soil is turned into humus by the actions of micro-organisms, insects and animals such as earthworms. This greatly improves the fertility and structure of the soil (or potting compost). Usually applied to bulky soil improvers and mulches such as compost, manure, or chipped bark, rather than to fertilisers of animal or plant origin (such as pelleted poultry manure). Does not necessarily refer to products certified ‘organic’ in their method of production.
Waterlogged: The state of a soil or a potting medium when it is saturated with water, displacing the air from the pores and spaces between the soil particles. This may result from compaction, natural defects in the soil profile (such as a soil pan), the lie of the land, the basic soil texture and to any other factors that inhibit drainage. Waterlogging is a common cause of root death, and may precipitate the development of root fungal diseases such as Phytophthora.