Humidity & watering

Requirements of Plants A very humid atmosphere reduces the rate of transpiration and evaporation to a level that may be harmful to some plants. They may suffer damage from overheating unless cooler, drier air is brought in by ventilation. Many tropical plants from humid climates, however, require high levels of humidity for healthy growth, and will not survive in a dry atmosphere.

Measuring Humidity Warm air is capable of holding more moisture than cold air before it becomes saturated. Relative humidity is a measure of the amount of water vapour in the air expressed as a percentage of saturation point at the same temperature. A "humid" atmosphere is defined as having a relative humidity of about 75 per cent; A "dry" atmosphere has a relative humidity of about 35 per cent.

Measuring Humidity Wet and dry bulb thermometers, used in conjunction with hygrometric tables, may be used to measure the relative humidity of the atmosphere. Hygrometers, which have a dial that gives readings for both the temperature and the humidity, are also available.

Measuring Humidity As a general guide, a relative humidity below 75 per cent, but above 40 per cent, is beneficial for most greenhouse plants during the growing season. At levels above 80 per cent, diseases such as grey mould (Botrytis) and mildew may become a problem. In winter, humidity should be maintained at a lower level, but the exact level required will depend on the types of plant grown and the temperature of the greenhouse.

Humidifiers Greenhouses may be "damped down" during the summer by splashing water -predominantly on the floor and on any staging -from a watering can or hose. This has the effect of increasing the level of atmospheric humidity. An automatic spray system simplifies humidity control, especially for plants that require very high humidity. In a small greenhouse, mist-spraying by hand or providing a tray filled with water that slowly evaporates into the air is usually adequate.

Watering A traditional watering can is still the best method of watering a mixed collection of plants in a small greenhouse. You can readily monitor the flow, so it ensures that all the plants are watered according to their individual requirements. An automatic watering system is a useful addition to a greenhouse in summer, if the greenhouse is left unattended on a regular basis. An automatic system becomes essential, since some pot plants may need to be watered several times a day in very hot weather.

Capillary systems Watering systems that rely on capillary action to draw up water are often used in a greenhouse. Plant pots may be placed on a 2-5cm layer of clean sand (which retains moisture well) that is placed on the greenhouse staging and kept permanently wet.

Capillary systems Capillary matting, which is widely available in rolls and is simply cut to the required size, is far lighter, easier to keep clean, and just as effective as sand. To keep the capillary matting continuously moist, trail the edge into a water trough or other reservoir of water. The water may be topped up by hand or supplied automatically from the mains.

Trickle irrigation systems This type of irrigation system consists of a series of small-bore tubes, each with an adjustable nozzle. The tubes are placed in the individual pots or growing bags, or near plants growing in the greenhouse border. –Most trickle irrigation systems are fed with water from a reservoir that is filled in turn from a hose connected to the mains water supply. It is possible, however, to use water supplied direct from the mains. The rate of water delivery must be monitored very carefuIly and adjusted according to the needs of the plants; these needs vary depending on time of year and the vagaries of the weather.