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.