Automatic greenhouse watering has made fast advances and there are now numerous systems on the market. A choice often has to be made to suit your particular requirements or the type of plants grown.
The capillary sand bench
This very successful method was first introduced by the National Institute of Agricultural Engineering. Its operation depends on the natural phenomenon of capillary attraction water will rise against gravity through a porous substance. In this case sand is used, and this is kept constantly moist by a water supply.
If pots are placed on the moist sand and pressed down so that the compost they contain comes into close contact with the sand, moisture will rise up into the compost. As the plants use water and deplete the moisture in the compost it is replaced from the sand bench, which is in turn kept moist from the mains or a water reservoir.
The system depends on arranging a water supply that will maintain the sand in a moist condition. There is a variety of methods, the most common being a constant-level Water supply, the water of which is fed to the sand bench by means of Wicks or similar means. To operate the constant level the ball-valve, well known to plumbers, can be used; or a float valve refinement usually made especially for the purpose. It is easy enough to make your own capillary sand bench. A number of proprietary kits and apparatus is also available. Some of these have neat float valves and utilise narrow-bore high pressure plastic tubing with simple connections so that plumbing becomes easy and elegant.
The tubing is often little thicker than electric cable so that the capillary apparatus can be used for house plants in rooms.
Most of the proprietary capillary units are now in the form of plastic trays that can be connected together to extend the area of watering as needed. Recently a plastic-backed textile material or type of matting has been introduced that can be used in place of sand . This has so far given promising results and may well render sand obsolete in the future. The matting is easy to use and is lightweight, and it is easier to sterilise if necessary. The matting can be kept moist by trickle watering via a siphon system, or by the photo-electric method described later. Vermiculite and similar mineral substances of a porous nature can be used in place of sand also.
Some proprietary tray systems use these because of their light weight. Some of the smaller systems also use inverted bottle water reservoirs so that a mains connection is unnecessary, but their running time is of course very limited.
The pots used on a capillary sand bench, or on the newer matting, must not be crocked. The compost in them must come into close contact with the porous base. The compost must be nicely moist when the pots are put on the capillary bench, or further moisture will not flow up into them.
Plastic pots are advisable, although clay can be used. Clay may become slimy and coated with lime or algae in a short time.
Algae are often a nuisance. They can be prevented by the use of Panasand. This product is mixed with the sand used for the capillary bench. It will discourage algae and slime and also help to prevent plants from rooting through the drainage holes and into the sand. Panasand can be lightly sprinkled on capillary matting, if this is used, and it will have the same function.
This can be manual, semi-automatic, or completely automatic. It is basically merely an arrangement whereby water is conveyed to the plants via plastic tubing. Sometimes the delivery tubes are fitted with adjustable nozzles so that the quantity of water each plant receives can be controlled. This arrangement can be very convenient in frames even if the flow of water is controlled manually.
However, an automatic water flow is easily possible. The simplest is a small cistern fitted with a siphon tube to which the trickle line or drip feed line is attached. The cistern is filled by means of a valve with sensitive control knob so that the rate of filling can be easily adjusted. This means that the frequency with which the water siphons over can be regulated to give a flow to suit the plants requirements. This method has the disadvantage that a good deal of experiment may be needed to get the siphon frequency right, and there is no automatic correction if there should be drastic changes in weather conditions likely to alter the plants needs for water.
The flow of water into a trickle irrigation pipeline can, however, be controlled in a fully automatic way by the use of the so-called electronic leaf or by the very recently introduced photo-electric method. These are described below.
For a greenhouse with many pots scattered about on staging and shelving, the trickle or drip irrigation system may prove inconvenient owing to a network of pipes having to be led to each plant container. However, a system with a reduced number of outlet nozzles can be used to irrigate a capillary sand or mat bench and to distribute water over it uniformly.
The electronic leaf
This is also known by other names according to the makers, but most consist essentially of a porous surface from which moisture is allowed to evaporate. When it reaches a stage of dryness it activates an electric circuit which controls an electromagnetic (solenoid) water valve which then allows water to flow into whatever system is used to distribute the water generally trickle or mist jets. At the same time the porous surface is re-wetted and the cycle is repeated. The process is automatic because the rate of evaporation of water from the porous surface depends on the humidity of the atmosphere and other atmospheric conditions prevailing in the greenhouse. With most designs of electronic leaf, provision is made for personal control of watering frequency and the amount of water delivered so that the method is very versatile. It is also used to control mist propagation.
The photo-electric method
This is probably the best automatic control so far introduced.
It depends on the fact that plants use water according to the amount of solar energy they receive. On bright days, when the temperature will also generally be higher in the greenhouse, plants take up water for their growth and the higher rate of photosynthesis that then occurs. On dull days and in the dark, very little water is needed since growth hardly takes place.
In the photo-electric method the controlling device is a photoelectric cell with a special electronic circuit combined. The amount of light falling on the cell governs the frequency with which a solenoid water valve delivers water to the plants. Thus on bright, warm days the water will be delivered very frequently, but as light fails, or on dull days, the frequency will change accordingly, and in the dark no water will be delivered. Again there is provision for considerable personal control to suit a variety of conditions, and the method can be used to operate the water supplying a capillary bench, trickle or drip irrigation, overhead sprinklers, or misting jets for watering or humidity control, or mist propagation.
Unlike the electronic leaf, the photo-electric cell is unaffected by slime, algae, or lime deposits from the water supply. It therefore gives reliable and accurate control over very long periods without any attention.
The tidal bench
This system waters pot plants by basal irrigation. The bench consists of a waterproof tray which can be made, if necessary, by draping plastic film over a frame of boards. At intervals that can be controlled by the electric methods already described, water is pumped into the tray so that the bottoms of the pots are standing in an inch or so of water, and then pumped out so that the pots are left without water at their base. The frequency of pumping can be controlled to give a high tide time, long enough for the compost in the pots to take up sufficient water, but not long enough to cause water logging.
Ordinarily, pot plants must not be allowed to stand in water too long. The water supply for a tidal bench comes from a constant-level tank, the water pumped out of the bench being returned to avoid waste. Modifications of this system where, for example, the water siphons out instead of being pumped out, can be made. This method of automatic watering is clean and relatively simple, and worth further experiment, especially where pot plants of similar size and in the same size pots are being grown.