Automatic Watering Equipment for the Greenhouse
The majority of home gardeners have to leave their greenhouse to itself during the day and most people have to be absent from the garden now and then. It is on these occasions that watering can become a real problem, especially if the summer happens to be very hot and sunny. Automatic watering can solve this.
Apart from the problems of absence, automatic watering will be invaluable if you have lots of plants to look after. Moreover, it generally keeps plants constantly moist and encourages the humidity essential to healthy growth. This eliminates the chore of damping-down, and avoids extreme changes between dry and waterlogged soils or compost. Erratic watering is the cause of many troubles like dropping of buds and flowers, and cracked fruit in tomatoes.
The first attempts at automatic watering were usually improvised wicks — textile lamp wicks or blotting paper, with one end dipped into a tank or bucket, and the other end in the soil around the plant roots. Such arrangements usually have the disconcerting habit of drying up, or ceasing to work, as soon as your back is turned. Alternatively they may flood the plants.
Watering through sand
The first really successful system was introduced by the National Institute of Agricultural Engineering and is called the capillary sand bench. This system relies on the capillary action of water rising through any fine material like textile fibres or sand, but the water is always below the level of the plants and there is no risk of flooding. The plants are potted in plastic pots, with large drainage holes, that are kept clear of any obstruction. The pots are then stood on staging spread with a few centimetres of sand, preferably of what builders call ‘washed grit’. The pots are pressed down firmly, so that the sand comes into direct contact with the compost in the pots through the drainage holes. The sand is kept constantly moist, by any one of several methods, and the compost in the plant pots will then take up moisture as required. The amount taken up will vary automatically with the needs of the plant and temperature and humidity conditions — provided the sand layer is always well supplied with water. One other important condition is that the potting compost must be nicely moist when potting, otherwise there will be no flow between the moist sand and the compost.
Regulating the level
Originally an ordinary ball valve cistern was used to provide a constant water level for the sand layer, a perforated plastic pipe being connected to the cistern and running the length of the sand bench. You can make such an arrangement yourself, but proprietary equipment specially designed for this purpose can be bought.
This is usually in the form of units that can be connected together as you require to extend the system. Instead of ball valves, neat little plastic float valves are available, with full instructions for installation. The sand has to be spread on plastic sheeting if ordinary slatted greenhouse staging is used for the bench, and the water level has to be just below the sand. The water is led into the sand by means of glass fibre or plastic wicks several centimetres wide. A good way to convey water along a considerable length of staging is to run plastic guttering against the edge; theis kept constantly filled with water by means of valve connected to the mains or a tank, and the wicks are laid in the sand and dipped in the at intervals along the bench.
A modern development instead of sand is a special kind of plastic porous matting. This is lightweight and very convenient to use. It can be cut and tailored to fit any shape of staging and even cleaned in a washing machine at the end of the season. It can be spread on polythene sheeting laid over the staging, and the edge dipped in a run of guttering as already described.
All capillary materials, such as sand, vermiculite, plastic matting, glass wool wicks, and so on, must be thoroughly moistened when being set up, otherwise the flow of water by capillary action may not start.
A neat little arrangement for simple distribution of water semi-automatically, which has now been in use for some years, is the siphon system. This has a small plastic tank fed with a drip feed valve connected to the mains or other source of. The valve is a sensitive one and by hand adjustment the rate at which the tank fills can be regulated. When full the tank siphons its water into plastic piping fitted with nozzles at intervals. These can be set over pots or along rows of plants, or along a sand or plastic matting capillary bench if desired. Of course with this arrangement you have first to experiment with the filling rate and adjust the amount of water according to the plants’ needs or the area to be watered. Various other proprietary systems using trickle-feed pipe lines and nozzles have also been introduced.
Electrically-controlled systems are, as you would expect, extremely accurate and efficient. Water flow in this case is governed by a water valve operated by electromagnet, and the water can be fed to nozzles or spray jets.
Two methods are now used to control the electromagnetic valve. The first is called an ‘electronic leaf’. There are several designs, but they basically estimate the drying rate of a surface and switch the valve on and off accordingly. The second is an important innovation using a photoelectric cell to estimate the solar energy reaching the greenhouse. An electronic circuit then controls the valve to issue the right ‘dose’ of water. At night or during very dull weather no water, or very little, will be given. On a bright sunny day, however, the photocell will ‘instruct’ the valve to water at frequent intervals. This photoelectric system can be adapted to almost any kind of watering — capillary bench, trickle-feed nozzles or overhead spray. It can also be used for mist propagation and when connected to mist jets can be further used for automatic damping-down. A selection of application methods can be controlled by one photoelectric cell if desired, and the system is as automatic as you can get. Once set up it requires virtually no attention.
Slimes and algae
With all automatic watering there is usually trouble sooner or later from slimes and algae. Good light encourages their growth, so remember never to use transparent plastic tubing to convey water. Unused areas of sand benches can be covered with black polythene. Fortunately, there are now products available, such as Algofen, that will keep water systems free from slimes and algae. This has been cleared as safe by the Ministry of Agriculture and can be used freely according to label instructions even where there are edible crops.