Greenhouse Benches and Greenhouse Shelving
The provision of benching or staging for a range of activities is invariably necessary, except when a greenhouse is being used exclusively for ground-grown crops. Even then propagation space is invariably demanded for a limited period of the year. Benching must be strong as it has frequently to bear a considerable weight. Fixed benching is not so popular as it was owing to management problems, but it certainly allows better presentation and utilizes space especially if tiered.
Conventional greenhouse benching employs slatted wood of 10-12cm (4-5in) and approximately 1.75-2cm (5/8-3/4in) section, set on bearers with approximately 2.5cm (1in) between the slats. Ideally the wood for the slats should be of good quality such as Western red cedar, although preservative treated redwood would suffice. Benches from about 67cm (2ft 3in) wide up to about 1.2m (4ft) permit ease of access, although centrally sited benches with both sides accessible can be much wider — up to about 1.5 or 1.8m (5 or 6ft). A height of 75cm (30in) (average table height) was previously thought to be ideal, but there is now commercial interest in knee-high benches of about 50cm (20in). These allow a much closer surveillance of the plants and their watering needs, a vital issue with the widespread use of plastic pots.
Wider spaced slats can be used where stock plants arc being maintained so that there is no contact or run of moisture between the plants, so cutting down the risk of infection. Wood slats can also be inserted into angle iron or alternatively patent angle steel with a mesh top used. Much more popular in recent years is aluminium benching, especially tailored to fit respective sizes of greenhouse. These generally incorporate aluminium gravel trays, which can be filled with gravel, sand or capillary matting if desired. Modular greenhouse shelving is also available. Solid benches can be made by using corrugated iron or wood; they are considered to be more suitable for certain types of pot plants by some gardeners, though a difference of opinion exists. Solid greenhouse benches are essential for mist propagation units or for the construction of capillary benches. Where solid greenhouse benches are used, there should always be a space between the bench and side of the greenhouse to allow the passage of air.
The usual arrangement is to have benches on both sides of a central path. With a greenhouse 2.4m (8ft) wide, two benches each 75cm (2ft 6in) wide would allow a 90cm (3ft) path. In narrower houses one 90cm (3ft) bench might be preferable, although two very narrow benches are possible but not always practical.36m (12ft) in width could accommodate two side benches each 75cm (2ft 6in) wide and a central bench does not restrict access through the door or doors. The free opening or entry from the door must also be taken into account when deciding on the width of the side benches. A sensible width of central path is always advisable to allow the entry of a barrow or trolley.
Greenhouse benches may be supplemented with greenhouse shelving at higher levels, useful for ensuring maximum light for plants and utilizing space. On vertical wooden-wall, they can be fitted with galvanized shelf brackets, while on alloy or metal houses manufactures offer special shelf fittings.
These involve the use of a completely level solid bench with 8-10cm (3-4in) sides which is lined with inert washed sand of acceptable capillarity: a coarse gravelly sand to a depth of 2.5-4cm (1-14n) to give a free water zone is topped by 2.5cm (1in) of finer sand. A perforated hose is laid down the centre of the bench before or during filling with sand, preferably under broken sections oftiles so that water can pass through the perforations in the hose without restriction into the free water zone, or sections of curved asbestos can be used. Water can in fact be led in almost as satisfactorily at a single point in the free water zone, when it will spread effectively, although care should be taken to ensure that sand does not block the outlet.
To control the level of water, a plastictank fitted with a ball and cock can be used so that the water level is held constantly 2.5-4cm (1 —1-1/2in) below the top of the sand. Alternatively a float control unit can be used connected to a storage tank, adjusting the supply of water to the necessary level by adjustment of the control unit. More simply a slowly dripping hose or bleed-type irrigation units can be used to supply the water to the bench, or it can merely be kept topped up at regular intervals, although neither of the two latter methods is entirely satisfactory, owing to the varying degree of solar heat.
Plants in pots spaced out on a capillary bench extract water by capillary pull, plastic pots being rather better in this instance than clay pots. Plants can also be fed with liquid nutrients by this method, although there can be problems of local build-up of salts, causing chemical damage to plants. In recent years individual drip systems have tended to supersede capillary watering, one reason being that roots from the pots develop into the sand on capillary action, especially if the sand is too dry, resulting in severance and checks to growth when the plants are moved. Fibre glass mats can replace sand, ideally covered with perforated black polythene to prevent growth of algae, which will grow on either sand or bare matting.
Greenhouse Benches and Soil Warming
The escalating cost of all fuels has thrown new emphasis on the highly efficient localized application of heat directly to the propagating material. The use of alkathene pipe at 23-30cm (9-12in) depth and 1.2—1.5m (4-5ft) apart linked to a pump-circulated hot water system has achieved certain favour in continental European countries for the production of early crops such as very earlyor demanding warmth. Similar systems are now used in conjunction with rock-wool or perlite methods of crop production using indented polystyrene slabs. These can also be used for benches.
Electric soil warming using 2.5mm (12 gauge) wire of known electrical resistance at 6-30 volts through a transformer at 23-25m (9-10in) depth and 8-23cm (3 — 9in) apart, to give a loading of 55-110w / m2 (5-10 watts/sq ft), has also received little attention since the 1950s, possibly because of the high cost of electricity when used on the scale necessary to heat large bulks of growing media such as . Such systems do, however, have an application for smaller border areas in many cases, using sheathed mains voltage cables.
The use of soil warming cables for bench application is, however, an entirely different matter, as the loading necessary at 86 — 130w/m2 (8-12 watts/ft) ensures that the level necessary (approximately 1kW per 10m2/ 100sq ft) is a highly efficient and economic operation. The cables are laid on a shallow bed of sand and covered to the depth of 5cm (2in) with more sand. Control of the soil warming cable temperature can be achieved by several methods: (1) at off peak tariff by time clock operation (8 hours in 24), (2) according to the weather, by inspection with a soil thermometer, or (3) the best method, and essential for the higher temperatures necessary for mist propagation, by the use of phial and capillary tube thermostats set across the run of the wires below the sand surface. It helps to induce warmth if spaces between pots or boxes on the bench are filled up with moist peat.
The local application of heat to either borders or benches (or plants or cuttings in pots) has little effect on the air temperature of the greenhouse, but such locally applied warmth to the media in which the plants are growing generally allows a lowering of the air temperature in the greenhouse, which has tremendous economic benefits. Indeed the success of mist propagation or white polythene “tents” with the necessary attendant warmth in the actual rooting media has revolutionized propagation of many previously difficult-to-root plants. Germination of seed, benefits too.