In climates where frost is severe or prolonged, heat is needed unless the greenhouse is to remain empty throughout the winter. Where winters are not severe, however, it is worth looking at the limitations of a cold greenhouse. From about mid-spring to late autumn, all the plants that need cold greenhouse conditions thrive happily. From late spring to early mid-autumn, even warm greenhouse plants will grow, so we are left with that period of the year where prolonged frosts are expected, that is from stayed autumn to early spring. At this time warm greenhouse plants must be kept near to their minimum temperature requirements or they will deteriorate rapidly and eventually die. Many cool greenhouse plants will stand temperatures down to freezing or even below for short periods, providing they be kept almost dry, but once the roots start to freeze, then severe damage or death results. Plants are growing in this respect and in areas which regularly suffer cold winters, the less hardy perennials, shrubs and climbers are best cultivated in this way. Large pots and tubs can be lagged with glass fibre (fibreglass), wood wool, dried bracken or any other insulating material that can easily be secured with wire, plastic netting, hessian, etc. This is not, of course, practicable with smaller pots, but these can be plunged in peat or sand.
During an average winter in maritime areas of the temperature zone, frosts occur mainly at night, the temperature rising at least just above freezing by day. Periods of prolonged day and night frost without daytime sunshine which will heat the greenhouse, are relatively infrequent. For night frosts, particularly the radiation kind, a surprising amount of protection can be given by covering pots with paper or cloth. Plastic sheeting can be used, but this encourages unwanted condensation around and on the foliage. Two or three thicknesses of newspaper or one or two sheets of thicker brown paper are easiest to deal with. Cover the plants completely, paying particular attention to the area between pots and the glass. For even better insulation, the polystyrene (plastic foam) sheeting used as a lining for wallpaper can be utilized, preferably over a sheet of newspaper to prevent too much condensation. Remove each morning once the temperature has risen above freezing and replace it in the afternoon before freezing starts again. Although one can bring a surprising number of the not too tender plants through the winter in this way, it is a chancy business owing to the vagaries of the winter climate. To get the best of winter sun and wind protection for a cold house, a lean-to against the south wall of a house or free standing wall is ideal. There is also a partial compromise in growing a few, favourite tender plants in one of the commercial heated propagation cases.
Even in the completely unheated greenhouse it is possible to have winter interest by growing hardy plants. Several garden annuals and perennials can be either specially produced or lifted from outside in autumn and brought in to continue flowering after those outside have succumbed to rough weather, winds, and frosts. Evergreen foliage plants can also be used, and practically all hardy bulbs will bloom ahead of their normal times without heat.
After considering the limitations of the chilly greenhouse and seriously deciding to install heating, the following should be considered. An extensive and satisfying range of plants can be grown if a minimum of 7c can be maintained. However, it costs twice as much to keep 13c than 7c, and three times as much to maintain the temperatures at 16c. These temperature examples above are not arbitrary but equal the three grades of heating recognized today: 7c represents the so-called chilly greenhouse, 13c the intermediate and 16c the warm greenhouse. Formerly greenhouses kept at minimum temperatures of 18c or above were known as stoves. However, as the majority of tropic plants, we cultivate survive at 16c or even down to 13c, the expense of running an oven in the greenhouse is no longer justified.
A careful look at the greenhouse should be made to see that warmth will not be wasted once the heater is installed. In maintaining a chosen minimum temperature, the problem is supplying enough heat to allow for losses through and around the glass, or other glazing material, and whatever building is used. These heat losses are obviously greater when it is cold outside, but particularly when it is cold and windy. The wind, as it blows around the greenhouse, can suck away as much as 50% of the heat because the speed of the air moving across the glass is a major factor.
It is, of course, important to choose a site protected as far as possible from the winds of winter. If this is not feasible or the greenhouse is already in position, a windbreak can be considered as long as it does not exclude any light or sun. A hedge or open lattice fence is best, and if situated at least three to four times the height of the greenhouse away from it, the shading factor will be minimized. Another reason for not putting the windbreak too close to the greenhouse is the problem of turbulence. This is most severe when the screen is a solid one such as is provided by a wall or a close boarded fence. If there is a good wall, it is best to use it like the back of a lean-to type, especially if one is lucky enough to have it on the north side facing south. The lean-to structure is the easiest to heat, as the wall will absorb and hold the sun's heat and continue to release it slowly after dark.
The way a greenhouse is glazed can also affect heat loss. Glazing can be dry, the glass merely sliding into grooves as in the light Dutch structures or it can be secured in a more airtight fashion with putty or a sealing compound or strip. Although there is more heat loss around the edges of the non-sealed glass, it is not too significant unless it fits very badly. There is much to be said for this method as it allows more air exchange with the outside and prevents extreme stagnation of the atmosphere even when there is no other ventilation. Double glazing reduces heat loss and is relatively easy to carry out, either with double-paned glass or using clear polyethene sheeting with single glass panes. This sheeting is used to line the greenhouse inside, creating an air gap between the plastic and the glass. There are snags, however. The polyethene sheeting cuts out as much as 15% of available sunlight, even when it is clean, and it also acts as a condensation collector and the water droplets cut out even more light. During the colder months of the year, the polyethene will probably be continuously damp, providing ideal conditions for the formation of green films of algae which will disastrously reduce available light. The sheeting also reduces air exchange through glass overlaps and other crevices and as a result the atmosphere will become more humid. At temperatures less than 7c this can do more harm than good, if only by encouraging the growth of grey mould. For these reasons it is now recommended that only the side exposed to winds, but away from the sun, should be lined. A special insulating curtaining composed of two sheets of polyethene sheeting sandwiching a layer of air bubbles is on the market. This is used to cover part or the entire greenhouse.
Fuel sources for heating are the same as those used domestically, and the equipment is similar though tailored to greenhouse requirements; household heaters should not, however, be utilized in the greenhouse as they are not insulated against the damper conditions. The primary fuels are coal and other variations of solid fuel, oil, gas, and electricity. Solid fuels are used in boilers to heat water. The hot water is then piped around the inner perimeter of the greenhouse close to ground level. The water circulates through the pipes and boiler by gravity using the principle whereby water lightens and rises when hot and sinks again when it is cold. The pipes must be installed correctly. Otherwise, there may be circulatory problems, though small pumps can be fitted to get over this. There is no doubt that the time-honoured practice of using piped hot water gives superior heat, but it is more expensive to install. The old hand-fed solid fuel boilers were messy to use, and the temperature was hard to regulate, but more modern, semi-automatic, hopper-fed solid fuel or oil-fired boilers provide a safe method of heating. If such boilers are already in use for domestic heating, it is always worth consulting a heating engineer or plumber to see if it is feasible to link the greenhouse to the existing system. Gas and electricity can also be used to heat water, but are best used in a more direct way.
Natural gas and propane can be burned in heaters which are very efficient. In some parts of the world, natural gas is also very cheap. Oil in the form of paraffin has also long been used in this way. Formerly, heaters gave off harmful fumes unless kept immaculately clean, but modern designs have eliminated this fault, and they can now be controlled with greater accuracy. Even if not chosen as the primary source of heat, one of these heaters should always be held in reserve in the event of a breakdown of the main method. Both oil and gas give off water vapour which significantly increases humidity unless the greenhouse is well ventilated. If maximum temperatures are kept above 7c, this extra air moisture is not harmful and can, in fact, be beneficial as long as cacti and succulents are not the main plants being grown.
Electricity has now come into its own for powering various items of greenhouse equipment including heating. Used in tubular heaters, it provides warmth similar in coverage to that of hot water pipes. Although the most expensive way of heating the greenhouse, with the aid of a thermostat it gives instant heat at the turn of a dial or touch of a switch and will run for months without attention. The vigorous air movement also prevents stagnation within the house and dries up any surplus water quickly. This is precious because the temperature is being maintained at or below 7c and definitely reduces the incidence of grey mould.
Heating costs can be reduced if the greenhouse is a lean-to or a sunroom with access to the house, or is linked for heating to the household system. By merely leaving the door into the room open, warmth will flow in, and frost can be kept at bay expect during severe spells when heat is lost from the house will be excessive.
Heating conditions have changed much during the last 50 years and continue to do so. Many of the larger commercial greenhouses are heated with warm air ducted, sometimes with the aid of fans, to all parts from a central heater, using polyethene sleeve. So far, though, this system has not proved easy to adapt for the smaller structure. In the USA, solar heaters which store and use the sun's heat are finding favour for heating both greenhouses and dwelling houses, and already have a stronghold in Britain.
Methods and designs of solar heating vary. Those in current use or in the developing state have heat collectors on the greenhouse roof and use water or air and stones to absorb and store the sun's warmth. Designs using water require a grid or series of black metal pipes or hollow plates. Water moves through the plates or pipes are warmed and passes into the very efficiently insulated tank. From the reservoir, water can then be circulated to the greenhouse as required. Methods using air work from a heat collector rather like a large, flat, glass box. Air enters the bottom of the box (usually assisted by a blower), is heated by the sun and passes into an insulated container full of stones. The stones absorb and hold the heat and are surrounded by the warm air which can be drawn off to heat the greenhouse. It is recommended that a supplementary, traditional heat source is coupled to both air and water systems in the event of long sunless periods or extra-cold- spells. Also, insulation ideally in the form of blinds which can cover the roof and walls at night or during cold spells will help to conserve heat. Naturally enough, these solar heating methods require a climate with a relatively high sunshine average in winter to make their installation worthwhile.
This discussion of heating has been so far entirely concerned with artificially raising the air temperature to a level essential for warmth demanding plants. Where for one reason or another, this sort of heating is not contemplated, electric soil or bench warming is beneficial. This type of warming is done with cables of known resistance either buried in ground level borders or placed in deep benches. The wires are arranged in flat loops 10-15cm apart. Those on benches are covered with an inch or two of stone, washed sand or grit kept moist. Those in the soil are buried 20-25cm deep. The cables have different resistances and electrical loadings for benches or soil and when purchasing makes sure that they are right for the purpose intended. In an otherwise unheated greenhouse, soil or bench warming not only prevents freezing of the roots media during prolonged cold spells but stimulates root growth at all levels. Indeed, it is invaluable in a propagating case or bench to aid the rooting of cuttings. It is essential under a mist propagating bench. A temperature of 18-21c is normal for such warming cables, but under a mist unit, it must be 24c as the rooting medium is continually wet.
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