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Archive for September, 2008

Greenhouse Glazing Materials

By admin On September 30, 2008 No Comments

Many materials other than glass have been used to create a clear covering for greenhouses. Indeed, the Romans some two thousand years ago use thin sheets of mica in frames to cover plants.

Sheets of polycarbonate, 3mm/1/8in thick are amazingly strong – thick sheets are said to be bulletproof! – and about one third of the weight of glass. But although tough and initially allowing about 85% of light to pass through, it deteriorates after about fifteen years and eventually its structure breaks down.

Acrylic is slightly cheaper but unfortunately also has the ageing problem of polycarbonate.

Hooped tunnels are covered with sheets of clear or opaque plastic, but they do not have the good warmth retention properties of glass and after a few years discolour and perish due to the action of ultraviolet light. In commercial horticulture, these structures are very popular and create inexpensive protection for plants.


The Greenhouse Effect

By admin On September 30, 2008 No Comments

Radiation from the sun contains infrared radiation as well as visible and ultra-violet radiation. When this radiation reaches a greenhouse, the glass reflects long wavelength infrared radiation, but allows short-wavelength infrared radiation as well as visible light to pass through. These are then absorbed by the soil and plants, thereby raising the temperature. Additionally, infrared radiation emitted by the soil and plants is of a longer wavelength and does not pass back out through the glass. Therefore it becomes trapped inside the greenhouse, thereby causing the temperature to rise.

If this continued, the temperature within a greenhouse would become unbearable. Therefore, provision must be made to open up ventilators to make the temperature suitable for the plants. Low external temperatures, and wind and rain are also responsible for decreasing temperatures within the greenhouse and make heating the structure very expensive.


Greenhouse Glass

By admin On September 30, 2008 No Comments

Although initially expensive during the early 1800’s, glass has proved to be the best covering for greenhouses, sunrooms and conservatories. The greatest impetus to its use in Britain was the abolition of the glass tax in the mid 1800’s and the construction of the glass-clad Exhibition Hall for the Great Exhibitions of 1851.

Early greenhouses had panes of glass 30 cm/12ins or less wide, but when glass-making techniques improved these became 45cm/1 1/2ft wide and are now frequently 60cm/2ft across. The use of wider panes was made possible by the introduction of extruded aluminium glazing bars, which are both stronger and lighter than wooden types.

Glass used in greenhouses must be free from bubbles, with a standard weight of 7.32kg/sq cm (24oz/sq ft). The total weight of glass in a greenhouse is considerable, giving the framework rigidity as well as weight to resist winds and storms.

When clean, only 85-80% of the available light passes through, but if dirty this decreases dramatically. In summer, this reduction is not a problem, as the glass will probably be covered in a shading material, but in winter and spring all available light is needed.


October Gardening

By admin On September 25, 2008 No Comments

By now autumn is beginning to make itself felt. In mild autumns the frosts may have not yet started but it is likely that there will have been a few, even if only light ones. This is your last opportunity to check that all plants that can be affected by frost have been lifted and stored or given some other form of protection.

You should, of course, always look after tools but at this time of the year, as the autumn rains start they may get muddier than usual. Because you may put them away and not use them again this year it is worth making doubly sure that they are clean and oiled to protect them from rusting. Do not just clean the metal parts, wipe down the handles as well. The same applies to any machines that you have been using.

In the fruit and vegetable gardens there is more produce to be harvested and stored. Storage can be a problem if you have not got a suitable outbuilding bulit of brick or stone. Garages and wooden sheds are suitable alternatives but they may get cold with the temperatures dropping below freezing in midwinter. If you can, line them withpolystyrene or use a thermostatically controlled heater to keep the temperature just above freezing. Another possibility is to use a defunct chest freezer. This will keep the frost out when the lid is closed but it should be closed only when the weather turns really cold. At other times it should be allowed to circulate freely around all stored produce.


September Gardening

By admin On September 25, 2008 No Comments

Autumn is a mellow time, colours change and the pace of garden annual cycle slows down. It is also a season of flavours, wonderful juicy blackberries come into season and apples and pears are coming into their own.

The lazy time is nearly over and it is time to start gardening again. The gardener must always be thinking ahead and planning. There is no such thing as instant gardening. Forethought and preparation are needed if you want to have a productive and attractive garden next year.

Now, as autumn approaches is the time to start tidying and getting the garden ready for winter. It is also time to start preparing the ground for the following year. This not only gives the soil time to weather but also allows any organic material to finish rotting down. Many plants, vegetables in particular, prefer to be planted into soil that was manured some months earlier rather than into ground that has only recently been treated.