Agriculture Reference
In-Depth Information
4
4.1
Introduction
caused by the cladding material in
exchanges between the soil, the substrate
and the canopy with the surroundings
(Villele, 1983).
The new regulation UNE-EN-13031-1
(
Greenhouses: design and construction
; the
Spanish version of the European regulation
EN-13031-1) defines the greenhouse as a
structure used for growing and/or to pro-
vide plant and yield protection, optimizing
the solar radiation transmission under con-
trolled conditions, to improve the crop
environment and whose dimensions allow
people to work inside.
The development of plastic materials has
been one of the most decisive factors in the
great expansion of greenhouse industries
around the world. During the second half of
the 20th century, the lower cost of plastic-
covered greenhouses in relation to tradi-
tional glasshouses, promoted their use
without heating in many mild climate areas,
but advanced designs also became common-
place in northern colder countries, because
of the significant savings in energy use.
The light weight of plastic materials
compared with glass have allowed for a
notable reduction in the supporting struc-
tures for the cladding materials and, as a
consequence, of the building cost.
It is difficult to define the distinction
between high tunnels and greenhouses, but
in this topic high tunnels (i.e. those struc-
tures that are high enough to allow workers
to walk inside them and work on the crop)
are included within the term 'greenhouse'.
However, facilities which do not use sun-
light as the main source of radiation (e.g.
growth chambers, phytotrons, etc.) are not
included in the term.
A greenhouse is a structure which
allows for the delimitation of a crop com-
partment, in which the climate differs
from that outdoors, due to modifications
4.2
Evolution of the Greenhouse
Concept
In greenhouses, the main function of increas-
ing the temperatures in relation to the open
field, as a consequence of the 'greenhouse
effect', was previously the main considera-
tion. However, this is not always the main
consideration now, although it remains the
most important factor during short periods
of low outside temperatures, as is the case in
many Mediterranean greenhouses. Indeed,
in some regions what is more important is
the 'shading effect' in the season of high
radiation, or the 'windbreak effect', at least
during some periods of the year.
