Agriculture Reference
In-Depth Information
temperatures varying over intervals from
15 to 20°C during the night and between
22 and 28°C during the day (Tesi, 2001).
Table 7.3 summarizes optimum temperature
intervals for the air and the substrate in dif-
ferent horticultural crops.
In sophisticated greenhouses, for opti-
mum fruit quality, with well-developed
crops, it is recommended, as a general rule,
that the day/night temperature thresholds
for heating, should be 19°C/17°C for tomato,
22°C/18°C for pepper, 21°C/19°C for cucum-
ber and aubergine, whereas on less demand-
ing crops, such as lettuce, temperatures
should be limited to 12°C/6°C (Urban,
1997a). These values can be slightly
decreased under low radiation conditions
(Kamp and Timmerman, 1996). In any case,
economic criteria must prevail when defin-
ing these thresholds for each specific case. In
low technology greenhouses these tempera-
ture thresholds are usually lower, as the
equipment and insulation levels are simpler.
During the winter, the management of
set temperatures in Mediterranean green-
houses at 22°C (day)/18°C (night) in the
beginning of the cycle, to continue at
20°C/16°C during the vegetative growth
stage, and later at 18°C/14°C during the pro-
ductive stage (Lorenzo et al ., 1997a) of a
cucumber or a green bean crop induces high
energy use, adversely affecting the eco-
nomic viability of heating.
When using air heating in low-cost
greenhouses that are not very airtight
(Table 7.1), some authors recommend estab-
lishing a low set point temperature for heat-
ing. For instance, in a cucumber or green
bean crop set points between 12 and 14°C
are recommended, to limit fuel consump-
tion (propane) below 5 kg m −2 (from
November until the middle of March) (López
et al ., 2000). Obviously, for each specific
case, the economic conditions must deter-
mine the management of the heating sys-
tem, with the aim of maintaining heating
expenses which can lead to profitable
results (Plate 13); energy consumption esti-
mates in Mediterranean greenhouses grow
exponentially with the set temperature
(López, 2003; López et al ., 2003a, b).
The combined use of heating and CO 2
enrichment enhances the effect of both,
allowing for yield increases (López et al .,
2000; Sánchez-Guerrero et al ., 2001), the
profitability of which depends on the spe-
cific conditions of use.
When using heating in Mediterranean
low-cost greenhouses (parral-type), it seems
necessary to improve how airtight they are,
in order to limit the thermal losses (López
et al ., 2000), with the subsequent increase
of the CO 2 fertilization efficiency.
In Mediterranean low technology
greenhouses, the use of heating is usually
limited to the winter months, in crops
whose price is higher at this time of year,
such as cucumber or green bean. Table 7.4
summarizes the results of a study of air
heating in a climbing bean crop and dem-
onstrates the interest, from an economic
point of view, of maintaining low tempera-
ture set points when they do not limit the
yield.
The use of fixed energy-saving screens
allows for an increase in night temperatures
but they decrease radiation so their use is of
no interest (Castilla, 1994; López et al .,
2003a, b). However, some growers do use
them, in order to limit the fall of water drop-
lets (from condensed water vapour or from
the rain in artisan low-cost greenhouses)
over the crop. Mobile thermal screens
improve the yield and adapt well to multi-
tunnel greenhouses, being of more interest
Table 7.3. Optimum air and substrate thermal
levels for different horticultural crops. (Source:
Tesi, 2001.)
Air (°C)
Crop
Day
Night
Substrate (°C)
Tomato
22-26
13-16
15-20
Cucumber
24-28
18-20
20-21
Melon
24-30
18-21
20-22
Green bean
21-28
16-18
15-20
Pepper
22-28
16-18
15-20
Aubergine
22-26
15-18
15-20
Lettuce
15-18
10-12
10-12
Strawberry
18-22
10-13
12-15
Carnation
18-21
10-12
10-15
Rose
20-25
14-16
15-18
Gerbera
20-24
13-15
18-20
Gladiolus
16-20
10-12
10-15
 
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