The Science Behind Polytunnels
Air
Carbon dioxide [CO2], is a tiny part of the air we breathe;
yet it is vital for the energy process. We suggest you open doors at both ends
of your polytunnel to make sure you get adequate ventilation.
Light
All coverings no matter how transparent affect the amount of light
entering a polytunnel. Polythene covering usually allows about 90 percent of light through, and far
less if the film is cloudy and old.
This cloudiness can be an advantage in the summer if high-density
light affects young fragile plants. Netting can be used as a shading material inside the polytunnel.
Nutrients
The plants take up nutrients when they are in a solution. Fertilisers
applied to a dry soil have no effect until rainwater or irrigation
system water washes the fertiliser down to the roots. It is
possible to control the nutrient supply more easily for plants
grown in a polytunnel. This means that plants are freer from
pest and disease, and more likely to be healthier.
Warmth
The polytunnel makes use of the greenhouse effect to trap heat energy.
Radiation from the sun is
trapped by the polythene covering, and absorbed by the inside materials;
pots, tools, structure etc.
As the night falls, and the outdoor temperature drops, so the inside
materials release the trapped
energy as radiation or heat. This helps to reduce frost damage.
Polythene covering on a cold night reaches the outside temperature,
so if frost protection is an
important requirement place a crop cover directly over the crops.
Bright frosty days could bring the
inside temperature up to 40 degrees, then below freezing at night.
Adequate ventilation helps keep
an even temperature.
Extension of the growing season is aided by the polytunnel being
a protection from cool winds,
and trapping daytime warmth. The soil and air will be enough inside
to support growth when it has
stopped outside.
Water
The only water to be in a polytunnel apart from condensation
is by irrigation. Water can be supplied at ground level and
is very effective by using the leaky hose method. For absolute
control of the moisture supplied to the plants an irrigation
system may be used.
|
