Part
II
Planning and Construction |
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4 |
Site
Selection |
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Light |
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Good sunlight is needed, especially early in the season |
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Calculating
the angle of incidence (angle between the sun's rays and
line perpendicular to the surface at the point of incidence)
at different latitudes is prudent for winter production
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Angles
of incidence from 0° to 50° have minimal effect on
light transmittance |
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As
the angle of incidence increases above 50°, light transmittance
decreases rapidly |
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For
example, when the angle of incidence changes from 0° to
90° in 10° increments, the respective transmittance
of light through 3mm glass is 0.86, 0.86, 0.86, 0.85, 0.77,
0.65, 0.40, and 0 |
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Avoid shadows cast from trees, adjacent high tunnels or other
buildings |
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Trees
and structures on all sides of the high tunnel should ideally
be set back 2.5 times their height
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As
a rule of thumb, multiple single bay high tunnels in an
east-west orientation should be 20’ apart
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As
a rule of thumb, multiple single bay high tunnels in a north-south
orientation can be as close as 4' apart
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Locating
high tunnels north of major structures is an undesirable
siting
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| Windbreaks |
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•Trees,
shrubs, and fabricated material like wood lath can be used as
a barrier to moderate and/or redirect wind |
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Air pressure builds
up on the side toward the wind (windward side) and air moves
over the top and around the ends of the barrier
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Windbreaks
could consist of woody shrubs designed to be harvested for
fruit or flowers |
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•Four
characteristics of a windbreak affect the area protected by
it: Height, Density, Orientation, and Length |
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The
interaction between height and density of the windbreak
determines the degree of 1) wind speed reduction, and 2)
downwind area protected
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•Height
of the windbreak is the most important factor in determining
the amount of area protected downwind |
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Wind speed reductions
are measured on the side toward the wind (windward side) for
a distance of 2 to 5 times the height of the windbreak
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Wind speed reductions
are measured on the side away from the wind (leeward side)
for a distance up to 30 times the height of the windbreak
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•Density
of the windbreak is defined as the ratio of solid matter in
the barrier to total area of the barrier |
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Windbreak
density of about 50% is rule of thumb for best wind speed
reduction
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Very dense windbreaks
result in low pressure developing on their leeward side
which pulls downward air coming over their top, creating
turbulence and reducing protection downwind
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Less Dense windbreaks
allow more air to flow through them which moderates the
build up of low pressure and turbulence to increase protection
downwind; however wind speed reductions are not as great
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Density
of windbreaks consisting of deciduous trees is about 25-35%,
of conifers about 40-60% , and of multi-row conifers about
60-80%
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•Orientation
of windbreaks
should be at right angles to prevailing winds that occur during
the most critical periods of the production season
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Examples
of a critical periods are cold winter winds carrying snow
or during the spring or summer when ventilation is required
and strong prevailing winds may damage plants near the sides
of the high tunnels
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Windbreaks
on the north side of a high tunnel should be about 100 feet
away to keep snow drifts back from the structures |
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•Length
of a windbreak determines the total area receiving protection |
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Length
of a windbreak should be 10 times longer than it is high
which reduces the occurence of end-turbulence
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Gaps
in the length of windbreaks decrease their effectiveness
by creating funnels or lanes that concentrate wind
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| Water
Supply |
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•Irrigation
source is required because crops will not receive moisture from
rainfall |
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•Adequate
water quantity is necessary |
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Two
quarts of water can easily be applied to one square foot
in a single application
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•Good
water quality is essential for successful production
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Well
water is often desired; however, it may be high in alkalinity
which can lead to clogged drip irrigation lines and high pH
problems |
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Municipal
or rural water is often cost prohibitive and may contain
undesirable additives like Fluoride
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Pond
or river water may contain disease organisms and may require
filtration to remove silt and other particulate matter that
might clog irrigation systems |
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Methods of irrigation depend on goals of production |
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Drip
irrigation reduces foliar diseases
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Overhead
irrigation increases cooling effects
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•If
soluble fertilizer will be injected through irrigation lines,
consider compatibility of water supply, delivery system and
fertilizer injector |
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Protect water supply line and hydrant from freezing in
winter by burying below frost line, or by installing drains |
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| Soil
& Topography |
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•Though
it may be tempting to select an unproductive field site for
the high tunnel, good soil quality is important |
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•Soil
should be well drained |
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Good
drainage improves quality of the plants
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Problems
with water logging and flooding are avoided
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Soil
warming is improved
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•Amend
soil with compost, peat, or other organic materials to improve
drainage and quality |
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•The
high tunnel is most productive if built on a slightly elevated
pad that is three feet larger than the structure in all directions |
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Level
interior of the high tunnel with a slight slope to the exterior
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The
pad should be high enough to prevent water from flowing
into the high tunnel from a rainfall or melted snow, especially
if the structure is built in a low-lying area
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| Accessibility |
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•High
tunnel location should be accessible by well maintained roads
and convenient to the grower because daily maintenance is often
necessary |
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•High
tunnel structure should be convenient to water source, tool
storage, cooler, packing shed and possibly electrical and /
or fuel source to facilitate labor efficiency |
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