Gardening with Greenhouses
Aaron Stierle
Solitude Springs Farm & Vineyard
Spring 2013
Greenhouses and Concept
• Transparent to translucent structures constructed
to trap heat and moisture
• “Greenhouse effect” - where the short
wavelengths of visible light from the sun pass
through a transparent medium and are absorbed,
but the longer wavelengths of the infrared reradiation from the heated objects are unable to
pass through that medium
• A major part of the efficiency of the heating of an
actual greenhouse is the trapping of the air so
that the energy is not lost through convection
Greenhouse Effect
Benefits
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Longer growing season
Warmer temperatures for heat-loving plants
Earlier harvests
Can grow greater variety of plants
Reduced water consumption
Reduced (in some cases) pesticide use
Disadvantages
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Can overheat – require ventilation
Excessive moisture can lead to fungal diseases
Require water during rainy season
Require annual and seasonal maintenance
– Annual maintenance: repair/replace transparent
cover, structural preservative treatment, cleaning
– Seasonal maintenance: winter snow removal, dust
removal during dry season, etc.
• Voles can be a problem
Construction Ideas
• Many options are available
– Frame: wood, metal, PVC pipe
– Shape: Gothic, peak roof, lean-to, high tunnel, low
tunnel
– Glazing: glass, rigid plastic panels, clear plastic
sheeting (Visqueen).
– Size: small (6’ x8’) to large (48’ x 96’)
Examples
Fig 1. Gothic, polycarbonate
Fig 4. Hoop house/high tunnel
Fig. 2 Peak, Visqueen, wood
Fig 5. Glass, wood
Fig. 3 Glass, metal
Fig 6. Lean-to, wood, polyc.
Foundation
• Permanent structures benefit from a
foundation
• Simple hoop houses (low tunnels) use 2’ or 3’
rebar pounded into the ground and pressuretreated wood to help hold the rebar in place
• Anchored cinderblock walls can also serve as a
foundation, but frost heaving on permafrost
can disrupt them.
Foundation/Floor
• “Floating” concrete pad – concrete pad
poured on gravel bed
• Typar/geotextile floor – use durable plastic
fabric tacked down with ground staples to
prevent weeds and form a floor to walk on,
self-draining
• Elevated wood floor – use pressure-treated
plywood over floor joists and posts
• Provide for drainage!
Framing
• Wood:
– Use pressure-treated wood to prevent rot from
moisture condensation. Do NOT use CCA treated
wood (not marketed since 2003)
– Readily available and easy to cut into shapes
– Relatively lightweight, but expensive
– Use appropriate dimensions to support snowload,
can intercept a significant portion of sunlight
Framing
• Metal:
– Strong and durable
– Needs proper treatment and maintenance to
prevent rust if aluminum is not used
– Can use old military-style double-hung aluminum
windows as walls of greenhouse
– Use caution if covering metal frames with plastic
sheeting to prevent tears or snags
Framing
• Plastic
– PVC pipe is readily available, relatively strong, rot
resistant, and easy to work with
– Typical cross-section is a half circle
– Maximum size is limited by strength of PVC pipe
– May require removal of the cover during the
winter.
– End wall construction can be complicated
– Can break during very cold weather if hit
Shape
• Gothic arch:
– Sheds snow, maximizes height
– Can be difficult to build end walls
– Requires bending glazing panels
• Peak roof:
– Sheds snow if roof pitch is 45 degrees
– Best if supported under peak, reduces accessibility
• Lean-to:
– Attached to a structure
Shape
• High tunnel:
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Rounded top (with or without sidewalls)
Greater vertical space available
Prone to damage from wind and/or snow; UAF
Most successful in snowy/windy climates if cover
removed during winter
– Maintenance and ventilation can be difficult due to
excessive height (often 12 ft high or greater) and
sheer volume of interior
– Costly to buy, ship, and erect; winter removal
– Popular with growers of specialty crops
Shape
• High tunnel (con’t):
– Some have elaborate mechanisms to roll up
sidewalls to assist with ventilation
– Hoop houses are low-cost version that use PVC
pipe and polyethylene plastic sheeting
• maximum height only in center and decreases rapidly
toward edges
• Moderately resistant to wind damage during growing
season; hail/heavy rain can be a problem
Shape
• Low tunnel:
– Cheapest, easiest to install; occasional cover
replacement
– Mini hoop-house design – half circle arch;
maximum height only in center and decreases
rapidly toward edges
– Resistant to wind damage during growing season;
hail/heavy rain can be a problem
– Usually 3-4 ft high and wide; no end walls or
modifications for ventilation
– Cover a single row or a raised bed
Glazing
• Glass:
– Long-lasting and no need to replace unless broken
– Durable, unaffected by mold or mildew
– Can have very high light transmission rates
– Very expensive, unless using old windows
Glazing
• Plastic greenhouse panels (polycarbonate)
– Moderately long-lasting; contain UV inhibitors
– Precut to standard sizes
– Replacement is usually not difficult
– Expensive ($1.40/ sq. ft), but typically more
durable than glass
– Double layer “corrugated” designs add insulating
quality
– Vary in amount of light transmitted
Glazing
• Visqueen (clear polyethylene plastic sheeting)
– Relatively cheap (5¢/sq. ft.), readily available in a
variety of sizes and thicknesses
– Lightweight and easy to work with
– Varies in light transmissivity, but generally lower
than glass and polycarbonates
– Least durable, lowest insulating value, tends to
billow in wind
– Rapidly degrades (<3 yrs) in sunlight
Size
• Smaller sizes are excellent for backyard
gardener
• Larger sizes used by commercial growers
• Bigger isn’t always better
– Ventilation requirements increase with size
– More work to keep clean and organized
– Repair/replacement costs increase with size
• But, sometimes . . . Greater thermal mass
Size
• Considerations:
– What do you want to grow? Compact vs. vining
vs. tall plants
– What are the heat requirements of your choices
– Rectangular shapes tend to generate more heat
than square shapes of same floor space when
oriented E-W.
– Is ground sloping or level? Erecting on sloping
surfaces may void greenhouse warranty
Size
• Considerations (con’t):
– What will it shade when erected?
– Can all gardening implements be stored inside for
the winter? (Dual purpose)
– Will it need to be portable/moved
– Others?
Drawbacks
• Excess moisture
– Air movement is key to avoiding moisture problems
– Ventilation reduces humidity, but also reduces
temperature
– Portable fans are useful, automatic ventilation
mechanisms
– Mold and mildew grow rapidly in wet, sunny locations
– Some plants (e.g., tomatoes) prone to fungal diseases
if foliage remains wet
Drawbacks
• Reverse greenhouse effect:
– Colder temperatures inside the greenhouse than
outside.
– Occurs during spring and fall when clear skies
allow heat to radiate out of the greenhouse, while
windy conditions maintain warmer temperatures
outside the greenhouse
– Occasionally can cause frost damage
– Opening greenhouse can mitigate; requires
vigilance
Drawbacks
• Insect infestations:
– Greenhouses often shield crops from insects and
reduce the amount of pesticides used
– However, introducing infested plants into the
warm, moist, and crowded structure can spell
disaster if not treated immediately; close
proximity of plants with similar resistance to
disease and insects allows them to spread fast
Benefits
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Growing varieties not suited to our climate
Extending the growing season
Reduced water and pesticide use
Reduced cost of food by growing it yourself
And it is Locally Grown!
And then there was plenty . . .