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Gardening with Greenhouses. Aaron Stierle Solitude Springs Farm & Vineyard Spring 2013. Greenhouses and Concept. Transparent to translucent structures constructed to trap heat and moisture
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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 re-radiation 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
Benefits • 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 • 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. 3 Glass, metal Fig 1. Gothic, polycarbonate Fig. 2 Peak, Visqueen, wood Fig 4. Hoop house/high tunnel Fig 6. Lean-to, wood, polyc. Fig 5. Glass, wood
Foundation • Permanent structures benefit from a foundation • Simple hoop houses (low tunnels) use 2’ or 3’ rebar pounded into the ground and pressure-treated 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: • 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 • 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!