Gardening with Greenhouses

Aaron StierleSolitude 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

Greenhouse Effect

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 1. Gothic, polycarbonate Fig. 2 Peak, Visqueen, wood Fig. 3 Glass, metal

Fig 4. Hoop house/high tunnel Fig 5. Glass, wood 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 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!

And then there was plenty . . .