Drive posts 18 to 24 inches into the soil

Use heavier gauge metal pipe for the ribs

Add more than one purlin to the structure

Decrease the spacing between the hoops

Decreasing rib spacing from 5 feet to 3 feet intervals increases the amount of load that the structure can carry by 66%

Fasten plastic glazing tightly to the structure

A gothic arched roof sheds snow better than a flat quonset roof, increasing structural carrying capacity by 15%

The gothic arch can be achieved in high tunnel designs by bending the typical hoop shape with a pipe bender or bending jig

No sidewalls or short sidewalls are stronger than structures with higher sidewalls

Quality ranges from electrical conduit on the weakest end to 1.9 inch OD (outside diameter) thick steel or schedule 80 water pipe as strongest materials used

Schedule 40 (3/4" galvanized water pipe is commonly used but results in a 20% weaker structure compared to using Schedule 80 pipe)

Metal ribs make the structure very heavy and difficult to maneuver

This lowest cost material is one of the weakest

PVC is lightweight and easy to maneuver

High tunnels made from plastic hoops are vulnerable to wind uplift and snow load collapse

PVC expands and contracts with temperature changes

Treated lumber should be used for baseboards and any other wood that will come in contact with the soil

•"Poly" is sold by thickness in mils (1 mil = 1/1000 of an inch) and rated for longevity in years

Typical specification for a high tunnel covering would be a single layer of greenhouse grade 6-mil poly rated for 5 years

Traditional greenhouses usually have two layers of poly, separated by air blown between the layers, to reduce heat loss during cold season production

Photosynthetic Active Radiation (PAR) is light that is utilized by the plant in photosynthesis and occurs in the 400-700nm wavelength range

Never purchase construction grade plastic because it does not contain a UV-inhibitor and will only last one growing season

Condensate will sheet and run down to the sides of the structure rather than bead and drop on foliage of plants below

Condensation is undesirable because it decreases light intensity

Polyethylene alone is a poor barrier to radiant energy loss, but IR treated poly can block IR heat loss by half, which is 15-25% of total heat loss at night

A single layer of IR-absorbing poly decreases PAR transmission to 82%

For example, installing a poly with photoselectively that blocks 280-320 nm UV light, which is essential for sporulation of Botrytis cineraria may prevent this disease

Shade cloth is used to reduce light intensity, temperature and plant exposure to wind

Shade cloth is often used in combination with plastic covering but in some applications such as cut flower production in Southern California, it is used as the sole covering for a high tunnel

White shade cloth may have a more profound cooling effect because it reflects more light than black and other darker colors of shade cloth

Seasonal and crop species requirements dictate what percent shade cloth would be chosen

For example, in spring little or no shade would be required for lettuce greens production: however, during the warmest summer months, 60% shade may be required to produce a quality product

•Secure attachment of the covering material is necessary to avoid tearing and excessive wear that shortens its longevity

However, this method may be the least secure, depending on the type of screws, spacing and installation technique

Wiggle wire is a continuously s-curving wire that is placed into the channel by "wiggling" it back and forth

These are the most secure and most expensive poly attachment systems

Some designs are reported to rip the plastic at the attachment site

•To increase ventilation in the summer, some high tunnel designs utilize completely removable end wall sections

If end walls are removable, sufficient structural support must be maintained

•Fixed side walls are applicable for winter production or in a climate where less ventilation is required

•Roll-up side walls offer a simple way to manage temperature and promote ventilation

Ventilation is accomplished by rolling up the sides and allowing warm air to dissipate to the outside

Temperature can be further manipulated by such actions as rolling up each side wall to different heights at different times of the day

There are many designs for hand-powered handles and cranks that can be inexpensively installed to roll up the length of the plastic wall

Purlin metal or fence toprail can be used for rolling up the length of the sidewalls