Greenhouses

Each stage has a check. Do not move on until it passes.

A greenhouse traps sunlight as heat and holds the air still. Everything else, the vents, fans, heaters and screens, exists to stop it being too hot, too cold, too wet or too dark for the crop. It earns its cost in four ways:

• Transplants and seedlings, raised under cover in the weeks before the field season.
• Season extension: the first and last crops of the year, when prices are highest.
• Warm-season fruit in a cool climate: tomato, cucumber, pepper, melon.
• Winter salad and greens, which need little or no heat.

The gain is large and so is the running cost:

Choose the cheapest house that keeps the crop within its range in your climate.

• Sun: no shadow on the house at noon on the shortest day. Every hour of winter light is yield.
• Orientation: a single house with its long axis east–west gets more winter light, about 13 % more than north–south with a 30° roof at 37° N; run the crop rows north–south so they are lit evenly (FAO). For a tunnel used mainly in summer, north–south spreads the light more evenly across the floor (University of Nebraska).
• Wind: shelter from the prevailing wind, but put roof vents across it; the narrow end of a tunnel faces into it.
• Ground: a pad raised slightly above the ground around it, with a 1–2 % fall; keep drains 45–60 cm clear of ground posts so the posts keep their grip (University of Vermont).
• Width: no wider than about 50 m if cooled by its vents alone, with space between houses for hot air to escape (FAO).
• Services: water for irrigation, power for fans and controls, and room for a standby generator.

Every greenhouse, from a tunnel to a glasshouse, is made of the same parts. Each one does one job:

• Footings or ground posts hold the house down against wind; kneewall and baseboard close the bottom edge.
• Columns, trusses or bows, and purlins carry the cover and the snow.
• Gutters drain the roof and join spans; glazing bars or a fastening channel hold the cover.
• Cover: glass, film or polycarbonate; its light, heat loss and life decide the rest (next section).
• Vents at the ridge and sides, with insect nets; exhaust and circulation fans; a wet pad in hot, dry climates.
• Screens drawn across at gutter height: shade by day, a blanket of still air at night.
• Heating: pipes at the crop, or unit heaters, each with a flue to the outside.
• Growing system: beds or slabs, drip lines, high wires for vine crops.
• Control: an aspirated sensor box at plant height, out of direct sun (FAO), a controller, alarms and a standby generator.

Choose by the light it lets in, the heat it lets out, and how long it lasts. Light transmission and life are from the University of Arkansas, heat-loss coefficients (U, the watts lost per m² of cover for each degree of difference) from FAO after ASAE; U rises with wind.

• Roof slope: at least 25–30° for winter light (FAO); 6:12 (26.6°) or a pointed gothic arch also sheds snow (University of Vermont).
• Tunnels: steel bows 1.2 m apart on ground posts driven at least 60 cm, longer in sand; purlins on the underside of the bows; a baseboard 10–15 cm high; a hip-board 1.2–1.8 m up carries the roof film and the roll-up side (Noble Foundation).
• Bracing: a diagonal brace from each end bow to the baseboard at every corner (University of Nebraska), and end walls tied to as many bows as possible.
• Wind: keep doors shut and roll-up sides tight in a storm; strap over every other bow to cables through eye-bolts at the ground posts (University of Vermont).
• Snow: heat an occupied house so snow melts as it falls; on an unheated one, clear it early and evenly from both sides, and prop the centre purlin at every other bow for the winter (Noble Foundation; University of Vermont).
• Life: FAO advises replacing a greenhouse after 15–20 years.

Vents first, fans second. Natural ventilation costs almost nothing to run; forced ventilation of a Mediterranean greenhouse takes about 70,000 kWh of electricity per hectare, by FAO's estimate.

• Vent area: 15–30 % of the floor, at the ridge, the sides and the gables; beyond 30 % adds little (FAO).
• Vent type: flaps facing the wind move the most air; roll-up roof vents move about a quarter as much per square metre.
• Insect nets cut the airflow by 40 % (anti-aphid) to 60–70 % (anti-thrips): add vent area, or fit pleated screens, which pass about 25 % more air than flat ones (FAO).
• Fans: at least one air change a minute in summer, holding the house about 4.4 °C (8 °F) above outside; at half that it runs about 8 °C above (University of Georgia). Put fans on the lee side, 8–10 m apart at most, with inlets opposite of at least 1.25 × the fan area and air slower than 0.5 m/s at the crop (FAO).

When vents and fans are not enough:

• Shade: white shade cloth of 30–50 %, which leaves more useful light than black, or a washable whitewash on the cover in the hottest months (University of Vermont).
• Fan and pad: fans draw outside air through a wet pad, which cools it by evaporation; it works best where the air is dry. About 1 m² of pad per 20–30 m² of floor, 100–200 mm thick, air through it at 0.75–1.5 m/s, fans at most 30–40 m from the pad, 120–150 m³ of air per m² of floor per hour. Wet the pad before the fans start and stop the fans before the water, or the pad clogs with salt (FAO). The modern greenhouse step has the plan drawing.
• Fog: high-pressure nozzles at about 40 bar, mounted as high as possible, with the vents kept 20 % open (FAO).

Size the heater for the design night: the average daily minimum of the coldest month, less 8 °C (University of Georgia).

Heat loss = U × A × ΔT: U from the cover table, A the whole area of cover, ΔT inside minus outside. Air leaking in adds 0.33 × V × air changes per hour × ΔT watts, V being the volume in m³; a new double-film house changes its air 0.5–1 times an hour, new glass 0.75–1, old glass in poor repair 2–4 (University of Georgia).

• 60–90 % relative humidity does little harm; long spells above 95 %, especially at night, let grey mould (Botrytis) spread (FAO).
• Dusk and dawn are when condensation forms. Bring the air down to 70–80 % as night falls by venting a little with the heat on, and keep the air moving with circulation fans.
• Anti-drip film turns droplets into a film of water that runs off.
• Cost: raising the humidity setpoint by 5 % cuts energy use by about 6 %, so do not dry the air more than the crop needs (FAO).

Light: the cover and frame take 30–50 % of the daylight (Virginia Tech). Measure the daily light integral (DLI), the total light a plant gets in a day, and add lamps where winter falls short of the crop's target.

Without soil, the solution is the whole root environment, and it can go wrong within hours (soilless growing). In return it saves water and fertilizer, and the crop starts free of soil-borne disease (FAO, 2013). Choose the system by the crop:

Exclude, monitor, then act, as on the farming guide. Insect nets keep most pests out, at a cost in ventilation. Yellow sticky cards just above the crop, near doors and vents, catch whitefly, aphids and fungus gnats; blue ones catch more thrips. Count them and graph the counts every week (FAO). Release natural enemies early, before numbers climb:

Most failures are missed checks. From FAO's heating checklist, the University of Vermont's storm guide and the Noble Foundation's construction guide: