Agriculture & Future Food Systems · Article

Greenhouses, tunnels and season extension

Understand how a cover changes heat, water and air movement, and choose shelter that fits the crop and the available operating capacity.

Preparation and related reading

Shared Foundations

This article helps you

Distinguish passive shelter from actively controlled environments.
Connect temperature, humidity and ventilation choices.
Evaluate season extension using observed production and inputs.

A sheltered morning

On a cool morning, seedlings beneath a tunnel cover may be warmer than an adjacent outdoor batch. By afternoon the same enclosure can become too hot. Shelter has changed the environment; it has not made the crop independent of weather or removed the need to observe it.

The growing season is the period in which conditions permit the intended crop and harvest. A cover can extend useful production by modifying exposure. Whether it does so depends on the crop, stage, weather and how the structure is operated.

What the cover changes

A conventional high tunnel is a walk-in covered structure commonly using solar heat and natural ventilation. A greenhouse can include heating, fans, shading, cooling and other controls. Names overlap in practice, so describe the actual cover and operating systems instead of assuming a label guarantees precise conditions.

A cover admits some sunlight and changes heat exchange and air movement. Soil and other materials can store heat during the day and release it later. Heat is still lost to colder surroundings. An unheated shelter therefore cannot promise a fixed night temperature or frost protection under every condition. Its material, condition and local exposure matter.

Temperature and moisture together

Plants transpire, and wet surfaces evaporate water into the enclosure. Relative humidity is the water-vapour pressure expressed as a fraction of the saturation vapour pressure at the air temperature. Temperature changes can change relative humidity without removing water vapour.

When a leaf or cover surface cools to the dew point or below, water can condense on it. Persistent leaf wetness can favour some diseases. A room-air sensor may not describe the coolest leaf or the humidity within a dense canopy. Record relevant places and times, particularly across day–night changes.

Exchange air with a purpose

Ventilation exchanges inside and outside air. Internal circulation moves air within the enclosure. A circulating fan does not, by itself, remove water from the structure. The diagram separates these operations. The moisture-removal effect of ventilation depends on incoming and outgoing water vapour and airflow, not merely on comparing their relative-humidity percentages.

Opening vents may remove excess heat but also lose heat needed during cold weather. Humidity control can therefore interact with heating. Use a crop-specific operating plan and observe the response. Structural loads, vent sizing and electrical or combustion systems require suitable engineering; the growing decision here is what environment must be maintained.

Movement inside versus exchange outside

Conceptual pathways, not airflow magnitudes or a complete heat balance. Ventilation performance depends on inside/outside conditions.

Read the diagram as text

Crop water vapour enters enclosed air; circulation redistributes that air while ventilation exchanges it with the surroundings.

Parts

Crop and wet surfaces: Transpiration and evaporation add vapour.
Enclosed air: Temperature and vapour affect conditions.
Internal circulation: Redistributes air without an export guarantee.
Outside air exchange: Ventilation can export heat and water vapour.

Connections

Crop and wet surfaces → Enclosed air: water vapour
Enclosed air → Internal circulation: air movement
Enclosed air → Outside air exchange: ventilation

Water and crop care still matter

Rainfall outside a closed roof does not directly irrigate the bed underneath. Provide and monitor a suitable water supply and drainage. Crop demand changes with radiation, temperature, growth and root conditions. Shelter cannot make an unsuitable irrigation schedule correct.

Retain pest scouting and hygiene. A barrier may reduce some exposure while its warm, crowded environment creates other opportunities for pests or disease. Check access for care, harvest and repairs. A more elaborate enclosure also needs materials, skills, maintenance and energy that must be included in a practical comparison.

Test season extension

Define the intended product, quality and harvest window before comparing sheltered and outdoor production. Record actual marketable output and losses, not just the presence of living plants. Include labour, water, heating, cover replacement and other relevant costs.

For a useful local comparison, keep planting identity, dates and management identifiable. A sheltered crop harvested earlier is an observed timing difference; one comparison does not isolate every cause or establish that year-round production is feasible. Use repeated seasonal experience to revise the crop and operating plan.

Worked examples

A night heat account

In a hypothetical one-hour interval, heat input to an enclosure is 3 kWh and total outward heat transfer is 5 kWh. Assume these are all heat transfers across the chosen boundary and no other energy changes. What is the change in stored energy?

Use input minus output: 3−5=−2 kWh.
The enclosure’s accounted stored energy decreases by 2 kWh.
Do not convert this to a temperature drop without the relevant thermal properties and distribution.

Stored energy decreases by 2 kWh under the assumptions. This is not an actual greenhouse heating requirement or a frost-survival prediction.

An observation that distinguishes fans

An enclosure has a fan moving air between benches but all external vents are closed. Has that fan established that moisture leaves the enclosure?

Identify the boundary: the enclosure and its air.
The fan redistributes internal air; it does not demonstrate outside exchange.
Check ventilation or another water-removal pathway and actual conditions.

No. Internal circulation and moisture export are different operations.

Check your understanding

It rained heavily outside a closed tunnel. Must the rooted bed inside be adequately watered?
Show answer and reasoning
No.
The cover changes the rain pathway. Observe the bed and its actual water supply.
Why can falling air temperature increase condensation risk without additional watering?
Show answer and reasoning
Cooling reduces saturation vapour pressure; a surface may reach the dew point.
Relative humidity and surface temperature matter together.
Does one earlier harvest prove that shelter is cheaper overall?
Show answer and reasoning
No.
Costs, output quality, losses, labour and other included inputs must also be compared.

Key ideas

Describe the actual shelter and controls.
Observe heat, moisture and crop conditions together.
Season extension has operating costs and crop-specific limits.

References

Sources supporting the explanations and examples in this article.