Climate and water in the polytunnel, step by step
Interactive lesson: thermometer and humidity, openings at two heights, summer shade, morning watering, drip from a water butt and automation with and without power.
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The whole lesson as text
1. Climate: what you measure
A tunnel without ventilation is an oven, and at night a damp chamber.
- Two instruments — A max-min thermometer and a hygrometer, now sold together in one unit for roughly £10–20 ($12–25).
- In shade, in the middle — Hang them in the middle of the tunnel, at the height of the plant tops, in a white box with vents. In direct sun a thermometer reads 10 °C too high.
- Target temperatures — Tomatoes 20–26 °C by day, peppers and cucumbers 22–28 °C. Above 32 °C (90 °F) tomato flowers drop.
- Humidity early in the morning — Good is 60–85 %. If you see drops on the film and leaves in the morning, open up as soon as the sun is up, even for 15 minutes.
Why: On a sunny May day a closed tunnel reaches 40 °C (104 °F) within an hour, tomatoes drop their flowers and cucumbers wilt. At night the moisture collects as droplets on the film and leaves, and within a few days grey mould has arrived.
- Lettuce and spinach want 12–20 °C by day; above 24 °C they bolt.
- Below 50 % humidity red spider mite moves in: damp down the paths in the morning and shade.
2. Openings
Warm air rises and leaves at the top, cold air comes in low down.
- At least 20 % of the floor — A 5 × 10 m tunnel needs 10 m² of openings. Two side openings 0.9 m high and 10 m long give 18 m².
- Two heights — Roll-up sides low down along the whole length, and a vent at the top of the end wall for warm air to escape.
- Doors aren't enough — Doors at both ends give a through-draught, but they are only enough for a tunnel up to 6–8 m long.
- Insect mesh — It keeps out whitefly, thrips and tomato moths, but cuts airflow by a third or more. With mesh, the openings must be bigger.
Why: So openings at two heights ventilate best: low side openings and high openings at the ends or in the roof. Doors alone aren't enough except in a small tunnel.
- In hot regions 25–30 % of the floor area in openings is better.
- A small fan inside mixes the air and dries the leaves in spring and autumn.
3. Day, year and heat · Locked
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4. Water: how much and when · Locked
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5. Drip irrigation · Locked
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6. Automation · Locked
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