Collection: Thermometer

Why Measure the Temperature

A greenhouse thermometer records what the space actually does rather than what you assume. That distinction matters because the two extremes that damage plants both happen when nobody is watching: the peak in the middle of a working day, and the low in the hours before dawn.

Every other climate decision follows from those two numbers. Whether you need ventilation, shading, insulation or heating is answered by the readings, not by guesswork, and a thermometer costs a fraction of any of them.

Maximum and Minimum Readings

A max and min thermometer holds the highest and lowest temperature reached since it was last reset. Check it in the morning and you know both what last night dropped to and what yesterday climbed to.

• A high peak points to ventilation or shading.
• A low overnight minimum points to insulation or heating.
• A wide daily swing stresses plants even when neither extreme is reached.

Reset it each day for a fortnight before deciding what to buy. A single reading tells you very little; a pattern tells you what the house needs.

Digital or Spirit

Traditional spirit thermometers use a marker within the tube and are reset with a magnet. They need no power, never fail and are accurate enough for any greenhouse purpose.

Digital models are easier to read, often include humidity, and some record over longer periods or send readings to a phone. They depend on a battery, which is worth remembering in a cold greenhouse where batteries fail sooner.

Where to Put It

Placement changes the reading more than the instrument does. A thermometer in direct sun reads the sun, not the air, and will show a figure well above what the plants experience.

Hang it in shade at plant height, away from the door and away from a heater. Head height at the ridge reads warmer than the bench; the bench is where the plants are.

Acting on the Readings

Once you know the peak, an automatic vent opener handles it without you being there, and shading cuts the input before it becomes heat.

Once you know the overnight low, the temperature control range covers what holds it where you need it.