You can’t manage a greenhouse climate you don’t measure. A max-min thermometer and a hygrometer (humidity meter) are the two cheapest, most useful tools in any greenhouse — they tell you how hot it got while you were out and whether humidity is in the danger zone for disease.
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Max-min thermometer
A max-min thermometer records the highest and lowest temperatures since you last reset it — so you learn how hot the greenhouse spiked at midday and how cold it dropped overnight, even when you weren’t there. That tells you whether you need more ventilation, shading or heating.
Hygrometer (humidity meter)
A hygrometer shows relative humidity. Persistently high humidity with poor airflow is what fungal diseases love; very low humidity stresses many plants. Many units combine thermometer and hygrometer in one, and digital versions store max-min for both.
Max-min thermometers
Thermometer-hygrometers
How to use them
Place the instrument at plant height, out of direct sun (which gives false-high readings). Check daily at first to learn your greenhouse’s patterns, then act: vent/shade if it overheats, add airflow if humidity stays high. See greenhouse temperature and humidity.
Features worth choosing
- Min/max memory — the essential feature; shows the overnight low and midday high you missed.
- Remote/wireless sensor — read the greenhouse from indoors; some log history.
- Combined humidity — a thermo-hygrometer covers both climate risks in one unit.
Where you place it matters
Put the sensor at plant height, shaded from direct sun — a sensor in sun reads far too high and misleads you. One badly-placed reading is worse than none.
When you need a thermostat instead
A thermometer measures; to act automatically (switch a heater or fan) you need a thermostat/controller — see greenhouse heaters and climate control.