A greenhouse is a small climate you are paying to control. On a sunny spring afternoon the inside can be 20 degrees hotter than the garden, and on a clear night it can drop close to freezing even when the forecast says it will be fine. You find out about both when the plants look wrong. A sensor lets you find out when it happens.

What to measure

  • Air temperature. The most useful reading by a long way. Overheating and cold snaps are the two most common ways to lose seedlings.
  • Humidity. High humidity that stays high tends to encourage fungal problems such as grey mould, and very low humidity stresses plants and speeds up watering needs.
  • Soil moisture. Greenhouse beds and pots dry out quickly, and rain does not top them up. See our guide to monitoring soil moisture remotely.
  • Light level. Optional, but useful if you want to see how much sun reaches each bench.

If you have to start with one sensor, make it a combined temperature and humidity sensor. Many inexpensive Wi-Fi and Bluetooth models cover both.

Where to mount the sensor

  • At plant height, not at the ridge. Hot air rises, so a sensor near the roof reports a number your plants never feel.
  • Out of direct sun. A sensor in direct sunlight reads the temperature of its own case. Put it in the shade of a bench, or inside a small ventilated shield.
  • Away from vents, heaters and doors. You want the typical air, not the air next to a draft.
  • Away from the watering area. Spray on the sensor makes humidity readings jump and can damage cheap models.

For a large greenhouse, use two sensors, one at each end. The difference between them shows you where air is not moving.

Connecting it

If the sensor lives inside a Wi-Fi range, the connection is simple. Ecowitt and Bresser stations can send to a custom server, and Govee sensors are collected from the Govee cloud with an API key. The Works with page has the steps for each brand. A greenhouse far from the house is a good case for a DIY board with a long-range option, or for a Wi-Fi extender placed halfway.

In SensorConsole, add the sensor as a Temperature/humidity device, name it after its location, and check that readings arrive before you walk away from it.

Use the daily lows and highs

Switch the device page to the 30 days view. Each day shows its average as a solid line and its lowest and highest values as faint dashed lines. For a greenhouse these two faint lines are the important ones.

  • The lowest line is the coldest the greenhouse got each night. Many growers treat tomatoes, peppers and cucumbers as at risk when nights fall below about 10 C (50 F), and any frost-tender plant as at risk near 2 C (35 F). Check what your own plants need.
  • The highest line shows how hot it got during the day. If it climbs well above what your plants like on sunny days, you need more ventilation, shade cloth or an automatic vent opener.
  • The gap between the two is the daily swing. A large swing stresses plants even when the average looks right.

The 24 hour view shows the same thing at a finer scale, which helps when you want to know exactly when the vents should have opened.

Set alerts

In the Alerts box on the device page, add a threshold alert for the temperature reading, such as an email when it goes below 5 C (41 F) on a frosty night, and another for when it goes above the limit your plants tolerate. You can add up to five per device, and each one sends at most one email every six hours.

Also set an email for when the device goes silent. A greenhouse sensor that dies in March is easy to miss, and a cold night soon after is exactly when you needed it.

What a season of data gives you

Your history is kept, so after a full year you can see which weeks were risky, how much the shade cloth helped, and when to start seedlings next time. That record is hard to get any other way.