A min/max thermometer is the most useful starting point for winter greenhouse checks. It records how cold the greenhouse became overnight, even if you do not get outside until the sun has warmed it up.
Choose a simple digital min/max thermometer if you can place the whole unit safely beside the plants and read it each day. A thermometer with a wired probe is more convenient when the sensing point and display need to be in different places. If you need a warning while you are away, look for a system with genuinely remote alerts.
Choose by how you will check the temperature
| What you need | Starting choice |
|---|---|
| Read the overnight minimum during a daily visit | Digital max/min LCD thermometer |
| Measure at plant height while keeping the display elsewhere | Digital max/min thermometer with a wired probe |
Both choices record temperature extremes. The wired option gives you more freedom over where the sensor sits, but its lead limits the separation. Neither of these is a phone-connected monitoring system.
You can also compare greenhouse min/max thermometers on Amazon. Look for clear minimum and maximum readings, an easy reset and instructions that explain where the instrument can safely be used.
Best for daily visits: a simple min/max display
Gardening Naturally’s Digital Max Min Thermometer LCD Display is a straightforward choice for a greenhouse you visit regularly. It shows the current temperature and records the highest and lowest readings, so a daily check can tell you how cold it became overnight.
It uses an AA battery and has a hanging slot. Put it somewhere you can read and reset it without moving pots. The unit is not waterproof: choose a sheltered, shaded position away from dripping condensation and splashes from watering.
This arrangement suits a small greenhouse where the display can sit close to the plants. If the only convenient mounting point is a sunny wall or a damp corner, a separate probe gives you more options.
View the digital max/min LCD thermometer at Gardening Naturally.
Best when the sensor needs its own position: a wired probe
The Digital Max Min Thermometer Remote Probe has an internal sensor and an external probe on a 1m lead. That lets you keep the display in a convenient dry position while measuring the air closer to the plants.
Measure the route the cable must follow before buying. A metre disappears quickly when it has to go around a bench or door frame. Avoid trapping it in a closing window or leaving it across the walkway.
The external probe is water-resistant. Keep the display itself protected, and check which sensor is selected when reading the minimum: the temperature beside the display may differ from the temperature beside the plants.
This model also has an audible alarm for set temperature limits. Treat it as a local warning; it will not send a message to your phone. For an isolated greenhouse you may never hear it from the house.
View the wired-probe max/min thermometer at Gardening Naturally.
Where to put a greenhouse thermometer
Measure the air around the plants you are trying to protect. Position the sensor at their height, in shade, with air able to move around it. Keep it away from a heater’s direct output and off cold, wet glass.
A sensor warmed by sunshine can report a reassuring daytime temperature while the shaded plants remain much cooler. Similarly, one above the staging tells you little about a pot sitting on the floor. If you have plants at very different heights, compare those positions with a second instrument or move a spare sensor between them over several checks.
If you are adding a lining or screening off a smaller area, the greenhouse insulation guide explains the material and fitting choices. Place the sensor inside the area you are protecting. Keep any heater-control sensor positioned according to the heater’s instructions, then use a separate thermometer to check the conditions the plants actually receive.
Read, record, reset: a useful winter routine
Check the minimum and maximum at roughly the same time each day. Note anything unusual, such as a door left open or a sunny spell, then reset the memory for the next period. Without a reset, an old cold night can remain on the display long after the weather has changed.
Check the instrument’s reset behaviour when setting it up. Some models retain extremes until you clear them; others use a rolling or automatically reset period. A minimum without a known recording period is difficult to interpret.
Read the accuracy specification as well as the number of decimal places. A display that changes in 0.1°C steps is not necessarily accurate to 0.1°C. Avoid setting a plant-protection decision right on a critical threshold: instrument error and colder pockets elsewhere in the greenhouse can both matter.
Choose the minimum temperature around the plants you keep. A plant that needs warmth through winter requires a different arrangement from one that only needs frost protection. Replace weak batteries before relying on the readings, and check the instrument again after moving it or changing the lining.
When remote monitoring or a compost probe is worth adding
If you cannot visit regularly, a connected sensor may be worth the extra setup. Before buying, establish whether you need a separate hub, whether the greenhouse has a reliable connection and whether alerts reach your phone when you are away from home. A Bluetooth-only display can be useful nearby without providing that away-from-home warning.
Check the sensor’s permitted temperature and humidity conditions, battery arrangements, alert settings and any ongoing service charges. Try the alert function while you can still go outside to investigate. For valuable tender plants, an alarm is most useful when someone can act on it.
For seed germination, measure the compost with an appropriate probe as well as checking greenhouse air. Warm air above a tray does not tell you the temperature around the seeds. The heated-propagator guide explains the differences between fixed-heat and thermostatically controlled setups.
For further positioning and heating advice, see the RHS greenhouse-heating guide.
