A greenhouse feels like one climate but rarely is: the corner by the door runs colder, the bench under the ridge vent dries out faster, and the zone in back holds heat overnight longer than anywhere else. This config puts a real number on one zone, cheaply enough that putting a few of them around the house is not a big decision.
The need.
What you want to know: what is my greenhouse actually doing, zone by zone?
Most climate decisions in a greenhouse (when to vent, when to run a heater, whether a zone is actually holding the target) are guesses until there is a number to check them against. A single cheap sensor turns "it feels dry over there" into "48% RH, and it has been for three hours," which is the difference between reacting to a feeling and reacting to a fact.
The solution.
When you are done, one zone reports its temperature and humidity continuously to a place you own. That is what this build delivers: the numbers, flowing. The graphs, the VPD calculation, and the "vent now" nudge come from your endpoint, Home Assistant or any endpoint you choose, working on the record this build feeds it. Nothing on this page vents, heats, or waters anything; it watches, which is the safe place to start and the layer this whole shelf builds.
The config: the parts.
| Part | Job | Cost |
|---|---|---|
| A BLE temperature and humidity sensor | Reads the zone. A Govee or similar Bluetooth sensor. | $10–20 |
| A small board (ESP32 family) | Runs the BLE Sensor Listener sketch; hears the sensor and pushes the reading on. | $5–10 |
| A USB power supply | Keeps the board running. A phone charger works. | $5–10 |
| A data USB cable | To flash the board and, if left plugged in, to power it. | $3–8 |
Same four items as the Cold-Chain Monitor config. The parts do not change between configs; the sensor placement and the endpoint's alert rules do.
The sketch and the build.
The same sketch runs this config: the BLE Sensor Listener. Flash it from your browser, mount the sensor at canopy height in the zone you care about (not on a wall, not in direct sun, see Your First Sensor for placement notes that apply here too), and point the listener at your endpoint.
Where the reading goes.
Same choice as every config here: your own server, a cloud service, or the live demonstration while testing. See Connectivity for the comparison. Once temperature and humidity are flowing to an endpoint that computes it, the same two numbers feed vapor pressure deficit, the number that actually predicts plant stress better than either reading alone; Air Temperature covers why the raw number matters on its own too.
Same need, other paths.
This is the one-zone, no-solder answer. Watching many zones with one board is its own build:
It lives in the Hardware Build Configs library; search the build's name there.
Variations.
More than one zone. One listener hears several nearby sensors at once; name each one by zone during setup and a single endpoint can show the whole house side by side.
Add CO2 or light. The same gateway pattern extends to other BLE-capable sensors; see the sensor catalog for what is available.
Graduate to Home Assistant. Nothing here blocks adding a hub later. Point the same sketch at Home Assistant instead of (or alongside) your own endpoint once you want dashboards and automations in one place; see Home Assistant.
Frequently asked questions.
What is the cheapest way to monitor greenhouse temperature and humidity?
A Bluetooth temperature and humidity sensor (around $10 to $20) and a few-dollar ESP32 board running the BLE Sensor Listener sketch, for roughly $25 to $50 total. The board hears the sensor and pushes the reading to an endpoint you choose, with no hub required to start.
Can I monitor more than one greenhouse zone with one gateway?
Yes. A single BLE Sensor Listener can hear several nearby Bluetooth sensors at once, so one gateway covers several zones as long as each sensor is within Bluetooth range (roughly 30 to 100 feet, less through greenhouse structure). Name each sensor by zone during setup.
Do I need Home Assistant to start monitoring a greenhouse?
No. This config pushes straight from the sensor to an endpoint you choose. Home Assistant is a natural place to graduate to later if you want dashboards and automations across many zones, but it is not required to get a working greenhouse monitor today.