You cannot be in the greenhouse at 2 a.m. when the heater quits — but a $12 sensor can, and it can text you before the crop is lost. Heat, humidity, and air are the conditions most likely to wreck a crop while your back is turned, and they are the cheapest to watch. For a lot of growers, this is where a first system pays for itself.
01The need.
Of everything that can go wrong on a farm, the air is the fastest. A heater fails on a cold night and a crop is gone by morning. A humid, still corner grows mold you do not see until it has spread. A cooler drifts a few degrees and a week of storage is lost. None of it announces itself, and all of it happens while you are doing something else — or asleep.
That is the bad news. The good news is that temperature, humidity, and air movement are also the cheapest things to measure and the first place a small system earns its keep. The 2 a.m. text that says the heater quit is, for a lot of growers, the whole reason to start.
02What's worth watching.
You do not need to measure everything. You need the handful of numbers that actually change a decision:
- Temperature, day and night — the first and most important.
- Humidity — paired with temperature, it drives mold and how the plant drinks.
- VPD — one number that combines temperature and humidity into "how thirsty is the air." More useful than either alone (more in a moment).
- Dew point — the temperature at which the air starts leaving water on your leaves and walls. Cross it and you invite mold.
- CO₂ — matters most in a sealed, enriched room; ordinary outdoor air is about 420 ppm.
- Air movement — still pockets are where trouble starts.
Match the tools to the need. A grower who just wants a frost warning needs a thermometer that texts, not a climate computer. A sealed indoor room running CO₂ needs more. Start with temperature and humidity; add the rest when a real question asks for it. That is appropriate technology applied to your own air.
03How it works.
Watching your environment is a Collect·Have·Use system like any other — three steps:
A temperature and humidity sensor in each area you care about, sending its readings somewhere you can see them.
Each reading stamped with which zone and when, from a sensor you trust — and yours to get back out.
A text when the heater quits; a chart that shows the humid nights before mold started.
The one idea worth learning is VPD. Plants drink through their leaves, and how fast they drink depends on temperature and humidity together, not either one alone. VPD — vapor pressure deficit — is the single number that captures that. Too low and the plant barely drinks and mold moves in; too high and it dries out and stresses. You do not have to do the math (the VPD tool does), but knowing the air your plant actually feels is temperature-and-humidity-together is most of the battle.
04Collect — what to put in.
Getting the readings in is the cheap part. Three appropriate paths, by what you need:
| The $12 start | Inexpensive | A temperature/humidity sensor on a small board, sending readings to a place you can see them. Catches the frost, the heat spike, the humid corner. Sketches and wiring live in the Build section. |
|---|---|---|
| The mid path | A modest tool | A couple of sensors per zone, dew point and VPD computed for you, a tidy dashboard, alerts you set yourself. |
| The commercial path | $$$ | Multi-zone sensing tied to the equipment that heats, cools, and dehumidifies — real money, real capability, when the operation needs it. |
Wherever you start, the readings are the same shape — so you can climb to the next path later without starting over, and without re-collecting a thing.
Tools for this stack
VPD Calculator · Dew Point · Air Exchange Rate · Greenhouse Cooling Load · HVAC Cost Estimator · Worker-Safety CO₂ · Mold (Botrytis) Pressure · Drying & Curing Schedule. Browse them all in the Library.
05Have — keep the readings.
A temperature reading is only worth keeping if it carries its context: which zone, when, and from a sensor you trust — placed out of direct sun, and checked against a known-good thermometer now and then. Keep those, and the readings stop being a live number and become a record: last winter's cold snaps, the run of humid nights that came right before last year's mold.
And keep them yours. As data is king puts it, ownership is not about where the readings are stored — on a box in the barn or on a service somewhere — it is about whether you can pull them out and take them with you. A season of air readings you own is worth more than the sensor that took them.
06Use — what it does for you.
This is where the value is, and it comes in two shapes:
- Act — the 2 a.m. heater text. A fan that kicks on when humidity climbs. A vent that opens before the afternoon heat. The alert that saves the crop.
- Make sense of it — a chart of the week, a dew-point warning before mold can start, this season next to last. The pattern you could not see by walking the rows.
A simple alert dashboard does the first and most of the second, cheaply — and the same readings can later feed something bigger without re-collecting. That is the payoff the sensor exists for: not the number on the screen, but the crop you did not lose and the trouble you caught early.
The shortest version
Watch the few numbers that wreck crops — temperature, humidity, VPD, dew point. Start with a $12 sensor that texts you, keep the readings (and keep them yours), and use them to act in time and to spot trouble early. The 2 a.m. alert is where it pays for itself.
Frequently asked questions.
The honest version.
What is VPD and why does it matter for growing?
VPD (vapor pressure deficit) is a single number that combines air temperature and humidity to describe how readily a plant can move water out through its leaves — essentially how thirsty the air is. It matters because that transpiration drives nutrient uptake and growth: too low, and the plant barely drinks while mold risk climbs; too high, and it stresses and closes up. Many growers aim for roughly 0.8–1.2 kPa in vegetative growth and a bit higher in flower, but it varies by crop and stage. You do not have to calculate it by hand — a VPD tool does — but it is the most useful single environment number indoors.
What temperature and humidity should a greenhouse be?
It depends on the crop and stage, but a common indoor range is about 70–80°F during the day, a few degrees cooler at night, with relative humidity around 55–65% — higher for seedlings and cuttings, lower as plants mature to limit mold. Leafy greens prefer cooler and a touch more humid; fruiting crops like it warmer. The pairing matters more than either number alone, which is exactly what VPD captures.
How can I get a text alert if my greenhouse gets too hot or too cold?
Put a temperature and humidity sensor in the space and have it send its readings to something that can text or email you when a number crosses a line you set — say, below 40°F or above 85°F. It can be a $12 sensor on a small board with a simple service, or a ready-made monitor. The three parts are a reliable sensor, a way to deliver the reading, and a threshold you choose. No AI needed — a simple scheduled check that watches the number does it.
What sensors do I need to monitor a greenhouse?
Start with one good temperature-and-humidity sensor per area you care about — that covers the most common problems: frost, heat spikes, and mold-prone humidity. Add a CO₂ sensor only if you run a sealed, enriched room, and consider a second temperature/humidity sensor for a separate zone like a drying room or cooler. Buy reliable over precise: a $12 sensor that reads consistently and catches a dangerous trend beats an expensive one placed poorly.
What humidity level causes mold in a greenhouse?
Mold risk climbs when relative humidity stays above about 60% with poor air movement and cool, damp surfaces — especially overnight, when the air reaches its dew point and leaves a film of water on leaves and walls. Different molds (gray mold or botrytis, powdery mildew) have slightly different triggers, but sustained high humidity with still air is the common thread. Watching humidity and dew point, and keeping air moving, is the cheapest prevention there is.
What is dew point and why does it matter in a greenhouse?
Dew point is the temperature at which the air can no longer hold its moisture and starts leaving water on surfaces — leaves, walls, glass. It matters because that film of water is where mold and disease take hold, often overnight as temperatures drop. If your surfaces cool to the dew point, you get condensation; keeping a gap between the air temperature and the dew point, and keeping air moving, prevents it. A dew point calculator turns your temperature and humidity into that number.