Growing under cover concentrates a small number of disease problems rather than creating new ones, and nearly all of them come back to the same physical condition: liquid water sitting on a leaf surface in still air for several hours. Botrytis, downy mildew, damping off and leaf mould all need it. Take it away and the diseases largely take care of themselves.
This reframes the whole subject. Humidity control in a greenhouse is not about running a dehumidifier. It is about temperature management, air movement and watering discipline, in that order, because those three are what decide whether water condenses on leaves.
Why relative humidity is the wrong number
Relative humidity is the amount of water in the air expressed as a percentage of what that air could hold at its current temperature. The second half of that sentence is the problem: the denominator moves with temperature, so the same reading means different things at different times of day.
Air at 85 degrees Fahrenheit and 70 percent relative humidity holds far more water than air at 60 degrees and 70 percent, and it is pulling water out of a leaf far harder. Two identical readings, two completely different conditions for the plant.
Vapour pressure deficit fixes this by expressing the same information as an absolute pressure difference: how hard the air is pulling water out of a leaf, in kilopascals. It is the number commercial growers actually control, and it is worth learning because it makes several otherwise confusing greenhouse behaviours obvious.
| Stage | Target VPD | What happens below it | What happens above it |
|---|---|---|---|
| Rooting cuttings and germination | 0.4 to 0.8 kPa | Fine, this is why cuttings go under a dome | Cuttings wilt before roots can supply water |
| Seedlings and young transplants | 0.6 to 1.0 kPa | Soft growth, damping off risk rises sharply | Growth slows and leaf edges scorch |
| Vegetative growth | 0.8 to 1.2 kPa | Weak stems, low transpiration, calcium transport suffers | Stomata close, photosynthesis stops, growth stalls |
| Flowering and fruiting | 1.0 to 1.4 kPa | Poor pollen release, botrytis on old flowers | Blossom drop and blossom end rot |
Two consequences fall straight out of this table. Low VPD is not a safe default: a plant that is not transpiring is not moving calcium, which is why blossom end rot and tip burn appear in houses that are too humid as well as in ones that dry out. And the same house needs different conditions in different corners, which is the argument for a propagation dome rather than a wetter greenhouse.
The dawn condensation problem
The highest-risk period in a greenhouse is not the middle of a humid afternoon. It is the hour either side of dawn, and understanding why explains most of the routine.
Overnight, the glazing radiates heat to the cold sky and becomes colder than the air inside. Air near the glazing cools, condensation forms on the glazing, and water drips. Meanwhile leaves are also radiating and can fall below the dew point of the surrounding air, so condensation forms directly on the leaf surface. Then the sun rises, the air warms faster than the plant tissue, relative humidity near the leaf spikes, and the leaf stays wet for another hour or two.
That is the window fungal spores germinate in. Everything useful you can do about greenhouse disease is aimed at shortening it.
- Run a circulation fan continuously, including overnight. Moving air prevents the still saturated layer forming over leaves and dries surfaces faster once the sun is up. A Roodike 5 in Clip-On Circulation Fans, 2-Pack ($35.99) costs a few watts and is the single highest-value disease control available.
- Vent for a short period in the morning, even in winter. A brief exchange of air after sunrise dumps the moisture the night accumulated. It costs a little heat and prevents a great deal of disease.
- Water in the morning, never in the evening. Water applied late sits on surfaces all night. Water applied at the start of the day is largely taken up or evaporated before dark.
- Keep leaves dry. Drip and soaker irrigation put water at the root zone and nowhere else. Overhead watering wets every leaf in the house and takes hours to dry.
- Raise the minimum a degree or two if you can. Warmer leaves are less likely to fall below the dew point in the first place, which is a heating decision with a disease benefit.
The specific diseases and what each one needs
| Disease | What you see | Conditions it needs | What actually stops it |
|---|---|---|---|
| Botrytis, grey mould | Grey fuzzy growth on stems, dying flowers and fruit | High humidity, still air, and dead or damaged tissue to colonise | Airflow, wider spacing, and removing every dead leaf and spent flower promptly |
| Downy mildew | Yellow patches above, greyish growth beneath the leaf | Leaf wetness at moderate temperatures, especially overnight | Morning venting, no overhead watering, and resistant varieties where available |
| Powdery mildew | White powder on the upper leaf surface | High humidity but a dry leaf, and poor air movement | Air movement and avoiding water stress, since stressed plants are far more susceptible |
| Damping off | Seedlings collapse at soil level and fall over | Cold wet medium, overcrowding, poor drainage, still air | Bottom heat, sowing thinly, a free-draining sterile mix, and taking the dome off at emergence |
| Tomato leaf mould | Pale patches above, olive-brown velvet beneath | A specifically greenhouse disease, needing high humidity for extended periods | Humidity control, leaf removal at the base of the plant, and resistant varieties |
| Root rots | Wilting despite wet medium, brown roots | Overwatering, poor drainage, medium that stays saturated | Drainage and watering discipline, not a spray |
When you actually need equipment
Most humidity problems in hobby greenhouses are solved by a fan, a watering change and a morning vent. Two situations genuinely call for equipment.
Raising humidity for propagation. Cuttings have no roots and cannot replace what they lose, so they need a VPD of 0.4 to 0.8 kPa, well below what the rest of the house wants. The answer is a small enclosed volume rather than a wetter house: a humidity dome such as the AC Infinity Heavy-Duty Humidity Dome Germination Kit, 5 x 8 ($89.99), or a fine misting system such as the MistKing Starter Misting System, 5th Generation ($199.99) over a propagation bench only.
Lowering humidity in a sealed winter house. A well-insulated house with the vents shut for weeks holds moisture that a leaky one lost. Venting costs heat. A small dehumidifier such as the BeOkay 1,657 ml Small Dehumidifier ($35.15) removes water without removing heat, which in a heated winter house is often the cheaper trade. Monitor with a logging hygrometer such as the Govee H5179 WiFi Thermometer and Hygrometer ($33.99) rather than guessing, because the overnight peak is what matters and nobody is standing there at 4 a.m.
Humidity monitoring and control
Humidity, temperature and airflow are one system rather than three. The airflow half is in ventilation basics, the watering half is in watering in a greenhouse, and the pest side of the same closed environment is in greenhouse pest management.