Quick answer: In cut flower greenhouses, humidity costs you in two ways. Wet petals at night start Botrytis, which spoils blooms in the house and in the box. And crops grown at constantly high humidity, roses especially, develop stomata that close poorly, so cut stems lose water faster and have a shorter vase life. Keep petals dry from dusk to dawn, avoid long periods of very high humidity, and add dehumidification where venting cannot hold the night target.
Quick answer
- Night: keep a margin below saturation (about 89 % RH at 16 °C) so petals never get wet.
- Avoid constant RH above about 85–90 % during flower development, especially for roses.
- Remove spent flowers and debris: Botrytis starts on dead tissue.
Two humidity problems in flower crops
| Problem | When it happens | What it costs |
|---|---|---|
| Botrytis blight on petals and buds | Cool, humid nights; after cloudy weather; dense crops; condensation on petals | Spotted petals in the house, and gray mold spreading in packed boxes and coolers |
| Poor vase life | Crops grown at constantly high humidity | Stems lose water faster after cutting; flowers wilt or fail to open |
UMass Extension's cut flower guidance links Botrytis to wet petals, spent flowers, irrigation, spacing and air movement together, and notes that aging flowers are most susceptible (UMass, Botrytis blight of cut flowers). Humidity control is one part of that list, not a substitute for hygiene.
Roses: why constant high humidity shortens vase life
Rose leaves that develop at continuously high humidity form stomata that respond poorly to darkness and drought. After cutting, the stem keeps losing water through those leaves, and vase life falls.
- In a controlled study on 'Asami Red', roses grown at constant high humidity had weaker stomatal closure, higher transpiration after cutting and shorter vase life. Plants grown with alternating day/night humidity performed close to those grown at constant moderate humidity (Environmental Control in Biology, 2006).
- Cultivars differ: in a Wageningen study, high humidity impaired flower opening and markedly reduced vase life in one cultivar, while two others were less affected (WUR).
What this means for growers: give the crop daily periods of lower humidity, avoid holding RH near saturation for days at a time, and test sensitive cultivars in the vase before committing to a climate strategy.
Setpoints for flower crops
| Period | Aim |
|---|---|
| Day | Moderate humidity with active transpiration; typical targets are around 60–80 % RH, by crop |
| Evening and night | Keep a margin below the dew point: about 83 % at 10 °C, 89 % at 16 °C, 91 % at 20 °C (UMass) |
| Sunrise | Raise air temperature gradually so cold petals and buds do not collect condensation |
The night values come from UMass guidance on reducing greenhouse humidity; see greenhouse humidity and disease for how they work.
Keeping petals dry at night
- Heat and vent at dusk and dawn when outside air is drier.
- Run horizontal airflow fans continuously so flowers stay at air temperature.
- Irrigate early so the canopy and floor dry before night; avoid wetting flowers.
- Remove spent flowers, leaves and stems from the crop, benches and floor.
- Dehumidify where screens stay closed, outside air is humid, or the crop needs a steady night target.
Field example: calla lily greenhouse
In a calla lily greenhouse in Yunnan, China, night humidity was the main problem in July. After adding RYCF-22S floor-standing dehumidifiers (starting with one unit and expanding to two), average night RH fell from 87.4 % to 77.1 % in July 2026. See the calla lily case study for the measurement points.
Harvest and cold storage are a separate problem
Postharvest coolers run at low temperature and high humidity to keep stems turgid; that is the opposite of the greenhouse night goal. Do not copy a greenhouse humidity target into the cooler. Follow handling guidance for your crop (UMass, Harvesting and handling cut flowers), and keep stems and boxes free of condensation when moving them between cold and warm areas.
Choosing dehumidifiers for a flower house
- Size for the night peak at the house condition, not the 30 °C/80 % RH catalogue rating.
- Spread units, or use air tubes, so dry air reaches every bed.
- Direct discharge along rows, not straight onto flowers.
- Link to the climate computer; start before sunset.
- Use coated coils where sprays and fertilizers are present.
See our greenhouse dehumidifiers and how to choose a greenhouse dehumidifier.
FAQ
Frequently asked questions
What humidity is best for cut flowers in a greenhouse?
Moderate daytime humidity with active transpiration, and a night margin below saturation so petals stay dry. Avoid constant RH near saturation, which shortens vase life in roses.
Why does my rose vase life vary between batches?
Cultivation humidity, cultivar and postharvest handling all affect it. Constant high humidity during development is a known cause of shorter vase life.
How do I prevent Botrytis on cut flowers?
Keep petals dry at night, keep air moving, remove spent flowers and debris, irrigate early, and keep stems dry in boxes and coolers.
Can dehumidifiers improve flower quality?
They help hold night humidity below saturation and avoid long humid periods. Quality also depends on cultivar, nutrition, hygiene and handling.
Sources
- UMass Extension, Botrytis blight of cut flowers; Reducing humidity in the greenhouse; Harvesting and handling cut flowers.
- Environmental Control in Biology 44(4), 2006: humidity regime and stomatal response in cut roses.
- Wageningen University & Research, Postharvest water relations in cut rose cultivars with contrasting sensitivity to high relative air humidity.
