Humidity introduces and promotes plant disease and pests. How can you control it? Environmental Engineer Kurt Parbst and Upstart University explain the problems and solutions to humidity in all climates. Learn more about Upstart Univeristy: https://university.upstartfarmers.com/become-a-farmer?ref=2

1. Humidity Management
HOW HUMIDITY WORKS AND POPULAR CONTROLS

2. Brought to you by:
Kurt Parbst
Director of Business Development
Envirotech Cultivation Solutions
Halle Brake
Director of Upstart University
Bright Agrotech
Kurt@agamgreenhouseusa.com halle@brightagrotech.com

3. Humidity - When is it a problem?
1) What is humidity, how does it work & what does it do?
2) Humidity measurement & management
3) Liquid Desiccants
4) More resources & support (stick around for a free
downloadable resource!)
In this deck:

4. Humidity causes many problems, chief of
which is promoting diseases and other pests.

5. But first, let’s learn the lingo
Here are some important ideas and
words to describe humidity and the
effects that it has

6. (Plant) Canopy
The collective
body of foliage in a
grouping of crops.

7. Transpiration
Loss of water vapor
from stomates plays an
important role in growth and
development, ensuring the
transport of nutrients from the
soil (or solution) to the plant
organs.

8. Relative humidity
The amount of
water in air/water
in saturated air
(expressed as a
percentage)
Humid
Saturated

9. Dew point
The temperature of an
object at which water
vapor condenses on
the surface of that
object

10. Psychrometer
A tool to measure
humidity

11. Vapor pressure deficit (VPD)
The ability (driving
force) for water to
evaporate from a
surface

12. Where does humidity
come from?

13. Evaporation & Transpiration
(evapotranspiration)

14. Unmanaged humidity can cause:
Disease outbreaks:
• Botrytis
• Powdery Mildew
• Downy Mildew
• Pythium
• Phytopthora
• Fusarium

15. How does humidity encourage
disease?
• Moisture promotes germination
• Water film is pathway for infection
• Dripping water can spread spores

16. • Significant damage
common
• Catastrophic loss a
threat
• Facilities become
contaminated
Fungal Threat is serious
Botrytis

17. In addition, humidity can cause:
-Hindered nutrient uptake
-Insect pest outbreaks

18. So how can we
dehumidify air?

19. Condensation is a key
concept for farmers.

20. How does condensation happen?
Sun increases
transpiration and
evaporation
Warm air holds
more vapor

21. ...How does condensation happen?
Temperature drops at night (esp. in fall
and spring)
Condensation forms and water droplets
form on cold surfaces like walls, and
plants (shoot tips can be 2.5o
C cooler)

22. *Temperature is key to humidity control
Relative humidity (RH) is temperature
dependent
-Alone, says little about plant’s ability to
transpire
For every 10C increase in temperature, water
holding capacity doubles (RH is halved)

23. When condensation
doesn’t work...

24. RH measurements not always accurate
Coldest point not indicated
on sensor for zone
Solid state humidity sensor
not necessarily accurate

25. Growers can monitor VPD instead
More accurate way of measuring ability of water
to enter the air
• VPD describes the ability (driving force) for
water to evaporate from a surface
• VPD of zero means a plant cannot transpire
as water will not leave the leaf, unless the leaf
heats up

26. • High VPD: high
temps, low RH
• Low VPD: low
temps, high RH
dry air
water vapor

27. VPD Rates
.2 .3 .4 .5 .8 2.2
Best for seedlings
Disease
Best for mature Stress

28. RH vs. VPD
Water holding capacity doubles with
every increase of 20F
70% RH can approach lower and upper limit,
depending on temp
Temperature, C RH, % VPD, kPa
16 70 .55
24 70 .90
32 70 1.45

29. RH vs. VPD
Water holding capacity doubles with
every increase of 20F
An RH ‘range’ is OK, at the right temperature
Temperature, C RH, % VPD, kPa
16 22 1.38
24 41 1.38
32 71 1.38

30. Let’s look at an example
What does VPD and
RH look like on a farm
throughout a day?

31. VPD Monitoring
Guatemala tomato farm: ‘beautiful’ weather. Unheated, natural ventilated greenhouse
Time Temp, F RH, % VPD, kPa
0:00 61.2 89 0.20
2:00 59.4 91 0.16
4:00 58.1 95 0.08
6:00 58.3 95 0.08
8:00 74.8 82 0.53
10:00 91.4 31 3.5
12:00 89.4 28 3.43
14:00 86.7 41 2.58
16:00 76.1 69 0.96
18:00 68 87 0.31
20:00 63.9 94 0.12
22:00 61.7 95 0.09
Target: 0.3 to
1.5kPa
Low VPD stress
2x per day
Water stress
midday
Poor quality,
yields
Need: Heat &
shade

32. What are the solutions to
Humidity?

33. Goal:
The key to suppression of disease is
to keep the canopy dry from dusk til
dawn.

34. 3 main methods of control
1. Ventilation (air exchange)
2. Condensation (cold surfaces, AC)
3. Desiccation (chemical absorption)

35. Option 1: Ventilation
• Water vapor removal from aerial
growing environment
• Ventilation: replace warm, high water
vapor content inside air with cooler,
drier outdoor air in intervals

36. Ventilation’s hidden cost
Indirect cost is heating/cooling new air

37. Option 2: Condensation
Condensation: Single-glazed
greenhouses typically have a good
ability to dehumidify themselves
satisfactorily (ventilation and
condensation)

38. The problem for condensation
Modern greenhouses
and other growing
spaces don’t have the
right materials for this
Double layer
polyethylene - great
for insulation, bad for
condensation
Glass -
increasingly
rare, bad for
insulation

39. What about thermal screens?
• Modern greenhouses utilize thermal screens for heat
conservation. Greenhouses with double glazing
and those with screens have a diminished ability for
dehumidification.
• Screens in houses are typically gapped for humidity
control. These leads to a loss of energy.

40. This can leave growers in a tough spot
• Ventilation introduces new heating
costs, and condensation only works
with some types of facility

41. A promising new solutions
to humidity control:
Dessicant systems

42. Dessicant systems
Dessicants can be a solution
• Keep air, walls and ceiling dry (less heat loss)
• No ventilation is required (no wasted energy)
• Condensing water releases energy
(generates heat)

43. Agam Greenhouses is a company involved
in making this solution accessible to
greenhouse growers
They built a dessicant-
based machine to deal with
humidity challenges in spas,
greenhouses for covered
cherries, eggplants, and roses

44. They called it the VLHC
• Liquid desiccant (salt solution)
dehumidifier
• Primary heat source for growing
environments
• Simultaneous dehumid. & direct air
heating
• Airborne Fungal decontamination
(Capture, Neutralize)

45. Learn more about
VHLC here.

46. Ready to get
empowered?