Part 3 - Tampering with Temperature
So far, we have explored the differences between vegetative and generative growth, and the role that water availability and EC play in steering the crop in either direction. Now it is time to explore the impact that temperature and humidity have on these two types of growth.
- Water availability
- EC
- Day/Night temperatures
- Vapour Pressure Deficit (VPD)
- Pruning
- Fruit load
Day/Night temps
While we are accustomed to focusing on the temperatures
through the day, it is also important to consider the greenhouse temperature at
night. With a high night temperature, you are going to see higher rates of
respiration, which results in a decreased fruit weight. Allowing the space to
cool down over the night is a good strategy to maximize yield. Be careful here
though, because too low of a night temperature will result in poor fruit
quality.
Not only does a cooler night temperature (compared to the
daytime temperature) reduce respiration in your fruit, but it also encourages
generative tendencies in the plant. Holding your temperature steady over 24
hours puts the plant into a more vegetative growth pattern.
A good strategy for transitioning from you night temperature
to your day temperature, is to allow the space to heat up 1-1.5 hours before
sunrise. This is important in maintaining the shelf life of your crop once it
has been harvested. Without this warm-up period, you will likely end up with
condensation in the production space, which can lead to a variety of issues.
Like irrigation, these highs and lows should be modified on
cloudy days – reducing that temperature gap when there is less sunlight will
prevent the crop from becoming overly stressed.
Vapour Pressure Deficit (VPD)
Vapour pressure deficit (measured in kPa) compares the
amount of moisture in the ambient air, to the amount of moisture in the air
surrounding the plant. It is a driving force for transpiration and determines
how quickly or slowly the moisture moves out of the plant.
High VPD
- low moisture content in the ambient air
- big difference in moisture content between the air and the plant
- water is rapidly pulled from the plant leaves, putting pressure on the roots to transport more water
Low VPD
- high moisture content in the ambient air
- small difference in moisture content between the air and the plant
- water is not pulled from the plant as quickly
- more vegetative plant
How do I know where I stand?
The chart posted below can be used to help visualize the
energy relationships of moist air. Based on your VPD reading and the
temperature of the grow space, you can determine what the ideal humidity is
(and vice versa). By maintaining a VPD in the target zone (shown in green in
the chart below), you can rest assured that the air-water relations have been
optimized within the plant and prevent encouragement towards vegetative
production. Once you start steering towards the red zones on either side, an
adjustment should be made to get back into the target zone. Fluctuation in
temperature or humidity is okay if the appropriate adjustments are made to keep
the system in balance.
Figure 1. The above chart serves as a guideline for
creating an ideal greenhouse environment. Taking regular VPD measurements, in
addition to temperature and humidity, will help to keep the system in check.
Table was sourced from https://scienceinhydroponics.com/2017/04/vapor-pressure-deficit-vpd-in-hydroponics.html
Posted by: Talia Plaskett
