This roundtable was originally published March 11, 2016.
About the Upstart Farmers Roundtables
Upstart Farmers Roundtables are an informal, monthly event. Using Google Hangouts video calling, a small group of Upstart Farmers comes together to share their experiences and learn from each other. Each week is guided by a discussion topic, with growers welcome to ask their own questions.
Roundtables are exclusively for Upstart Farmers, and recordings are only released to the Upstart Farmers Community.
Roundtable: Humidity
Humidity is the amount of water vapor in the atmosphere. If humidity levels are very high, plants have a hard time getting rid of water or pulling water up from the root system because transpiration drives water uptake. This can cause all kinds of problems, so it becomes a very important variable to monitor.
Measuring humidity
Relative humidity takes into account air temperature, wet bulb temperature (the temperature at which water condenses), and the amount of water vapor in the air as a function of condensation at certain temperature points. The point varies quite a lot depending on air temperature and the amount of water in that air.
Sensors in our growing environment can help us measure relative humidity to a specific number.
How do we control humidity?
- Temperature control – temperature is one of the biggest, and most effective ways to control humidity. Warmer air holds more water, so to decrease humidity, you need to take the temperature of your growing environment down (of course within the appropriate range for your plants).
- Dehumidifiers – dehumidifiers are best used in tight spaces and colder locations where lots of CO2 enrichment is happening. This is usually the best option for confined growing environments like Freight Farms. (We spent lots of time chatting about this in the recording).
- Ventilation – Ventilation is best when you are only concerned about cost, you’re not doing CO2 enrichment, and the average outdoor humidity level is below or at your target humidity.
- CO2 – CO2 reduces transpiration which reduces humidity. But when does this not work well? If you’re already at high humidity (like 100% relative humidity) CO2 is not a good method. If you’re below that are you’re actively working to decrease the humidity, then enriching your CO2 levels in the environment allows you to drop humidity faster because it slows transpiration.
What are the effects of humidity on plants?
- Calcium deficiency
- Fungal outbreaks
- Bacterial disease & pathogens
- Nutrient absorption issues
- Many more scary things we aren’t ready to get into
What’s the tradeoff between airflow and transpiration?
Airflow averages everything. The point of airflow is to average the temperature and to average CO2 concentrations in your growing environment. Around the plant, CO2 is depleted, humidity is higher, and temperature can vary from the general environment (and is often warmer, which is bad).
Unfortunately, airflow typically increases transpiration rates which can contribute to higher humidity levels (a catch-22!) However, even in relatively high humidity, it’s still worth increasing airflow as a general rule.
If I’m using a dehumidifier, is it okay to reuse reclaimed condensation?
Yes! Dehumidifiers usually do not use lead as the brazing material on the coils anymore. Nowadays it’s typically copper which is safe to use. Assuming it is copper, then you are safe to reuse this in your hydroponics system. Reclaimed condensation is essentially distilled water. If you have an aquaponic system, Nate hypothesizes that it’s still okay to use this water because the copper levels are so small and insoluble that it won’t cause problems. However, we do recommend keeping a close eye on your fish if you do use reclaimed condensation just to be safe.