This roundtable was originally published July 9, 2015.

About the Upstart Farmers Roundtables

Upstart Farmers Roundtables are an informal, biweekly 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: Transpiration and Water Loss

What is transpiration?

Simply put: Transpiration = Water Loss 

Transpiration is evaporative water loss from the plants. Most of the time this is happening as the stomates open and close to let CO2 in and let O2 out. When that happens, the plant loses water.

Why is this important to Upstart Farmers?

Transpiration and water loss in general is probably one of the most important considerations for Upstart Farmers. It tends to be a topic that people breeze over, but it is important to take the time to understand it in order to ensure the health and efficiency of your system. 

In traditional agriculture, you lose water from flood irrigation or drip irrigation when:

• it evaporates or soaks deep into the soil profile and is lost,
• it is consumed by weeds,
• or through water loss in plants you are not able to sell.

If you are a hydroponic or aquaponic grower, most of your water loss happens through transpiration (assuming you don’t have any major leaks). 

Understanding transpiration and water loss is important for Upstart Farmers because we don’t want to lose any more water than necessary, but also because it drives the uptake of certain nutrients (like calcium, for example). Plants do not uptake all nutrients at the same level. Nutrients are transported into the plant through something called mass flow, where water flows into the plant and the nutrients flow into the plant with it.

So, can we manipulate or control transpiration and water loss?

Transpiration is really at the core of almost all our problems and growth processes.When we talk about many of the variables in the growing environment, what we are really discussing in some way or another is transpiration so manipulating this can be very useful.

Some factors affecting transpiration:

• Humidity 
• Heat and sunlight 
• EC in the solution
• Air circulation around the plant 
• Carbon dioxide

There’s a million things we can do to influence transpiration. But, understanding the effect that it has on the environment, as well as the plant itself, is super important for building the right kind of environment to grow in and for minimizing your costs. 

What do Upstart Farmers need to consider when planning for transpiration?

Unfortunately there aren’t any really good predictive models for transpiration – just observational models (which are limited in their usefulness). For the time being, planning really just comes down to innovating in this area and doing some trial and error.

Dr. Nate described two examples of things people are trying to do to control and plan for transpiration:

1. Testing different CO2 levels to change humidity and therefore transpiration
2. Alternating EC levels during the day and night 

Although right now there aren’t any hard-fast solutions or plans, some day Dr. Nate intends to write a book on the subject! 

Measuring transpiration 

There are two ways to measure for transpiration – one ridiculously complicated way and another more realistic way (which we highly advise).

1. The first method is to go through a lab where you can leverage some high-tech equipment. For example, they make clips that clamp onto the plant leaves that measure certain things which allow us to extrapolate how much water the plant is losing. This is ridiculously complicated.
2. The realistic, and best method for growers, is just to monitor your system. Start out with the assumption that in a well managed system with low humidity, almost all of the water loss is from transpiration. Then, work backwards from there. Look at how many towers you have and how many gallons or plants you are losing and then it’s just factoring water loss to transpiration from there.

If you are an Upstart Farmer losing water, most of it is going to transpiration and some of it is getting used by plants which are splitting water molecules and using it for hydrogen atoms.

Measuring water loss

There are several ways to measure your water usage and water loss.

For us, we top off our system by hand and operate out of a large sump, so we can measure our losses pretty cleanly. Other people have automatic top-off. If you went with a low-tech system with a float value, it can be pretty tough to figure out how much water you are using unless you have it on a separate meter. Now, if you have a more high-tech system with a nice automatic top-off or dosing system, then some of these come with water sensors which will meter the water for you.

If you are doing purely hydroponics and don’t have any other components to your farm (like soil growing), then your water consumption is your water consumption -you know how much water you using on a monthly basis.

Dealing with high humidity and water loss

Carey Martin brought up a good discussion about having issues with high humidity, high water temperatures, and high water loss. In this discussion he asked about RO filters and whether or not farmers really need to be concerned with chlorine or chloramines.

According to Dr. Nate, farmers don’t need to worry about chlorine or chloramines. The only thing you really need to be concerned about in the water (especially when you are going through a lot) are carbonates, and only to a certain point. If you are below 300 to 350 ppm then you don’t need an RO filter.

CO2 and Transpiration

CO2 levels can have a big impact on transpiration. High CO2 can cause less transpiration and vice versa. So here’s a few things to keep in mind when supplementing CO2:

The carbon content of natural gas is MUCH higher than pure CO2. For every pound of natural gas that you burn, you get over three times that weight in CO2. It’s a nice trick to remember. People tend to look at the heat and humidity produced by that, but the thing to remember is the efficiency is so much higher and you are actually consuming a lot of the oxygen that is being produced. Plus, you are tripling the CO2 that you are dumping in the room. A little bitty flame adds a lot of CO2. It’s a good option and worth experimenting with because you save a lot of money on carbon.

Another tip is to keep your CO2 sensor lower to the ground, it will have a more accurate reading because CO2 is heavy and sinks to the ground. To help evenly disperse this, it can help to place a fan in the lower area of the growing space. 

How can you control high humidity? 

High humidity can occur when transpiration rates are really high. A safe range for humidity is 55 to 70 percent. You really don’t want to get higher than about 90 percent.

In high humidity:

• plants become more susceptible to fungal and bacterial pathogens,
• become weakened,
• there is reduced ability to manipulate the plants’ nutrient uptake,
• and the plants stop producing certain compounds to protect their tissues in low humidities.

Low humidity is definitely preferable. The good thing is, it is possible to remove this humidity through dehumidification and reclamation to dump the moisture back into your system.This can be done with condensation-based dehumidifiers or desiccants.

• Condensation-based dehumidifiers are good for hydroponics, but should be avoided in aquaponics because of copper components.
• Dry or liquid desiccants are another option. These are chemical compounds which bind to the air and grab on to the water molecules. Then, through another chemical or mechanical process, the water molecules are removed from the desiccant. Generally this water cannot be reused.

Should dehumidifiers be used in greenhouses?

The short answer is you can, but you really shouldn’t need to. Greenhouse humidity can primarily be controlled with proper ventilation. Sometimes the reality is you just won’t get it any less humid and there’s no way around it.

It’s more worth while to concern yourself with temperature control than humidity control. Temperature control comes first on priorities because it limits your humidity (unless you are somewhere super muggy). 

If you are somewhere super muggy, there are quite a few Israeli companies and a few US companies that create greenhouse dehumidifiers. Unfortunately, most of these also warm up the greenhouse environment. You want to compare this affect with the humidity and decide when it is worth it or not. Excel is your friend! Know your thresholds and make your decision that way. 

Climate Control and Dosing Systems

Remember: Upstart Farmers get a discount on IntelliClimate and IntelliDose systems from AutoGrow. These are not cheap, but once you scale your business, it will be a lot cheaper to have these systems than to do it all by hand – especially if you ever want to take a vacation some day. 

IntelliClimate systems will control your environmental details and is good for hydroponics and aquaponics. However, Intellidose is only applicable to hydroponic systems. For an aquaponics system, just get a simple dosing system that just does pH adjustment, and maybe a few other supplements.