Determining how to irrigate

Departments - Production pointers

Here’s how to decide when to water plants and when to hold off.

Fig. 1. By removing a plant from its container, a thorough inspection of the rootzone can provide more information than looking simply at the substrate surface. The container on the top looked like it had sufficient water, but in fact, the substrate was dry; the rootzone on the bottom was thoroughly irrigated and had sufficient moisture.
Photos by Christopher j. currey

“The person holding the hose has the most important job in a greenhouse.”

Yes, that is hyperbolic. However, there is a fair amount of truth in that statement. Irrigating greenhouse crops, whether by hand or automated, is one of the most important responsibilities, even if the act itself may seem mundane. One of the biggest challenges to irrigating greenhouse crops is determining when irrigation is required. Over- and under-watering crops each result in a different set of problems, but both are detrimental to crop quality. This article will cover how to approach making the determination to irrigate greenhouse crops.

On a schedule

The most rudimentary approach to irrigation is on a schedule.

Yes, it may be easy to remember, and it can also make scheduling labor and tasks more straightforward. However, the fundamental flaw to irrigating on a schedule is that it does not take a biological approach to growing plants.

Watering on Monday and then Wednesday — regardless of what happened on Tuesday — does not take into account the myriad of variables that affect water uptake and, subsequently, irrigation needs. These include, but are not limited to, the daily and seasonal fluctuations in crop age/size and growth stage, light, temperature and humidity.

Ultimately, irrigation is based on how much plants are using or taking up water, which can be directly related to transpiration, and the amount of water available in substrate. When crops are younger and smaller, there is less water uptake compared to when crops grow larger, since transpiration increases with the more numerous and larger leaves. Warmer air temperatures, higher light intensities and lower humidity promote transpiration and water uptake, whereas cooler air temperatures, lower light intensities and higher humidity reduces transpiration and water uptake. Taking a more dynamic approach to determining when to irrigate matches the dynamic nature of plants and the environment they are grown in.

“Look and feel”

The “look and feel” method of determining when to irrigate may be low-tech, but it is a great way for growers to assess plant water needs. It is also probably the most common approach taken to determining when to irrigate. What exactly does “look and feel” entail? It incorporates several different assessments that, when taken together, fairly accurately help determine if irrigation is required.

“Look” refers to several things, including the color of the substrate and the appearance of the plants. For peat-based soilless substrates, the color reflects the amount of water in it; i.e. darker substrate has more water while lighter substrate has less water. Since “light brown” and “dark brown” can be considered subjective from person to person, a numerical scale is becoming more common to describe substrate color. Substrates that have been thoroughly irrigated are saturated and described as a “5,” with a dark color and sometimes free water visible at the surface. Alternatively, substrate at a “1” is light brown in color and will be pulling away from the container edge. Developing your own in-house visual aid to teach new growers what this scale looks like helps them learn and adopt this practice.

In addition to substrate color, the color of plants themselves can also be a good indicator. When plants are well-watered, foliage will look more or deeper green and almost have a sheen or shine to it. Alternatively, when plants start do dry down, foliage will appear lighter green — almost bluish- or gray-green for some species — and look slightly dull. Each plant will have their own nuanced appearance when well-watered or dry, so this will take some experience to learn for different crops. Tomato and petunia are two species that demonstrate these differences quite well. Finally, the turgidity of plants is also a clear indicator of water needs. Particularly, as plants dry down and lose water, they start to wilt, commonly called “flagging.” The degree of allowable flagging varies among different crops — flagging should be avoided on poinsettias but can be tolerated (even desired) on packs or pots of tomato plants.

Using visual aids alone is not going to give a sufficient assessment, as appearances can sometimes be deceiving. “Feel” consists of several assessments, including the weight of containers, how moist substrate feels, and removing plants from their containers, and will complement the visual cues we already covered. First, picking up containers is an informal assessment of how heavy or light a container weighs, and is directly related to the amount of moisture in the substrate. Sometimes the surface of the substrate may look lighter in color, but that just may not reflect the amount of water in the rest of the substrate. Another way to judge the amount of substrate moisture is to simply stick your finger into the substrate! Clearly this will not be an option for small-volume containers (i.e., plug trays) but for containers that are 4-inches or larger, it can be effective without harming plants. Finally, take plants out of their containers to inspect the substrate. By removing plants from containers and looking at the color of the substrate throughout the root zone, you can make a more accurate determination of substrate moisture. Make “look and feel” assessments on several containers throughout a crop, from the edge, the middle, etc., so you can make an accurate representation of the crop as a whole.

Fig. 2. Weighing containers provides an objective measurement of the amount of water available to plants. By using a predetermined weight threshold, it is easy to determine when to irrigate.

By weight

Making decisions is much easier when objective measurements are made, as opposed to subjective determinations. As an extension of the “feel” approach to picking up containers, weighing containers takes a more objective approach by generating quantitative data (the weight) to determine if irrigation is required.

By measuring container weights, tracking them, and having pre-determined values or thresholds that indicate irrigation is necessary, the guesswork or subjective nature of determining whether or not to irrigate is removed- we are using numbers (the weight) to make the decisions for us. Yes, as plants grow larger, they will weigh more. However, substrate moisture will account for a large proportion of the weight for any container, flat or plug tray. Target weights to warrant irrigation will need to be determined for different container sizes and crops to account for the variation in substrate volume and moisture requirements for different plant species. As with assessing containers using the “look and feel” method, be sure to measure container weights on several samples from throughout the crop.

By measuring substrate moisture

Finally, using substrate moisture sensors can be a very accurate way to measure when plants require irrigation. Sensors placed into substrate can accurately measure the substrate volumetric water content and can provide a quantitative measurement that can be used to determine if irrigation is necessary. These sensors are not the “water meters” sold to consumers at retail outlets. Rather, these are very accurate sensors which can be tied into your environmental control to measure and record substrate moisture and, potentially, control automated irrigation equipment such as drippers. As with the previously mentioned strategies, the different plant species will have their own substrate water content setpoints that reflect the species’ water requirements. Place sensors in several containers throughout the crop to get a representative assessment of substrate moisture.

The take-home message

Successfully determining when to irrigate crops can be one of the most important skills to develop in greenhouse crop production. Take away the mystique surrounding this determination by taking one of the methodical approaches outlined here. Most importantly, make sure those who are new to “handling the hose” are given the tools they need to succeed from the start.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University.