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Irrigation essentials: basics, biology, and best practices

Irrigation management is often one of the most overlooked factors in horticultural production. While every grower understands that crops need water to perform basic biological functions, there are still misconceptions as to how and when a crop should best be watered. Ornamental and vegetable growers have tackled this issue for years. As more and more medicinal growers move to container-based production in a controlled environment, the assumption is that watering should be roughly the same as the traditional soil based methods. The reality is, irrigation methods should be tailored to growing style and cultivar.

Why proper irrigation matters
Irrigation is only one piece of the puzzle when it comes to high-quality crop production, but it can easily become a limiting factor if ignored or mismanaged. Container-based production gives growers a high degree of control over every aspect of the crop's root zone, but with this control comes an increased need for precise and properly managed irrigation. This irrigation management is important for avoiding "drought stress."

"Drought stress" is a general, qualitative term used to describe how a lack of available water affects a plant's physiology. Most growers are familiar with what a plant looks like when it is under extreme, acute drought stress. The plant appears wilted and the leaves droop downward because the force that keeps the leaves upright, known as turgor pressure, has dropped due to a lack of water within the plant's tissues. If left in this state, the plant will continue to lose water until individual cells die, leading to necrosis and potentially death of the entire plant.

When a plant's roots experience a lack of available water in the media or soil, one of the first responses is for the stomata—the small openings on leaves that allow gas exchange to occur—to close. This restricts transpiration and prevents carbon dioxide from entering the leaf cells, in turn slowing photosynthesis and growth. Stomata closing is a relatively short-term strategy to prevent moisture loss, but plants exposed to consistent drought stress have long-term strategies to adapt to a lack of moisture in the root zone which are detrimental in a horticultural setting.

Even a single drought stress event can begin to change the way a plant grows and functions. The plant will begin to adapt in order to more efficiently use the water that is available. This is known as a crop's water use efficiency (WUE). Plants exposed to drought stress will increase their water use efficiency so that the amount of water they have been exposed to in the past will be sufficient to maintain physiological functions, thereby preventing stomata from having to close.

This means the plant will grow smaller leaves with fewer stomatal openings per leaf area, and change their metabolic functions. The plant uses less water, grows slower, and will have a reduced yield. For growers aiming for a highly productive plant, this is a significant problem.

How growers can help ward off drought stress
Continuing with our car analogy, in order to keep the "engine" of photosynthesis and growth moving (assuming we have the right amount of light, temperature and humidity), we need a steady supply of water and nutrients. If the gas tank represents available water to the plant in the growing media (or substrate), then it should never be allowed to run dry. A lack of available water in the media leads to the aforementioned drought stress symptoms, and a reduced yield. The best way to prevent reduced yield is through more frequent, lower volume irrigation events throughout the day rather than large, infrequent irrigation events.

Keep in mind, this method, often called "pulse irrigation," works best with a growing medium like coconut coir or rockwool. This is because the properties of these media is excellent in terms of water-holding capacity, while maintaining enough air porosity to keep the root-zone oxygenated, or with adequate space of air-filled pores.

The future of irrigation systems
The focus on high-quality irrigation system design will increase as the industry continues to move toward more hydroponic-style production. This is because soilless production requires a finely tuned approach to irrigation when compared to traditional soil-based production.

One of the most efficient and accurate ways to deliver water and nutrients to a crop is drip irrigation. This method is designed to add water to the system slowly, thereby allowing growing media components to become fully saturated while minimizing leaching or runoff.

According to Hawthorne, a company specialized in supporting growers in their horticultural endeavors, has some of the largest irrigation equipment manufacturers as partners, is very much aware of how a well-designed drip irrigation system should look like. "First of all, a well-designed drip irrigation system features the ability to alter the frequency and duration of irrigation events," the Hawthorne team explains. "Then, the system should deliver uniform flow rate and total amount of water and nutrients to each individual plant. The system should also include the ability to provide targeted water programs for each plant growth or crop stage. It is also equally important that it'd provide adequate water supply or storage to maintain consistent irrigation. If all is designed accordingly, a drip irrigation system can help minimize costly wastewater removal treatment and minimize humidity load for indoor growing operation, thus helping to reduce dehumidification load." 

For more information:
Hawthorne Gardening