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Whether it’s hydrangeas grown in large containers as seen here, or containerized annual shuttle trays, sub-irrigation is flexible for crops with different spacing requirements.

Photo: Christopher J. Currey

Watering greenhouse crops is arguably one of the most important tasks of any production facility. How and when crops are irrigated affects the growth and development of plants and, ultimately, the quality of finished plants.

Irrigation also affects other aspects of production, such as plant diseases and the efficiency of water and nutrient use. The way crops are watered can also have a big impact on labor costs. There are a variety of ways to irrigate greenhouse crops, and this article will focus on sub-irrigation and some of the tips and tricks or best management practices to keep in mind.

Sprinkler irrigation, where water and nutrient solutions are delivered by hand from a hose or automated from a boom, is the most common form of irrigation.

Drip tubes or drippers irrigate the surface of the substrate and are also widely used. Subirrigation is a technique where water or a nutrient solution is supplied to the bottom of the container and is absorbed or absorbed into the growing medium by capillary action. There are a number of different methods for irrigating plants, including ebb and flow benches or dutch trays, flood soils or capillary mats.

While these different sub-irrigation systems vary in cost and lifespan, providing options for any scale of production, the principles of using any sub-irrigation system remain the same for everyone.

Why use sub-irrigation to grow greenhouse crops? There are a few advantages to taking this approach. First, subirrigation can help minimize disease, especially leaf disease. High humidity and standing water promote diseases such as botrytis and powdery mildew. Sprinkler irrigation leaves the foliage moist and drying is promoted by watering early in the day and allowing the foliage sufficient time to dry out, as well as by using horizontal airflow fans (HAF). With sub-irrigation, the foliage stays dry regardless of the irrigation. Sub-irrigation also allows for more flexibility in the irrigation schedule – watering the crops later in the day is not likely to let wet foliage in in the evening.

In addition to minimizing leaf diseases, sub-irrigation can improve plant quality. As mentioned earlier, the water or nutrient solution is absorbed by capillary action in the root zone until the substrate is saturated. Compared to sprinkler irrigation, where the same or a similar amount of water is applied to each container whether wet or dry, sub-irrigation allows containers to absorb different amounts of water up to that they reach the same level of saturation, improving uniformity across the crop.

Flood tables are effective in irrigating container plants like these potted roses, and can also be effective in moving plants around the greenhouse.

Sub-irrigation is also flexible and adapts to different growing spacings. Whether it’s shuttle trays of tightly placed 4-inch containers or 8-inch poinsettias on 18-inch centers, it doesn’t matter for sub-irrigation. This flexibility makes changing between crops throughout the year seamless and easy, especially compared to using drip tubes and fixed-spacing emitters.

Finally, like any form of automated irrigation, labor savings can be significant for sub-irrigation systems. By eliminating manual watering, sub-irrigation can irrigate hundreds to thousands of plants at the same time, depending on the size of the sub-irrigation area.

Certain cultural practices will need to be modified or carefully monitored when using sub-irrigation, regardless of the system used. First, the concentrations of fertilizers must be reduced. Whether it’s water soluble or controlled release fertilizers, less is needed with sub-irrigation. Since the water rises in the container by capillary action, there is virtually no leaching when the crops are irrigated.

While this can be considered a good thing, especially to improve water and nutrient use efficiency and prevent unwanted runoff, there is no possibility of leaching excess salts from the water. ‘fertilizer out of containers and reduce electrical conductivity. Using less fertilizer will help avoid supra-optimal growing medium EC while providing enough nutrients for plant growth. How much should the fertilizer concentrations be reduced? A good place to start is to reduce the concentrations by 50% from what would be used in sprinkler irrigation. When in doubt, choose less fertilizer. Plants can still be given higher concentrations of water soluble fertilizer (WSF) to increase growing medium EC in irrigated systems, but providing too much can be difficult to manage.

Another consideration for systems subject to irrigation, especially those that capture and recirculate water and nutrient solutions, is excessive runoff from pesticides, including plant growth regulators (PGRs), insecticides, and pesticides. fungicides. When these solutions have the potential to be collected in the same tank where the water and nutrient solution are stored, the active ingredients can be absorbed during subsequent irrigation events. Ultimately, this can lead to unwanted and / or excessive applications of these chemicals. When spray applications are made to containers grown in irrigated systems, try to minimize any leaching of the pesticide solutions out of the container. Likewise, avoid excessive runoff from awnings and applications to the floor or bench tops.

Finally, sanitation should be a priority in any sub-irrigation system. While greenhouse sanitation is important for any operation, regardless of their irrigation system, it takes on added importance for sub-irrigation systems where water and nutrient solutions are captured and reused. Diseases will generally not be transmitted from plant to plant by transmission in sub-irrigation systems, as the water is absorbed by the substrate and no leaching occurs. However, if pathogens are introduced to the water by other means, such as infected plant material placed on the bench or the ground, the infested water that is recirculated and supplied to the plants can cause rapid spread. diseases.

Maximizing crop quality and minimizing labor are common goals for any greenhouse grower. No matter what scale you produce, there are sub-irrigation strategies that can fit your facility and budget. The best management practices described in this article can help achieve optimal results with sub-irrigation systems.

Christopher is Associate Professor of Horticulture in the Department of Horticulture at Iowa State University. [email protected]