Subirrigation strategies

Departments - Production Pointers

When done correctly, subirrigation can be an effective watering method for greenhouse crops.

October 21, 2021

Whether it is hydrangeas grown in large containers as seen here, or shuttle trays of containerized annuals, subirrigation is flexible for crops with different spacing requirements.
Photo: Christopher J. Currey

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

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

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

Drip tubes or drip emitters irrigate on the surface of the substrate, and are also widely used. Subirrigation is a technique where water or nutrient solution is provided to the bottom of the container and is absorbed or taken-up into the growing substrate by capillary action. There are a number of different methods of subirrigating plants, including ebb-and-flood benches or Dutch trays, flood floors, or capillary mats.

While these different subirrigation systems vary in their cost and lifespan, providing options for any scale of production, the principles of using any subirrigation system remain the same for all of them.

Why use subirrigation for growing greenhouse crops? There are a few advantages to taking this approach. First, subirrigation can help minimize diseases, specifically foliar diseases. High humidity and free-standing water promote diseases such as botrytis and powdery mildew. Overhead irrigation leaves foliage wet, and drying is promoted by watering early in the day and allowing sufficient time for foliage to dry, as well as through the use of horizontal air flow (HAF) fans. With subirrigation, foliage stays dry regardless of irrigation. Subirrigation also allows for more flexibility in timing irrigation — watering crops later in the day won’t run the risk of wet foliage going into in the evening.

Aside from minimizing foliar diseases, subirrigation can improve plant quality. As previously mentioned, water or nutrient solution is taken up by capillary action in the root zone until the substrate is saturated. Compared to overhead irrigation, where the same, or a similar, amount of water is applied to each container regardless of how wet or dry it is, subirrigation allows containers to absorb varying amounts of water until they reach the same saturated level, improving uniformity across the crop.

Flood tables are effective for irrigating containerized plants like these potted roses, and can be efficient for moving plants around the greenhouse as well.

Subirrigation is also flexible and accommodating for different crop spacing. Whether it is shuttle trays of 4-inch containers placed pot tight or 8-inch poinsettias on 18-inch centers, it doesn’t matter for subirrigation. This flexibility makes the switch between crops throughout the year seamless and easy, especially when compared to using drip tubes and emitters that have fixed spacing.

Finally, like any form of automated irrigation, labor savings can be significant for subirrigation systems. By eliminating hand watering, subirrigation allows for hundreds to thousands of plants to be irrigated at the same time, depending on the size of the subirrigation zone.

There are some cultural practices that will have to be modified or monitored carefully when using subirrigation, regardless of which system is used. First, fertilizer concentrations have to be reduced. Whether it is water-soluble or controlled-release fertilizer, less is required with subirrigation. Since water moves up into the container through capillary action, there is virtually no leaching when crops are subirrigated.

Although this can be considered a good thing, especially to improve water- and nutrient-use efficiency and avoid unwanted runoff, there is no opportunity to leach excess fertilizer salts out of containers and reduce electrical conductivity. Using less fertilizer will help avoid supra-optimal substrate ECs and still provide sufficient nutrients for plant growth. By how much should fertilizer concentrations be reduced? A good place to start is to reduce concentrations by 50% compared to what would be used in overhead irrigation. When in doubt, err on the side of less fertilizer. Plants can always be provided with higher concentrations of water soluble fertilizer (WSF) to increase substrate EC in subirrigated systems, but providing too much may be challenging to deal with.

Another consideration for subirrigated systems, especially those that are capturing and recirculating water and nutrient solutions, is excess pesticide runoff, including plant growth regulators (PGRs), insecticides and fungicides. When these solutions have the potential to be recaptured in the same reservoir where water and nutrient solution is stored, the active ingredients can be taken up during subsequent irrigation events. Ultimately this can lead to unwanted and/or excessive applications of these chemicals. When drench applications are made to containers grown in subirrigated systems, try to minimize any leaching of pesticide solutions out of the container. Similarly, avoid excessive runoff from canopies and applications to floor or bench areas.

Finally, sanitation should be a priority in any subirrigation system. While greenhouse sanitation is important for any operation, regardless of their irrigation system, it takes on additional importance for subirrigation systems where water and nutrient solutions are captured and reused. Diseases are generally not going to be transmitted plant-to-plant via transmission in subirrigation systems, as the water is taken up by the substrate and no leaching occurs. However, if pathogens are introduced into the water from other means, such as infected plant materials laying on the bench or floor, then infested water that is recirculated and provided to plants can cause diseases to spread quickly.

Maximizing crop quality and minimizing labor are common goals for any greenhouse grower. Regardless of what scale you are producing on, there are subirrigation strategies that can fit your facility and budget. The best management practices outlined in this article can help deliver optimal results with subirrigation systems.

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