Fig. 1. These Easter lilies are irrigated with the boom at the top of the image. Note the plastic saucers underneath the containers to catch excess irrigation and allow it to be utilized by plants.
Photo: Christopher J. Currey

Watering is one of the most important responsibilities in the greenhouse. It can also be one of the costliest. Automating irrigation is one step you can take in the greenhouse to reduce costs. However, which system will work best for you?

Overhead irrigation

There are a number of different forms of automated overhead irrigation. The most ubiquitous automated overhead irrigation are mist nozzles used to root cuttings. Impact sprinklers are the simplest form of automated overhead irrigation for containers. These are not widely used inside greenhouses, but are more common outside for crops such as fall mums. Booms are the best automated overhead irrigation system for greenhouses (Fig. 1). Unlike impact sprinklers, booms can turn on and off as they pass over crops, irrigating only those blocks of plants that need it.

These overhead irrigation systems are best-suited to high planting densities, such as plug and liner trays and flats of bedding plants; the more dense the planting, the less water is wasted. For larger (i.e. 6-inch) containers, square saucers can be placed underneath the plants to catch excess water not applied to the container, and allow it to be taken up through capillary action (Fig. 1). Another benefit for booms is that they can be used to apply insecticides, fungicides and plant growth regulators with the addition of an injector.

One of the drawbacks with any of these automated methods is that foliage gets wet. Just like overhead irrigation by hand, irrigate as early as possible to allow ample time for foliage to dry so plants are not wet at night; keeping foliage dry is key to minimizing diseases.

Fig. 2. Drippers provide water directly to the surface of the substrate without wetting foliage, which can help reduce foliar diseases.
Photo: Christopher J. Currey

Substrate surface irrigation

Irrigating the surface of the substrate is a great way to improve water conservation and minimize foliar diseases. Drippers are an automated method of applying water to the substrate surface, providing water to the plants while keeping leaves dry (Fig. 2). Small volumes of water are applied to plants and capillary action draws the water throughout the substrate. This works very well in conventional greenhouse substrates comprised of peat and perlite; however, it is less effective for substrates with coarse components like bark, which reduces the capillary movement of water throughout the substrate. For coarse substrates, use an emitter that sprays water over the surface of the substrate for even distribution.

When selecting drippers, be sure to use pressure-compensated emitters. With this pressure-compensating feature, all drippers in a line need to be pressurized to the same degree before water is emitted. This ensures that the plants at the end of the irrigation line will receive the same amount of water as those plants that are first in the line.

One drawback to using drippers is that they can take some time to install with each planting, ensuring each plant has a dripper. However, this is the only labor-intensive task until the drippers are removed from containers and prepared for the next crop.

Fig. 3. These hydrangeas on the flood floor were recently irrigated and the zone is now draining. Note the zone adjacent to these hydrangeas has not been irrigated yet.
Photo: Christopher J. Currey

Subirrigation

In addition to providing water overhead or directly to the substrate surface, water may also be provided to the bottom of the containers and taken up by capillary action with subirrigation. Capillary mats are a fabric mat that can be placed on top of any bench, allowing for simple retrofitting for subirrigation for any bench. A more popular option for larger commercial facilities are Dutch trays or ebb-and-flood benches. These are solid benches that are temporarily flooded with nutrient solution; an additional benefit is that these systems are often constructed to be mobile and move throughout the greenhouse.

Flood floors are the largest-scale subirrigation systems in the greenhouse. To create different irrigation zones, a rubber border or “lip” partitions the greenhouse floor so sections can be irrigated individually. Since flood floors have a gentle slope, the containers at the highest point receive water last and are the first to drain. You will want to watch plants in those areas and provide edge watering when needed for a uniform crop.

Compared to drip emitters, these subirrigation systems are very flexible, with respect to plant spacing. Unlike drippers, where you need to have enough for high planting densities or the ability to turn some off to save water for sparse planting densities, subirrigations will be equally effective for 4-inch containers in shuttle trays to large patio containers.

Regardless of which subirrigation system you may use, you will need to adjust your fertilizer practices for each of them. Since water is taken up into the container, there is no leaching from irrigation that may occur with overhead or substrate surface irrigation where water moves down through the containers. With no leaching, fertilizer salts can concentrate in the container higher than we’d like. By reducing your fertilizer concentration for subirrigation, you can avoid salt stress.

The take-home

If you are looking to save on labor with your irrigation practices, consider automating your irrigation. The different systems available — booms, drippers, ebb-and-flood benches and flood floors — each have their own advantages and disadvantages and vary in cost. Consider which of these systems could work for you in your facility. Water you waiting for?

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu