Chapter 15

Irrigating Greenhouse Crops

Greenhouse Irrigation Systems

The irrigation system you select will depend a great deal on the range of plants you intend to produce, the size and complexity of the operation, the quality of the water, and how much of it there is available and when it is available. Small greenhouses and those growing a variety of species may prefer hand-watering for irrigation, whereas large greenhouse operations growing only a few species may use sophisticated sem-automatic systems that can be programed to water an entire range of greenhouse crops, such as with overhead booms, micro-sprinklers, or flood floor irrigation. The two types of irrigation systems used in greenhouse production systems are overhead systems (which apply irrigation solution to the substrate surface) and subsurface systems (which apply water to the base of the root zone and relies on capillary action to bring water into the root zone from below).

Overhead Irrigation Systems

The most common form of irrigation found in greenhouse operations is overhead irrigation by applying water to the media surface by hand watering with a hose, micro-irrigation, overhead sprinklers, or booms. In overhead irrigation systems the nutrient solution is applied to the upper surface of the substrate in a bench or pot, and any excess solution applied can drain from the bottom of the container out to the environment. Overhead irrigation systems, also referred to as open irrigation systems, can prevent fertilizer salt accumulation in the medium and be installed with relatively little expense. From a water and nutrient management perspective; however, these systems can be inefficient, may result in significant fertilizer leaching, runoff, irrigation inefficiency, and heavy use of water.

Subsurface Irrigation Systems

Subirrigation systems bring irrigation solution into the root zone from below. In subirrigation, a water-containing structure is flooded until the water level contacts the medium. Once contact is made, capillary action (the attraction of water molecules for one another and other surfaces) moves water up through the medium and throughout the container. Growing medium pore space and medium type (e.g., sphagnum peat) are the primary factors dictating saturation height and speed. Subirrigation is widely used in the European greenhouse industry. In the United States, however, many greenhouse growers have indicated that high initial investment costs and a lack of technical production information impede the adoption of this technology. Many different subirrigation systems are available to greenhouse operators.

Advantages and Disadvantages

The most commonly cited advantage is the savings in labor needed for watering the plants—a single person can water thousands of plants by operating the flooding system manually or with the help of a computer. Watering is highly uniform; labor for watering is minimal; and flood floors (and benches, to a slightly lesser degree) allow great flexibility in crop spacing, choice of containers, and efficient use of space. Many growers report more uniform plant growth and less foliar disease with subirrigation. The increase in plant uniformity may be the result of more even and complete moistening of the growth medium and better distribution of nutrients absorbed by capillary flow.

Closed Irrigation Systems

In a closed irrigation system (e.g., capillary mat, ebb-andflow, flooded floor), the nutrient solution is recirculated. Nutrients are not allowed to leach into the ground. Water is pumped from a storage tank and delivered to the plant root zone; when the irrigation cycle is completed, the water returns to the reservoir tank. Typically, water is held in the until the medium is brought to field capacity; however, a range (that is, low to high cost) of equipment is available to facilitate and fine-tune this process.

Types of Containers. Pots with holes that extend up the sides work well. Pots with holes on the bottom work best only if there is a ridge on the bottom of the pot to elevate it off the bench floor. Situating the pot drainage hole off the floor of the bench/floor permits thorough drainage of water from the pot when the bench is emptied.

Media. The physical characteristics of media used with subirrigation systems are important. Very light and open mixes do not work well because the reduced capillary pores do not move water to the upper regions of the media very well. Smaller containers should have a much coarser media to provide more air space, and deeper pots should have a finer textured media to facilitate capillary action.

Water Storage Tanks. Irrigation water in subirrigations systems most effectively stored in closed storage tanks. This provides a means to better control water quality as it protects water from outside contaminants in addition to minimizing loss of water due to evaporation. Filling a group of benches or a floor requires that large amounts of water be supplied in a short time.

Irrigation Scheduling. To maximize performance in a subirrigation system, the medium should be allowed to dry out to the point just before stress to the plants occurs. This will vary greatly depending on the type of media used and its moisture-holding capabilities. Many growers irrigate by the “Growers Eye” approach

Nutrient Solution Adjustments. The nutrient solution should be tested for pH and salt (EC) levels periodically. If these levels have deviated, adjustments should be made. Control of the nutrients and flow can be manually. In more sophisticated systems, the solution is automatically tested in the holding tank after each zone is watered. If the EC is too low, due to plants removing fertilizer elements from the solution for example, a concentrated fertilizer stock solution may be added to the nutrient solution to increase the EC to the desired level.

Diseases. There is a risk of spreading soil-borne disease-causing organisms such as Pythium and Phytophthora species in recirculated fertilization solutions. Therefore, after the solution is drained from the root zone and filtered, it may be treated with UV light or ozone to kill any organisms present.

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