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Drip Irrigation for Greenhouses

Posted November 18th, 2015 by Eric Hopper in

Commercial horticulturists are constantly trying to find ways to most efficiently use their resources. Efficiently utilizing water and nutrients not only promotes healthy plant growth but also saves money and makes horticulture more sustainable. Considering the rising costs of water and electricity, teamed with a heightened sense of personal responsibility in regard to our planet’s health, it is no wonder that greenhouse hobbyists and home gardeners are actively implementing techniques aimed at increasing efficiency. One particular system that can significantly increase efficiency in multiple ways is a drip irrigation system.

Drip irrigation systems are horticultural watering systems that localize the watering (and fertilization) process in order to provide a precise amount of water (and nutrients) directly to the root zone of the plants. The key benefits to drip irrigation systems include less water loss during irrigation, more control over “zones” in the greenhouse or field, and precise watering and fertilizing that can be tailored to a specific crop or a specific stage of growth.

The biggest advantage of drip irrigation has to be its efficiency in terms of water loss. On average, the typical overhead irrigation systems are only 50% efficient. When you consider that drip irrigation is over 90% efficient, it is easy to see why more horticulturists are turning to drip irrigation systems for their crops. Drip irrigation systems are straightforward in design and relatively easy to maintain. Drip irrigation systems can generally be broken down into three parts: the water source, the pumping system, and the distribution system.

Water Source

The first thing any gardener who wishes to set up a drip irrigation system should consider is the water source. Typically, a small greenhouse grower will use city water or well water to provide water to the plants. If using a well or a holding tank, it is important to make calculations or to consider the volume of water that the system will require. The last thing a gardener needs is the well or holding tank running dry during the summer because the system wasn’t properly sized. In other words, the reliability of the water source should be a major consideration.

The quality of the water source is another significant consideration. A water source that is dirty and requires filtration or other treatments will create additional costs in the long run. As with any other horticultural application, a grower looking to set up a drip irrigation system should look for the least expensive, yet highest quality, water source available.

Pumping System

Many home hobbyists will connect their drip irrigation systems directly to an outdoor water spigot. For smaller drip irrigation systems, this will work just fine. However, for larger systems or for growers not on a municipal water system, a holding tank and a water pump will be necessary. The pump size will be determined by the size of the irrigation system and the type of distribution system that is being used. The emitters will generally determine the required amount of pressure (pounds per square inch or psi) and the required volume (gallons per minute or GPM) that is needed in the garden space.

The psi of the system is determined by the psi of the spigot or the pump manufacturer’s rating. Although the pump will have its own GPM rating, the length of the main water line and the feed-lines will reduce the manufacturer’s rating. To accurately determine the GPM of a drip irrigation system at the garden site, a grower must measure the volume his or herself. This can be done by operating the system and filling a container for 15 seconds. After 15 seconds, multiply the volume of water in the container (in gallons) by four and you will come up with the gallons per minute.

Distribution System

The distribution system for a drip irrigation system consists of a series of pipes, hoses, and, in most cases, emitters. A typical distribution system starts with the main water line. This line is the line that is directly connected to the water source or pump. The main line’s purpose is to provide water to the growing area at full psi. The main line can be made of various materials but is most commonly made of PVC pipes. The sub-main lines of a drip irrigation system are the lines that branch off the main line and run down the length of the crop rows.

A typical sub-main line will be made of 1/2” or 3/4” poly-tubing. For large systems with a lot of pressure, a hose bib assembly, which includes a pressure reducer and a pressure gauge, is connected to the main line prior to the sub-main lines. This allows the grower to monitor pressure at every sub-main connection. For small greenhouses, the main line and the sub-main line will be one in the same and will generally not require hose bib assemblies with pressure gauges.

Attached to the sub-main lines are the feed lines. These are normally 1/4 inch poly lines that run from the sub-main line to the base of each plant in the greenhouse. Typically, at the end of each feed line is a drip emitter. The emitter is usually the size of a quarter and is designed to drip a particular volume of water right at the base of the plant. For some larger crops, like tomatoes, a grower may want two or more drip emitters at the base of the plant to ensure the entire root area is provided with water.

For commercial growers, a popular substitute for drip emitters is drip tape. Drip tape is essentially feed lines with holes every 10-12 inches. These work great for growers with long beds where the line can be run the length of the crop row. When configuring a drip irrigation’s distribution system there are no rules. Water lines can be above or below ground and the configuration can vary greatly from system to system. As long as each drip emitter is receiving adequate pressure, there is no right or wrong way to configure a drip irrigation distribution system.

Emitters and Drip Tape

As previously mentioned, the final piece of the drip irrigation system is the component that actually delivers the water. This is typically done through an emitter or drip tape. Drip tape seems to be more popular with larger operations that would otherwise require so many feed lines off the sub-main water line that it would create a spaghetti-like mess of lines. Drip tape operates at 10 psi and each “line” of drip tape can be operated by an in-line valve. Most emitters will operate at 35 psi and can have individual in-line valves placed at each emitter. In other words, the emitters give the horticulturist increased control over irrigating specific plants or specific zones. It is important to remember the difference between a drip emitter and a sprayer.

A sprayer is another component used for water delivery in a pressurized watering system. Sprayers are usually staked a couple of inches into the soil and create a spray of water directly above the soil’s surface. Many plants will grow just fine when using sprayers but there are a few things to consider. First, a sprayer is generally not pressure controlled. This means that sprayers placed closer to the main water line will have a greater output volume than sprayers placed farther away. Also, thought must be given to the disruption of the soil and the potential damage to the plant’s stem created by a pressurized spray of water.

A final consideration is that sprayers introduce the water to the air which creates a higher loss of water through evaporation. In my opinion, if a grower is going through all the trouble to set up a pressurized watering system, he or she might as well gain all the benefits of the drip irrigation system and use drip emitters.

Drip irrigation is one of the most effective and efficient ways a grower can automate watering the plants in a greenhouse. When done correctly, a drip irrigation system will be 90% efficient in terms of water usage and allows for precise control. With the use of in-line valves placed at each feeder line, a grower can easily set up the system to water specific areas or plants in the garden.

Heightened control and efficiency are two things any greenhouse gardener can get excited about. My favorite aspect of drip irrigation is that it is relatively easy to set up and maintain. In other words, don’t be afraid to experiment with drip irrigation in your greenhouse. Even if you just set up one raised bed or a few planting containers with a drip system, you can quickly experience the multitude of benefits drip irrigation has to offer.

Special Drip Irrigation Vendor Offer

The new #548 Digital Mist Timer can be used for propagation, green houses and mist systems. This versatile and affordable timer requires only 6 psi to operate, has 16 preset programs starting as little as 5 seconds. The Digital Mist Timer features an easy to set push button display, weather proof clear cover, a 9V battery for long life and a 1 year warranty. Save $5 by entering code GGM at MrDrip.com. Offers ends 6/1/15.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.

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