With the concerns over the availability of water and the potential for groundwater and surface water pollution, growers are expanding their use of low-volume irritation. Since the 1950s, drip tape irrigation systems have been continually improved and are adaptable to many crops.
Drip irrigation can be a valuable tool for accurate growing medium moisture control. Besides saving water and labor, drip irrigation reduces the potential for groundwater pollution.
One of the most common applications is to supply water and nutrients for bench-top, capillary mats. Two or more drip lines are placed atop the mat. Water is dispersed laterally by the mat material, making an even supply available to each pot.
Vegetable crops, such as tomatoes and cucumbers, when grown in beds, bags or pots, are frequently watered with drip tapes. Tubing is placed atop the ground or container or woven through the bags. Uniformity of application and low cost make this system an excellent choice.
Drip Tape for Hanging Baskets, Outdoor Crops
A more recent use of drip tape in the greenhouse is for hanging baskets. This has proven to be a great labor saver. The tube is usually attached to the overhead pipe that supports the baskets.
Where limited water supply is available, a drip system is a good choice for outdoor container crops such as pot mums, perennials in larger pots and nursery stock. It will save as much as 80 percent of the irrigation water compared to an overhead sprinkler system. One disadvantage: unless emitters with a shutoff are used, water is wasted if individual containers are removed.
Types of Drip Tape, Tubing
There are many variations available, including double-chamber, porous tubing and in-line emitters. In all cases, the goal is to take low-pressure water (8 to 20 pounds per square inch) and reduce it further so it exits the dripper as droplets.
Double-chamber tapes are fabricated of thin black poly, 4 to 20 mil thick. The inner tube distributes high-pressure water the length of the tube. Small holes punched at even intervals allow some water to flow into the outer tube. Here, tiny holes (about their size of a pinhole), spaced from 4 to 24 inches apart allow the water to drip onto the crop. The double tube provides even pressure and flow over longer lengths and is generally referred to as pressure compensating.
Porous tubing is made from recycled rubber. Water supplied to the inside of the tube oozes out at a slow rate. It is generally best suited to landscape applications as it is expensive and has limitations on lateral length and output uniformity.
There are several systems that use in-line emitters. Some manufacturers make the emitters from the same material as the tape. Others insert molded plastic emitters into the tape wall.
In the above systems, flow rate is measured in gallons per minute per 100 feet of length. Rates from 0.2 to 2 gpm/100 feet are common. Variations in pressure have little effect on flow rate but higher pressure will increase the length of tape that can be used.
Thin-wall tubing is used for more permanent systems. In one method, the tubing is cut into sections, and an in-line emitter is inserted. Another method is to punch holes in the supply tubing and insert separate button-type emitters. With these methods, spacing and emitter output can be selected by the grower. In all systems, the purpose of the emitter is to reduce water pressure.
Thin-wall tapes (4 to 8 mil) are used by growers who discard them after each crop. Thicker-wall tapes and tubing can be reused. These also give greater mechanical protection when installed outdoors or in ground beds.
Uniformity of Water Application
For most applications, irrigation uniformity is important, especially for small containers. The way these systems are manufactured and installed can affect this uniformity.
Some points to remember:
- The coefficient of manufacturing variation (CV) is a measure of quality control, the amount of variation in output between individual emitters. It is established by comparing a large number of emitters at the same pressure. A variability of less than 0.05 is excellent. A CV of greater than 0.2 is considered unacceptable for most applications. CV values are available from manufacturers or from university research reports.
- For most greenhouse applications, pressure-compensating emitters should be used. Output is not affected by pressure changes, length of line or elevation difference.
- Water filtration with a 150 or 200 mesh screen is important to remove particulate matter. Where chemicals are present in the water, select tapes or emitters with greater flow rates as fine holes or passages can be clogged. Provisions for flushing the lines by removing the end plug should be made.
- Length is usually not of concern in greenhouse applications. Most tapes and tubing can be several hundred feet before there is a pressure drop due to friction loss.
Extension Professor Emeritus and Agricultural Engineer
NRME Department, University of Connecticut, Storrs CT 06269-4087