Landscape Irrigation Equipment Part 2: Drip Irrigation

Prepared by W. Bryan Smith, Area Extension Agent, Agricultural Engineer, Clemson University. (New 5/08.)

HGIC 1811

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Drip irrigation is quite different from “conventional” irrigation methods. Sprinklers and spray heads apply water to an entire area, while drip irrigation components apply water only to the root zone area near the plant. This allows a drip system to provide needed water to the plant without irrigating large portions of unplanted ground that may harbor weeds and weed seed.

Drip irrigation systems apply water directly to the ground (with some exceptions). A drip irrigation system or zone can be operated at any time of day with little concern for disease pressure, since the plant foliage is not wetted during irrigation. Applying water directly to the ground also makes drip systems highly efficient. Figures of 90% to 95% efficiency are regularly associated with drip.

Drip irrigation has gained a wide acceptance in the residential irrigation market over the last several years. Initially, many irrigation installers did not include drip irrigation in their designs simply because they were not familiar with the equipment or proper installation practices. However, the benefits of drip have slowly made it an indispensable part of the irrigation designer’s toolbox.

There are a number of pieces of equipment that make up a successful drip irrigation system. Understanding what these pieces are and why they work better than simple “holes in a pipe” will help us utilize a drip irrigation system to its fullest advantage.

Drip Emitters

The drip emitter is the heart of a drip irrigation system. A drip emitter “emits” water, but more importantly it regulates the water flow.

1/2 gallon per hour drip emitter in operation
Figure 1. A 1/2 gallon per hour drip emitter in operation.
(Photo credit: Bryan Smith)

Consider a long pipe placed in a flower bed with holes drilled every foot or two. If we attach a garden hose to this pipe and turn on the water, quite a bit of water will squirt out of the first hole in the pipe. The second hole will also provide a good bit of water, but not quite as much as the first due to a slightly lower pressure. Each successive hole will provide slightly less water than the one before it. The last hole in the pipe will be applying substantially less water than the first one.

The drip emitter helps us overcome this problem. Each emitter provides a given flow rate of water. The emitter is designed so that the flow rate will change very little with changes in pressure. If we were to install emitters in each hole in the pipe in that flower bed, the first emitter would regulate the water flow exiting the first hole to some level, which would be quite a bit less than the open hole was providing. The second emitter would regulate the flow from the second hole to approximately the same flow rate. The last emitter would also regulate flow from the last hole – to approximately the same flow rate as the first emitter. Now we have an irrigation system that applies the same amount of water to each plant simply by installing emitters.

Drip emitters come in a myriad of shapes, sizes and colors. Some are pressure-compensating, meaning their flow rate changes very little with variations in pressure. Others are of a turbulent-flow design, which is less expensive but allows a little more variation in flow with pressure change. Either type will work well in a typical landscape.

Emitters of the same flow rate from diffrent manufacturers
Figure 2. Emitters of the same flow rate from different manufacturers.
(Photo credit: Bryan Smith)

Emitter Flow Rates

There are three standard emitter flow rates:

  • ½ gallon per hour (2 liters per hour)
  • 1 gallon per hour (4 liters per hour)
  • 2 gallons per hour (8 liters per hour)

These flow rates are used internationally. Emitters made in the United States will be sold with the gallon per hour flow listing, while emitters made overseas will be sold with the liter per hour flow listing. A single manufacturer may make a number of different emitter styles, but each style will normally be offered in these three flow rates.

The manufacturer will normally use the same emitter body of a given style for all three flow rates of emitters sold. Sometimes the emitter body will be color-coded based on flow rate, other times there will be a tiny number stamped in the emitter body indicating the flow rate (such as “1” or “4” depending on where it was manufactured).

Multi-Outlet Emitters

Multiple outlet emitters are also available. These are simply five or six separate emitters placed inside a single body or housing. These are normally used with 1/4 inch distribution tubing (called “spaghetti” tubing) to provide water to several plants or pots in a single location.

The Drip System - Flexibility in Action

The incredible flexibility of a drip system is due in part to the three emitter flow rates. A one gallon per hour emitter may be placed near a small shrub, a pair of two gallon per hour emitters may be placed around a small tree, and a one-half gallon per hour emitter near a small flower – all on the same section of drip tubing. If the area is irrigated for one hour the flower receives one-half gallon of water, the shrub receives one gallon, and the small tree receives four gallons. This incredible flexibility on a single zone of drip irrigation makes it quite attractive.

Drip emitters typically have a plastic barb on the bottom that is inserted into a hole punched into the drip tubing (more in tubing later). No glue, thread sealant, or Teflon tape is required. The hole in the tubing is made with a punch sized for that particular emitter. Extra holes punched into the tubing can be plugged with an inexpensive “goof plug.”

Installing an emitter is as simple as punching a hole, then inserting an emitter
Figure 3. Installing and emitter is as simple as punching a hole, then inserting an emitter.
(Courtesy Dr. Tony Tyson)

Many emitters have a small shank or barb outlet on the top suitable for ¼ inch distribution or “spaghetti ” tubing. This allows a small amount of tubing to be added to the emitter to distribute water some distance from it. Be careful when adding spaghetti tubing to an emitter outlet. The emitter provides only a tiny bit of pressure to the outlet, which is not enough to “push” the water very far. One or two feet of spaghetti tubing can be used on a level surface with few problems, but 6 to 10 feet of tubing will insure the water does not reach the end of the tubing. Also, the small amount of pressure on the emitter outlet is not enough to “push” the water very far vertically. The water may rise 6 to 12 inches, but will certainly not reach a hanging basket 8 feet above the emitter. Barbed couplings or “transfer barbs” will allow the spaghetti tubing to be attached directly to drip tubing for this application, with the emitter placed on the end of the spaghetti tubing. Care should be exercised even in this case not to use a long piece of spaghetti tubing.

Emitter with spaghetti tubing attached
Figure 4. An emitter with spaghetti tubing attached.
(Photo credit: Bryan Smith)

Drip emitters in the landscape are always placed above ground and under mulch. If the emitter is buried, dirt may plug the outlet or roots may grow into it. If the emitter is placed above a layer of mulch, the mulch must first be completely saturated before any water reaches the soil. This will degrade the mulch more quickly and require much more water to properly irrigate a plant. Since mulch eventually degrades into a soil-like material, it is always a good idea to pull the drip tubing up on top of the old, degraded mulch before adding new mulch to a bed.

Drip Tubing

Drip tubing comes in a number of sizes ranging from ¼ inch up to 1.5 inches or more (in various styles). It is always black in color, sometimes with a colored stripe or identifying mark that is a trademark of the company (and serves no other purpose). It is normally sold in 100, 500, or 1,000 foot rolls. It is sunlight resistant and will last 30 years or more in direct sunlight.

1000 foot roll of 16mm drip tubing.
Figure 5. A 1000 foot roll of 16mm drip tubing.
(Photo credit: Bryan Smith)

Drip tubing is made for use with drip emitters and microsprayers see (HGIC 1812, Landscape Irrigation Equipment Part 3: Microspsprayers & Microsprinklers.) It is quite flexible and will expand and contract with heat. If the tubing is placed in a bed on a hot summer day, the installer should run cold water through it before inserting emitters. A hot piece of drip tubing 100 feet long may shrink up to one foot when cold water is added to it, pulling the last emitter one foot away from where the installer intended it to be. The cold water (normal temperature irrigation water in this case) will cause the sun-heated, expanded tubing to shrink or draw up to the length it will maintain during irrigation. Emitters can then be placed in the tubing with confidence (after first turning the water off!).

The most common drip tubing diameter sizes are:

  • ⅜ inch (10 mm or “380” size)
  • ½ inch (13 mm or “500” size)
  • ⅝ inch (16 mm outside diameter or “600” size)
  • ⅝ inch (16 mm inside diameter or “630” size)

The most popular sizes are 16 mm diameters (600 and 630 sizes). These sizes are large enough to provide adequate flow for an entire foundation planting of a normal house, yet small enough to be inconspicuous in the landscape. The 10 mm size is also commonly sold, but is small enough to restrict flow (due to friction loss) if a large number of emitters are used.

Each tubing size requires a specific size fitting. Fittings are available in tees, elbows, and adapters to male garden hose thread (MHT) and female garden hose thread (FHT) as well as a few others. The fittings attach to the drip tubing using a compression-type connection which does not require tools, thread sealants, or glue. A sharp knife will easily cut the tubing to the desired length (exercise care with all tools and sharp objects – safety first).

The open end of the tubing is plugged with a “figure 8” clamp that actually looks like the number 8. Two to three inches of the tubing is inserted through the bottom hole of the “8,” and then the tubing is bent back to form a “kink.” The free, “kinked” end of the tubing is pushed through the upper hole of the “ 8” to hold it in place. There are no tools required and it is quite easy to do.

Drip tubing is generally “snaked” or curved through the landscape bed, placing the tubing near the various plants to be irrigated. Tees and elbows are inserted where necessary to irrigate more than one row of plants in an area. A bed of closely-planted groundcover (such as liriope) may be irrigated with a 2 foot by 2 foot “grid” of tubing and emitters. Place the drip tubing in rows through the bed with the rows 2 feet apart, then insert 1/2 gallon per hour emitters in each row of drip tubing spaced 2 feet apart along the tubing. The 2 x 2 grid provides coverage to the entire area. (One note of caution - this “grid” may apply water more quickly than the rest of your drip system. You may want it on a separate zone.)

Dripline

Many companies provide the “600” size drip tubing with emitters pre-installed inside the tubing on an even spacing, such as 12, 18, or 24 inches. This dripline greatly reduces the amount of labor required to install a drip irrigation system. It is only sold in 1,000 foot rolls (or larger).

1000 foot roll of dripline with emitters pre installed.
Figure 6. A 1000 foot roll of dripline with emitters pre-installed.
(Courtesy Rainbird Corporation)

Dripline may be a perfect fit for an orchard or an area with regularly-spaced plants. In the landscape dripline may provide too many or two few emitters to water randomly-spaced plants in a bed. If the homeowner plans to irrigate the entire bed with a 2 x 2 grid of emitters, dripline would be an excellent choice. If their preference is to water a number of different plants in various areas, drip tubing with separate emitters would be a better selection.

Filtration & Pressure Regulation

All drip irrigation systems require some type of filtration and pressure regulation. Drip systems operate quite well with a water pressure of 20 to 30 psi, but water pressures higher than 40 psi can cause ruptures in the tubing or emitters to “pop” out. Pressure regulators maintain the optimum pressure for the system, regardless of fluctuations in the irrigation system pressure.

Filters are also required for drip systems due to the tiny openings in the emitters. More information on pressure regulators and filters is provided in brochure HGIC 1814, Landscape Irrigation Equipment Part 5: Filters & Pressure Regulators.

Problems for Drip Systems

There are two special problems that may cause difficulty for a drip system regardless of the filtration system used. The first is pond or surface water. Surface water naturally contains a large amount of sediment and organic matter. The high load of sediment can plug a normally-sized screen filter in a short time. Usually some type of self-cleaning screen filter or a sand media filter (similar to a pool filter) is used to prevent frequent plugging.

Surface water will also contain algae that may grow on the filter screen, causing frequent plugging problems. Usually a small but continuous injection of chlorine into the irrigation water will prevent algae problems. Chemical injection of any type requires certain safeguards to be installed in the irrigation system to prevent backflow into the water body. For a small drip system in a landscape it will be less expensive (and much more convenient) to use well or municipal water for the drip system and surface water for the sprinkler system.

The second potential problem for a drip system is iron in well water. The iron will remain in a liquid form and flow through the filter regardless of the filter mesh size. When the iron leaves the drip emitter or microsprayer and contacts the air, it will oxidize into iron oxide, which is a solid. In a normal sprinkler system with large nozzle openings this is not a problem, but iron oxide deposits will plug an emitter or microsprayer in a very short time.

If the homeowner suspects that iron is present in the well water to be used with a drip system, the first course of action is to have the water tested for iron. If the iron content is less than 0.1 parts per million there will not be a plugging problem. If the iron content is 0.3 parts per million or more there will definitely be a plugging problem. Test the water before installing a drip system if there are reddish-brown stains in the sink or tub.

Summary

Drip irrigation is an excellent addition to any landscape - easily installed and modified. However, it must be managed properly, just as any other irrigation component must be, for good results.

Excerpted from the 2007 South Carolina Master Gardener Training Manual.

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