Because water pressure is not usually a limiting factor as it is in sprinkler systems, and because they can be added to or changed at any time, drip systems are not usually planned with as much care as sprinkler systems. You will need to plan where you will get the water, how you will get it to the plants you are watering, and what kind of emitters you will use. Unless you are planning a very large system with hundreds of emitters, there is no need to measure water pressure or flow rate, as there is with sprinkler systems.
For Large Systems
If you are planning a very large system, measure the flow rate of water from the valve you are intending to use for the drip system. With the valve turned on all the way, see how long it takes to fill a 1-gallon bucket, in seconds. Divide that number into 60 to get the flow rate in gallons per minute (gpm). Multiply by 60 to get gallons per hour (gph), which is the figure used with drip irrigation. To allow for friction loss in the pipes, dropping water pressure while somebody takes a shower, and the like, use 75 percent of the gph to get a real-world estimate of the flow rate available.
Example:
- The bucket fills in 5 seconds
- 60 divided by 5 equals 12 gpm
- 12 gpm times 60 equals 720 gph
- 720 gph times 75 percent equals 540 gph maximum usage
You will use this number to plan how many emitters you can have on a single zone.
Getting Water
Select a spot to attach the drip system to the household water. This might be a hose bibb, an electric or manual sprinkler valve, or you can cut in a new valve at any place in the line. If you select a hose bibb, attach a Y connector to the bibb so you can still use it for a garden hose if you wish.
You can use the sprinkler controller to run the drip system if it can be adjusted for the long periods of time that drip irrigation needs — sometimes several hours a day. If it can’t be adjusted to those long periods, or if you don’t have a sprinkler controller, you can purchase a simple battery-powered controller for the drip system that is perfectly adequate. Because they are powered by batteries, these controllers can be placed anywhere in the garden. They are commonly sold with hose threads for attaching to hose bibbs.
Water can be transported from the source to the plants through 1/2-inch polyethylene hoses, or a more permanent PVC pipe system can be installed. Polyethylene drip hose is not as strong or stable as PVC pipe. You can cut it with a shovel without noticing you have cut it, and rodents are fond of chewing on it. But it is cheap and is simple to install. PVC pipe is usually buried, which entails digging ditches, so is more trouble to install, but it is tougher and more secure than drip hose.
Emitters
Several types of emitters are available for emitting water at the plants. These can be mixed on the system, using different types for different situations. It isn’t necessary to plan them in detail, other than to have enough of the right parts available when you install them.
In parts of the garden where plants are close together, such as flower and vegetable beds and groundcovers, emitters are most easily placed on a regular grid. In places where plants are more scattered, or isolated from the rest of the garden, emitters are placed at each plant.
Laying Out Mass Plantings
In mass plantings, drip irrigation pipe is generally laid out as a series of relatively parallel lateral lines that eventually form a rectangular shape. Using parallel lines set 12 to 24 inches apart (that is, at distances where two water spreads meet) is the most efficient way to ensure even watering. Set the lines closer (12 inches) in sandy soils, farther apart (18 to 24 inches) in loam or clay soils.
Connect parallel lateral lines to a single 1/2-inch supply line, called the header line, which connects back to the main supply line. Do not add emitters to the header or main supply lines. Connect the supply and lateral lines using tees and elbows. The end of each lateral line must be closed off. This is done with an end closure.
Each line can terminate with its own end closure, or you can close the rectangle by adding another piece of supply line that leads to a single end closure per zone. The latter option takes more effort to install, but is less work in the long run because drip irrigation lines must be drained periodically, and it is easier to remove two or three end closures than several dozen.
For areas watered by parallel lateral lines, you can use punch-in emitters and emitter lines interchangeably. Porous pipe can be substituted as long as you take its slightly irregular water delivery into account. Some manufacturers suggest 1/4-inch vinyl tubing with in-line drip emitters for areas where short lateral lines are needed, but such tubing is more delicate to work with than standard 1/2-inch poly pipe and is best restricted to hanging baskets and containers, where thicker pipe is undesirable.
Laying Out Isolated Plantings
It is inefficient to water isolated or well-spaced plants, such as trees, using parallel line irrigation M you’d have more pipe than emitters. instead, run a single 1/2-inch supply line to within the drip line of the plant. If there are several well-spaced plants in the vicinity, snake the supply line toward each, or use tee connectors to attach additional lateral lines. There is no need to use straight lines because poly pipe is flexible.
Isolated plants will absorb water from anywhere within their root area, but, for more efficient results, place emitters or an emitter line a few inches from the base of smaller plants and at about one-third to one-half the distance away from the center of the foliage canopy for larger plants. Watch the plant over time and adjust the number or size of emitters as needed. For example, a small shrub needs only one emitter, but a tree with a 15-foot canopy requires six.
Adding Emitters to the Layout
There are several ways to water a plant once the pipe has reached it. Punch emitters directly into the line. Use a short section of line as a feeder line leading from the supply line to within the plant’s canopy, and add emitters. Use either 1/2-inch poly pipe or 1/4-inch vinyl. For plants where four or more emitters will be needed, loop a short section of line around the plant and attach it to the supply line with a tee connector. This is called a pigtail. You can use emitter line, porous pipe, or plain 1/2-inch poly pipe and emitters.
Number of Emitters per Plant Based on Canopy Diameter
| Plant Canopy Diameter | Emitters per Plant |
| Up to 3′ | 1 |
| 3′ to 5′ | 2 |
| 6′ to 9′ | 3 |
| 10′ and more | 1 per 2-1/2 feet of canopy diameter |