Permanent Raised Bed Gardening: Installing a Drip Irrigation System

Freeze-proof hydrant with components, top to bottom: hose bib, anti-siphon valve, pressure regulator and filter

Once you've determined the amount of emitter line to purchase, calculate system flow rate. The flow rate for any type of emitter line will be given in either gallons per minute (gpm) per 100 feet of tubing or gallons per hour (gph) per dripper.

For example, the manufacturer of one popular hard hose product lists flow rate on a per dripper basis. Calculate system flow by multiplying total number of drippers by dripper flow rate. If your system contains 400 drippers and each is rated at 1 gph, the water source would need to supply 400 gph or 6.6 gpm to operate the system.

Most typical house faucets are rated at 5 gpm. In the above example, one faucet would not supply enough water to operate the entire system at one time. To remedy this problem, you can add another faucet or, probably a better choice, water half the garden at a time. This can best be accomplished by installing two sub mains, or "header" lines, each controlled by a valve. Another option is to install a small valve on the supply line to each bed.

Assuming beds are arranged side-by-side, the most practical design consists of the main supply line extending the width of the garden down one end of the beds. The individual emitter lines that service each bed arise from the main line.

Where sub mains are required, the main line need only extend half the width of the garden. At this point, each sub main is connected to the main via a "T" fitting and valve with each sub main extending in opposite directions.

The system flow rate will determine the size of main line required. A 1/2-inch line is sufficient for flow rates up to 2 gpm; a 3/4-inch line is sufficient for flow rates up to 4 gpm; and a 1-inch line can handle flow up to 8 gpm. This rule, however, applies only to pipe less than 100 feet in length.

For design specifications on drip systems requiring in excess of 8 gpm and/or having main lines in excess of 100 feet in length, consult an irrigation design manual or seek the advice of a knowledgeable individual.

If you have designed your drip system to be supplied by a house faucet or freeze-proof hydrant, plan on using components equipped with standard hose fittings. This will greatly simplify things, not to mention speed up the installation process.

All the major components of a drip system, including anti-siphon valve, pressure regulator, filter, timer, fertilizer injector and line connectors can be purchased with hose fittings. Irrigation stores, mail-order irrigation supply companies, garden centers and many home improvement centers carry a complete line of system components and drip hose.