Black polyethylene (PE) and polyvinyl chloride (PVC) pipe are used most commonly in drip irrigation systems. Low costs and handling ease are primary factors. Low pressure, rigid polyvinyl chloride pipe often is used for supply and header lines because the connections and fittings can be solvent bonded. Polyethylene connections, however, must be clamped or held by a tight fit. Polyethylene pipe normally is used for emitter laterals because it is flexible and easy to handle. Flexible polyvinyl chloride pipe is less sensitive to high temperature and sunlight and is more durable, but it is also more expensive. Water for processing and human consumption should not be supplied through non-sanitation-approved pipe.

A ½ inch polyethylene pipe is adequate for emitter lateral lines. Where manufactured emitters are used, ½ inch pipe will provide equal water distribution and uniform water application throughout the usually 100 to 150 foot greenhouse irrigation runs. Of course, other irrigation system components must be properly designed and selected.

Plan a ½ inch lateral emitter line for each plant row unless experience suggests a successful alternative. Place lateral pipe containing emitters at the base of plants. Keep the pipe in line on the inside of the plant row with respect to the working aisle. Soil in the working aisle can become compressed by traffic. With emitter lines on the aisle side of the plants, water tends to flood the aisles.

When drip irrigation is installed in an existing greenhouse with rows spaced 20 inches apart or closer, one emitter line may be sufficient for two plant rows. However, production is normally significantly higher where plant rows are spaced further apart. Consider wider row spacing for future crops and plan the system accordingly.

Do not place a single emitter line serving two rows in a furrow or ditch. Water will not move up the soil incline and across the row to properly wet soil in the aisles. In this case, no roots develop in the aisle soil and plants tend to stress for water sooner during high water requirement periods.

Polyethylene pipe, which is sensitive to high temperature, will contract and expand and can move out of position. Light springs, such as screen door springs, can be attached to the lower end of emitter lines with the other end fastened to a stake or wall. This helps hold the pipe next to the plants. Adjust the fasteners tension while the system is irrigating and the pipe is cold and contracted.

Pipe 3/4 to 1 inch in size is usually large enough for header lines, not main supply lines, for 5,000 square feet of growing area. Make the connection to the supply pipe at the center of the header pipe rather than at the end. A tee connection into the center of the header line equally divides the water supply and reduces water pressure losses.

Plug or tape the ends of all piping and fittings during installation except when making the final connection. This keeps soil and other particles out of the system and reduces drip emitter plugging.