Drip irrigation should be considered the primary method of providing water to your raised bed garden. It is the only method compatible with plastic mulch culture.
Drip irrigation is defined as the frequent, slow application of water to the soil through mechanical devices called emitters. Operated properly, drip irrigation delivers the ideal amount of water to your crops at the ideal rate, thus avoiding the drench or drought phenomenon associated with other watering methods. Less plant stress translates into optimum growth and greater yields.
Every drip system has three general parts: 1) the head unit, which includes all control components and a filter; 2) a transmission system of plastic hose or pipe; and 3) the emitters. Sources of water to supply the system might include a municipal water supply, well or holding tank.
Components of the head unit include a valve to turn water on and off; an anti-siphon valve designed to prevent contaminated water from flowing back from the system into the domestic clean water supply; a pressure regulation device designed to maintain the optimum working pressure; and a filter to prevent entrance of damaging foreign particles such as sand and silt into the drip system.
Water flows from the head to the emitters through plastic pipe or hose or a combination of both. This main line should be buried to protect it from light and physical damage, and to keep the entire installation less cluttered.
The kind of emitter recommended for use in permanent raised bed gardens is referred to as a line emitter or emitter line. They are ideal for closely spaced crops such as flowers, vegetables and small fruits.
Several types of emitter lines are on the market. Some of the more popular include double-walled polyethylene collapsible tubing, soaker hose and hard hose with pre-installed drippers.
The collapsible tubing is commonly referred to as tape. This type of emitter has openings in the outer wall every 12, 18 or 24 inches.
The soaker hose type of emitter is equipped with very fine pores throughout its length. This type of emitter is commonly manufactured from recycled automobile tires.
We have used both the tape and the soaker hose emitters extensively in our raised bed operations. The soaker hose emitter proved unsatisfactory due to its lack of application uniformity. Tape emitters provide uniform water distribution. However, because of their thin wall design, they are susceptible to rodent and bird damage.
The emitter line of choice for use in permanent beds is the thick-walled hard hose with pre-installed drippers at 12- or 18-inch intervals. While initial cost is greater than that of tape, the hard hose emitter has a long service life and can be overwintered in the beds, thanks to its rugged design. Experience has shown hard hose to be the easiest of all the emitter types to install in our raised bed growing system, as well.
In order to develop a shopping list for your drip system, you should first design the system. Refer to the scale drawing of your raised bed garden.
Start by locating the source of water in the drawing. Ideally, the faucet or hydrant should be within 100 feet of the farthest bed. This shouldn't be a concern in most backyard garden situations, however.
If the source needs to be closer to the garden, consider installing a freeze-proof hydrant nearby. Freeze-proof hydrants permit irrigation during the winter months, if necessary. When not in use, water in the hydrant is automatically drained off, preventing freeze-up.
To determine the amount of emitter line to purchase, multiply the number of beds by the length of line per bed. The length of emitter line per bed can vary, depending on the number of lines per bed.
Certain crops such as tomato, squash, cucumber, cantaloupe and eggplant are typically planted in single rows when grown in 40-inch beds on 5-foot centers. In such situations, one emitter line per bed will suffice.
In situations where multiple rows of closely spaced root and leafy green crops are distributed over the entire surface of the bed, two evenly spaced emitter lines per bed provide for a more uniform wetting pattern. Plan on installing two emitter lines per 40-inch bed. You will be pleased with the added performance and flexibility the dual lines provide.
For beds more than 4 feet in width, a third emitter line should be considered. One line should suffice in beds less than 24 inches in width.
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.
When selecting system components, make sure they are adequate or even necessary. Many name brand manufacturers furnish a do-it-yourself guide to assist with component selection.
Anti-siphon valves are mandatory in most municipalities. Check local ordinances to determine the type of device required.
When selecting a pressure regulator, make sure it is matched with the system operating pressure. Excess pressure can cause rupturing of the emitter lines. Inadequate pressure will result in a lowered flow rate, increasing the time required to water.
Drip systems using city water do not require a filter. However, contamination can occur as the result of water line breaks. Therefore, protect your system by installing a filter having a minimum 150-mesh screen size. If you plan to obtain water from a river, pond or lake, the drip system will require a more sophisticated, and expensive filter. Consult an irrigation specialist or product representative for details.
A water timer may sound like a good idea, but, if depended on exclusively, can lead to a false sense of security, not to mention poor crop performance. Plants require water based on need, not on any particular schedule. A plant's need changes based on maturity and weather conditions. Therefore, use a timer only as a last resort.
A fertilizer injector is required if you plan on fertilizing. As mentioned earlier, this is the preferred method of fertilizing crops grown under plastic mulch. Before you purchase an injector, make sure it can operate at the low pressure and flow rates common to small garden drip systems. See the section 'Selection and Use of Fertilizer Injectors' for details.
Placing Emitter Lines
Emitter lines may be buried in the beds or placed on the surface, depending on if plastic mulch is used. For best results using plastic mulch, the emitter lines should be buried. Their presence on the soil surface prevents the film from hugging the surface tightly, a condition necessary for rapid and efficient warming of the beds. On unmulched beds, emitter lines may be buried or left on the surface.
Locate emitter line to best accommodate planting configuration. Plan on spacing emitter lines 12 inches apart (6 inches from centerline) when planting one, two or three rows on 40-inch beds. Examples of crops planted on one, two and three rows per bed include tomato, pepper and strawberry, respectively. Plan on spacing emitter lines 18 inches apart (9 inches from centerline) when planting four or more rows per bed. Examples of crops planted four, six and eight rows per bed include lettuce, spinach and carrot, respectively. Refer to Tables 2-A and 2-B for emitter line placement based on planting (row) configuration.
Regardless of how or where emitter lines are spaced, it is critical they be kept straight. Knowing the exact location of emitter lines is possible only if the lines are installed in a straight, precise manner. This will help to avoid damaging buried lines when planting or cultivating. For surface application, use landscape staples or mound soil over the emitter lines at strategic locations to keep them straight.
In cold weather, hard hose emitter tubing can be difficult to work with. To increase flexibility, warm the tubing by exposing it to sunshine.
The easiest method of burying hard hose emitter line is to press the tubing into the soil with your hands. Press the tubing into the soil at least 1 inch. You'll need to dig a trench to bury thin-walled drip tape or if you plan on burying hard hose deeper than 2 inches.
Be sure the soil profile has been shaped and the soil is in a loose, easy-to-crumble condition prior to pressing the tube into place. Once the lines have been installed, go ahead and firm the soil surface in preparation for mulch application.
Installing emitter lines on the surface of unmulched beds has one advantage: if laid out in a straight line, the tubing can be used as a planting guide. Push planters directed alongside emitter lines will insure straight rows of crops. Uniformly spaced drippers permit emitter line to substitute for a measuring tape, providing uniform placement of transplants. Be sure to firm soil prior to surface application of emitter lines.
To prevent the plugging of drippers by foreign material that might have entered the system during installation, be sure and flush emitter lines prior to closing the ends. It's also a good idea to flush mains and header lines prior to attaching emitter lines.
To extend the life of your system, remove the head unit and store out of the elements during cold weather. The emitter lines can be rolled up and stored or left in the beds. If left in place, protect the tubing from sunlight degradation by covering with soil.
To prevent pipes and tubing from bursting during the winter, expel excess water from the system. On small systems, blow water out using your mouth. On larger systems, use a compressed air tank equipped with a hose fitting.