# Taking the guesswork out of drip irrigation.

You can figure out how much to water each plant.

Turn the page, and use our charts as a guide

It only makes sense to avoid guesswork in drip watering. You need to decide how much water the plants require, then run the system (automatically or manually controlled) so they get just the right amount when they need it.

To accomplish this, you first have to

change your conception of watering plants. With drip irrigation, one trick is to think in gallons per hour (gph)-not the gallons per minute (gpm) characteristic of conventional sprinkler rates. For instance, water flows out of most sprinkler heads at a rate of at least 1 gpm. If you run such a system for 10 minutes, each head will spew out at least 10 gallons of water. A 1 -gph emitter would have to run 10 hours to achieve that amount.

Also, most people aren't conscious of how much water they give their plants. They turn on the system and let it run for what seems like the right length of time. This may work with sprinklers. It's harder to do with drip, since you can't see the water.

Use the charts to help

determine water needs

To help you figure out how much water your plants need and how often, we've developed the easy-to-use charts on page 175. We've done the complicated calculations. You simply locate the right numbers and then do some simple arithmetic. The top chart tells precisely how many gallons of water each plant needs per day in midsummer. To determine the correct quantity, locate your region, figure out which category-from A to D-your plant fits into, determine its size (measured in canopy diameter), and then locate the corresponding number.

(If all plants on one valve have similar water requirements-native shrubs, for example-it's necessary to calculate the needs of only one plant.)

For instance, if you live in coastal northern California and you want to know how much water your 8-foot-diameter lemon tree requires, locate the coastal chart and find the number where category C and 8 feet intersect. The lemon tree needs 4.2 gallons of water per day.

In most cases, you don't need to water plants every day. So now you have to figure out how often to water them and multiply tbis number by the gallons-perday figure.

Schedule irrigation to encourage deep roots

Your goat should be to water as infrequently as possible, for as long as possible. This encourages deep rooting, so plant roots have a greater reservoir for water. The chart at lower right gives you a conservative estimate for how often to water an average water-loving plant (category A in the top chart).

This chart considers climate and soil type, but it does not factor in rain, wind, sun, and plant type-all of which greatly affect your watering schedule. Also, you'll need to water new plants more frequently until they're established.

It won't harm any kind of established plant to be watered according to this schedule, as long as you apply the right amount each time (from the top chart). Once you become familiar with your drip system and the plants watered by it, you can adjust the schedules. (Remember also that you need to adjust the frequency seasonally.)

For instance, in the Sacramento Valley you may need to water a drought-tolerant tree growing in loam soil only once or twice during the summer instead of every eight days, as shown in the chart. On the coast, a native tree may not need any water other than normal rainfall once it's established.

One way to become more familiar with your plants' water requirements is to learn about each plant. Also, check soil moisture periodically with a soil probe or by digging down with a trowel between, not right below, emitters. As a rule of thumb, if the top I inch (flowers), 2 inches (shrubs), or 4 inches (trees) of soil is dry, you need to water.

Also, monitor foliage color for signs of under- or overwatering, and check for wilting and leaf drop. It may help to set out indicator plants in the garden. These are plants tbat need more water than their neighbors. If you notice that they're wilting or changing color, it's a good indication that the soil is getting dry and your other plants will soon need water.

Getting back to the lemon tree example: according to the lower chart, a 6-foot or taller lemon tree growing in loam soil in coastal northern California needs water every 12 days. Multiply 12 times the gallons-per-day figure, 4.2, from the top chart. It needs 50.4 gallons every 12 days, How do you apply this much water to a plant? If you've designed an adequate system (for information on how to design a drip system, see page 72 of the July Sunset), you would have at least three 1gph emitters on a J-Ioop around the tree. Add up the total gph of emitters around the plant (3 gph if the tree has three 1 -gph emitters around it). Then divide the total number of gallons the plant needs (50.4) by the total output of the emitters (3). The tree needs to be watered for 17 hours every 12 days.

If you don't want to run your system for this many hours, add more emitters (with five 1 -gph emitters, you need to run the system for only 10 hours) or change them to a higher gph (2 gph each, instead of I gph, for example).

Some controllers can be scheduled for only 7 days; others run 15 days. Sophisticated controllers can be scheduled by setting the number of days between waterings. More modest types can only be set to run on certain days of the week (every Tuesday and Saturday, for example). This means that on a seven-day clock, the system will water at odd intervals (first three days apart, then four).

If you have this kind of controller, you'll have to modify your calculated irrigation schedule according to how the controller operates. As long as you determine how much water your plants need in gallons per day and then space irrigations the best way you can, you'll water plants properly.

Watering in saline soils

If your soil or irrigation water is high in sodium or chloride (typical of regions with low annual rainfall), drip systems may not adequately flush the salts away from plant roots. To prevent salt burn, add 10 percent more time to your normal irrigation schedule and flush the soil once a year by running the system overnight.

Even with longer watering times, salts may accumulate (check for a white film on the soil surface); these can leach into the root zone wben it rains. To avoid injuring roots, it's best to leave your system on during periods of light or infrequent rainfall that is not sufficient to soak the soil.