Most homeowners tend to overlook their ornamental shrubs and plants when they consider watering. This may be because these plants in the landscape usually need the most watering during July and August, when homeowners find lots of other demands on their time, like going to the beach. These plants, and lawns too, go through a dry/wet/dry cycle that stops and starts their growth over and over again. This stop/start routine seriously undermines the plants' health and vigor.
The One Inch Rule
So, the objective of regular watering is to replace the moisture that plants lose from transpiration, so that their moisture levels remain more or less constant. We know that water loss begins at the top of the soil and works its way down. We also know that a plant's root system is generally as deep in the soil as the plant is tall. Research on watering has indicated that 1 inch of water penetrates 24 inches in sandy soil, 16 inches in loamy soil, and 11 inches in clay soil. So when you water by drip irrigation for just 20 minutes, you still may be sending water down fairly deeply, because the water will seep down toward that soil needing replenishment.
A general rule of thumb is to be sure your landscape overall receives about 1 inch of water a week from rain and/or and supplemental watering systems in the spring and fall; two inches in the heat of July and August. This standard may be too much water for some properties and too little for others. The minimum weekly requirement of plants in loamy soil that are mulched will be something less than an inch a week, probably between 1/2 inch and ;3/4 inch, depending on the season. Conversely, those in very sandy soils, containing little organic matter and unprotected by mulch, may need 2 inches of water a week. If you miss a week in the spring or fall, there should be no serious problems for all but the smallest and most tender plants. Soil with lots of compost and mulched garden beds that have a drip irrigation system hold water longer and lose little or none from evaporation.
Exception to One Inch Rule
If your flower beds, trees, and shrubs are well mulched, then the one inch rule shifts down to ½ inch in spring and fall and goes to one inch in the hot summer.
One inch of water puts 1/2 gallon of water on each square foot of a bed of annuals, perennials, and some foliage plants. Likewise, 1/2 inch will then put 1/4 gallon or 1 quart of water on each square foot. To get 1 inch a week on a 200 square-foot garden bed, you need about 100 gallons of water, minus any rain that falls.
If you water deeply, shrubs and small trees can manage fine on once weekly waterings during drought periods. But watering annuals and other tender plants in a garden bed only once a week when there is no rain is not necessarily a good idea. Remember, for small plants consistency is more important than quantity. You do not want the plants to go dry/wet/dry over and over again. If it does not rain, you may need to water your garden beds and lawn two or three times a week. That means putting about a quart of water on each square foot of garden space, resulting in about 1/2 gallon of water a week, less evaporation and runoff. With a drip irrigation system in your beds, run the system three times a week for about 20 to 30 minutes each time. If you get 1/2 inch of rain, wait 2 days and start your 20 minute watering cycle again.
If you are not sure how much water your drip irrigation system releases in 20 minutes, simply place a coffee can under it while it runs and time how long it takes the coffe can to fill up. Then compute how much would have been released in 20 minutes. A 1 pound coffee can holds 1 quart of water. Use this same technique to determine how long it takes an above ground sprinkler system to release 1 inch of water on the lawn or garden bed.
The 20 minutes, two or three times a week, (depending on rainfall), guideline is just that, a guideline. In the summer months, during very hot periods with little rain or breeze, you may need to increase watering to a half hour or more two or three times a week. Feel the ground under the mulch to double check. It should always feel moist. It should never feel dry.
In the spring you may need to water new seedbeds by hand for a week or two until their root systems get down past the 2 or 3 inch depth where they can reach the water from the drip system.
In the fall, you can probably cut back watering to only 10 or 15 minutes twice a week, since plant transpiration slows down as the days get shorter and growth slows.
Of course, overwatering the lawn or garden, causes just as much harm as underwatering. If the soil gets saturated, it has less space for critical oxygen, and plant growth suffers. Overwatering is not a great problem if your soil drains very well. The excess water just passes on down past the roots of the plants. It is a serious problem in poorly drained soil. If puddles form on the surface of the bed and remain for more than a few minutes, you can suspect your plants have too much water. The best solution is to improve soil drainage using the techniques described in chapter 7. In the short term, you must stop all watering and let the soil dry before beginning your watering program again.
Master gardeners who know which plants need extra water at certain times in their life cycles, use the watering guideline outlined above as a base. They offer supplementary water to certain individual plants by hand. For most of us, just the basic watering guideline represents a significant improvement in watering that will be reflected in the health and vigor of the the plants in our landscape.