By Dan Gill

LSU AgCenter Horticulturist

(07/28/17) Although Louisiana receives on average about 50 to 60 inches of rain annually, it doesn’t occur evenly throughout the year. As a result, dry spells are not uncommon, particularly during the heat of mid- to late summer. At those times, we may need to provide supplemental irrigation to flower beds, shrubs, lawns and newly planted trees.

How often we need to water our landscapes varies depending on such factors as temperature, rainfall, humidity, season, plant material and light intensity. Plants need to be irrigated more frequently, for instance, when temperatures are high, the plants are growing in full sun and there is a lot of root competition for the water in the soil, for instance when a tree is nearby or in a thickly planted bed. When you do water, water thoroughly and deeply. Then learn to evaluate the condition of your soil and plants to know when you need to water again.

When the weather is very dry, watering by hand is not effective for irrigating plants growing in the ground. When you water holding the hose with a nozzle or your thumb over the end, water is applied too quickly over too short a time for it to penetrate deeply into the soil. For proper irrigation, water needs to be applied slowly over a sufficient period of time to allow it to soak into the soil.

Enough water should be applied to penetrate the soil about 6 inches to irrigate thoroughly. Applying about an inch of water to medium-textured soils will generally accomplish this. Sprinklers are most frequently used to water landscapes, so the question is, “How long should I leave my sprinkler on to apply about an inch of water?”

Most gardeners irrigate with various types of sprinklers attached to the end of a hose. To figure out how long to leave your sprinkler on, place several empty cans in the spray pattern of the sprinkler. Turn on the sprinkler and check the time. When about an inch of water has accumulated in most of the cans, check the time again. That’s how long it takes your sprinkler to apply an inch of water and how long you should leave it on to thoroughly irrigate an area. The best check of how thoroughly an area has been watered is to go back about 15 minutes after watering and dig into the soil with a trowel. Did the water penetrate about 6 inches? Check several places. This procedure also works to calibrate an installed irrigation system.

In some situations, such as on slopes and heavy clay soils, the water may need to be added even more slowly to reduce runoff. Run the sprinkler on for 10 to 15 minutes and off for 15 to 20 minutes and then on again for 10 to 15 minutes until you’ve applied an inch of water.

Morning is the preferred time to irrigate. This provides plants adequate moisture going into the hottest time of the day when they need it most and also allows the foliage to dry rapidly, reducing the possibility of foliar disease problems. Watering plants while the sun is shining on them will not injure them.

You can use a variety of methods to irrigate your landscape. You may even use different methods in different areas. Soaker hoses are made of a material that oozes water slowly and are ideal for watering beds. They apply water very efficiently and do not wet foliage (reducing potential disease problems) and can be left in place or moved easily. Soaker hoses must be laid fairly close to the plants to be effective, so snake them throughout a bed around the plants. Then, cover them over with a mulch so they are not noticeable.

Many types of hose-end nozzles are available for hand watering plants in containers and newly seeded beds as well as rinsing off foliage. Although hand watering is excellent for plants in containers, remember this is not the best way to irrigate plants growing in the ground.

The most common and popular method of irrigation is sprinklers, which are the only practical way to water lawns. Installed underground systems and hose-end sprinklers are the two basic types. Underground systems are effective and convenient, but they are expensive to purchase and must be installed professionally. Make sure the company or individual who installs your system has a Louisiana state irrigation contractors license. Often these systems are put on an automatic timer for convenience, but they are frequently set to come on too often and for too short a time.

Most home gardeners use hose-end sprinklers. They are less expensive and do not require professional instillation, but they are inconvenient to use because they must be moved around to cover large areas and generally must be picked up after use. Many types of sprinklers cover areas of various sizes and shapes. Watch spray patterns and include sufficient overlap to ensure even watering of an area.

Remember, proper watering is a function of applying the right amount of water at the appropriate times. If you water thoroughly as recommended but too often, you will keep the soil saturated with water and reduce oxygen availability to the root system. This will impair the function of the roots and may lead to root rot. Overwatering, then, is more destructive than watering too little. Never irrigate if you check the soil and it is still damp.

Finally, don’t forget to keep your landscape beds and vegetable gardens well mulched to conserve soil moisture and reduce the frequency of watering.

Automatic sprinklers can provide irrigation to planting areas during periods of low rainfall. Photo by Rick Bogren/LSU AgCenter

Underground systems are effective and convenient, but they are expensive to purchase and must be installed professionally. Photo by Rick Bogren/LSU AgCenter