Watering with a hand-held nozzle or fan may be enjoyable for you, but it's usually inadequate for plants--it takes too long to truly soak the soil. Hand watering is, however, useful for new transplants, seedlings, and container plants, since you can apply the water gently and exactly where it's needed.

Sprinklers, which essentially produce artificial rainfall, offer the simplest way to apply water over a large surface. Many plants, particularly those that like a cool, humid atmosphere, thrive with overhead sprinkling. And this method rinses dust from foliage and discourages certain pests, especially spider mites. But sprinkling has some negative aspects as well. First, it's wasteful: wind can carry off some water before it even reaches the ground, and water that falls or runs off onto pavement is lost too. In humid climates, sprinkling encourages some foliage diseases such as black spot and rust (though you can minimize this risk by sprinkling early in the morning, so that leaves dry quickly as the day warms). Another potential drawback is that plants with weak stems and/or heavy flowers bend and can break under a heavy load of water.

MEASURING SPRINKLER DELIVERY RATE AND DISPERSION

To sprinkle effectively, you need to know how fast water penetrates your soil and the delivery rate of your sprinklers. One inch of water (from sprinkling or rainfall) moistens about 12 inches in sandy soil, 7 inches in loam, and 4 to 5 inches in clay. Thus, if you want to water to a depth of 12 inches, you'll need to apply about an inch of water to sandy soil, 2 1/2 to 3 inches to clay soil.

To determine delivery rate, place a number of equal-size containers (straight-sided coffee cups, for example) at regular intervals outward from the sprinkler, as shown above. Then turn on the water and note how long it takes to fill a container with an inch of water. This test will also show you the delivery pattern: that is, the containers will typically fill at different rates. To ensure that every area ultimately receives the same amount of water, you'll need to move the sprinklers so that the coverage overlaps.